Proceedings Volume 9912

Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II

Ramón Navarro, James H. Burge
Proceedings Volume 9912

Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II

Ramón Navarro, James H. Burge
Purchase the printed version of this volume at or access the digital version at SPIE Digital Library.

Volume Details

Date Published: 24 October 2016
Contents: 30 Sessions, 215 Papers, 0 Presentations
Conference: SPIE Astronomical Telescopes + Instrumentation 2016
Volume Number: 9912

Table of Contents


Table of Contents

All links to SPIE Proceedings will open in the SPIE Digital Library. external link icon
View Session icon_mobile_dropdown
  • Front Matter: Volume 9912
  • Atmospheric Compensation
  • Segment Mirror Technologies
  • Optical Fabrication
  • Materials
  • Large Optics Manufacturing
  • Test and Metrology I
  • Test and Metrology II
  • Technologies for Cryogenic Instruments
  • Telescope Structures and Domes
  • Active Instruments (Active Structures, Active Optics)
  • Optical Fibers and Positioners I
  • Optical Fibers and Positioners II
  • Multi Object Spectroscopy
  • Slit Spectroscopy and Image Slicers
  • Coatings, Filters and Gratings I
  • Coatings, Filters and Gratings II
  • Coronography and High Contrast Imaging
  • Poster Session: Coatings, Filters and Gratings
  • Poster Session: Materials
  • Poster Session: Optical Fabrication
  • Poster Session: Test and Metrology
  • Poster Session: Telescope Structures and Domes
  • Poster Session: Optical Fibers and Positioners
  • Poster Session: Technology for Spectroscopy
  • Poster Session: Technologies for Cryogenic Instruments
  • Poster Session: Active Instruments (Active Structures, Active Optics)
  • Poster Session: Coronography and High Contrast Imaging
  • Poster Session: Atmospheric Compensation and Polarimetry
  • Poster Session: Revolutionary Technologies
Front Matter: Volume 9912
Front Matter: Volume 9912
This PDF file contains the front matter associated with SPIE Proceedings Volume 9912, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.
Atmospheric Compensation
A prototype of the NFIRAOS to instrument thermo-mechanical interface
NFIRAOS is the first light adaptive optics system for the Thirty Meter Telescope (TMT). NFIRAOS components are maintained at a stable -30°C ±0.5°C by embedding an actively cooled refrigeration system in the walls of the NFIRAOS enclosure. Three instruments are attached to interface ports in the NFIRAOS enclosure and are required to be thermally stable while the instrument rotates in place. Additionally, instruments must be installed and removed while NFIRAOS is cold to avoid lengthy cool-down cycles. A portion of the actively cooled enclosure system and the interface has been prototyped at NRC-Herzberg. We present a description of the design of the interface and results of testing so far and lessons learned.
Polarization dOTF: on-sky focal plane wavefront sensing
Keira J. Brooks, Laure Catala, Matthew A. Kenworthy, et al.
The differential Optical Transfer Function (dOTF) is a focal plane wavefront sensing method that uses a diversity in the pupil plane to generate two different focal plane images. The difference of their Fourier transforms recovers the complex amplitude of the pupil down to the spatial scale of the diversity. We produce two simultaneous PSF images with diversity using a polarizing filter at the edge of the telescope pupil, and a polarization camera to simultaneously record the two images. Here we present the first on-sky demonstration of polarization dOTF at the 1.0m South African Astronomical Observatory telescope in Sutherland, and our attempt to validate it with simultaneous Shack-Hartmann wavefront sensor images.
Electromagnetic deformable mirror development at TNO
Stefan Kuiper, Niek Doelman, Evert Nieuwkoop, et al.
Over the last decade TNO has developed a deformable mirror concept using electromagnetic actuators with the main advantages of having very low non-linearity and hysteresis, low power consumption, and high inherent reliability of the actuators. TNO recently started a program to redesign the electromagnetic actuator to improve the actuator efficiency, allowing higher actuator force per volume and per wattage. The increased actuator efficiency gives improvement of the DM performance in terms of dynamical performance, actuation range, and power dissipation. With this technology various applications in the fields of ground-based astronomy and space missions are targeted.
NFIRAOS beamsplitters subsystems optomechanical design
Frédéric Lamontagne, Nichola Desnoyers, Reston Nash, et al.
The early-light facility adaptive optics system for the Thirty Meter Telescope (TMT) is the Narrow-Field InfraRed Adaptive Optics System (NFIRAOS). The science beam splitter changer mechanism and the visible light beam splitter are subsystems of NFIRAOS. This paper presents the opto-mechanical design of the NFIRAOS beam splitters subsystems (NBS). In addition to the modal and the structural analyses, the beam splitters surface deformations are computed considering the environmental constraints during operation. Surface deformations are fit to Zernike polynomials using SigFit software. Rigid body motion as well as residual RMS and peak-to-valley surface deformations are calculated. Finally, deformed surfaces are exported to Zemax to evaluate the transmitted and reflected wave front error. The simulation results of this integrated opto-mechanical analysis have shown compliance with all optical requirements.
Segment Mirror Technologies
Development and test of the Ball Aerospace optical frequency comb: a versatile measurement tool for aerospace applications
The Ball Fiber Optical Comb Demo is a lab-based system which is used to develop space applications for optical frequency combs. These developments utilize the broadband optical coherence of the frequency comb to expand the capabilities of ground test and orbital systems used for optical wave-front measurement, control of adaptive optics, precision ranging, and reference frequency stabilization. The work expands upon a NIST-developed all-fiber frequency comb that exhibits high stability in a compact, enclosed package.

Previously demonstrated applications for frequency combs include: Spectroscopy, distance and velocity measurement, frequency conversion, and timing transfer. Results from the Ball system show the characterization and performance of a frequency comb system with a technological path-to-space. Demonstrations in high precision metrology and long distance ranging are also presented for application in adaptive and multi-body optical systems.
A soft actuator for Prototype Segmented Mirror Telescope
Prasanna Deshmukh, Padmakar Parihar, Deepta Sundar Mishra, et al.
The Segmented Mirror Telescopes (SMT) are built using small hexagonal mirror segments placed side by side to form a monolithic primary mirror of very large size. The effective figure of such a segmented primary mirror is maintained against external disturbances introduced by gravity, temperature, wind and vibration with the help of primary mirror active control system. This active control system consists of two levels of control – global and local level. At the global scale, three actuators per segment and two edge sensors per intersegment sides are used to maintain the shape of the primary mirror. At the local level, actuator control system executes the commands generated by the global control loop. Every mirror segment is controlled with the help of three actuators, where the major role of these actuators is to provide a tip, tilt, and piston to the mirror segments. In this paper, we describe the actuator developed for 1.5m diameter Prototype Segmented Mirror Telescope (PSMT). The actuator for this telescope is a soft actuator based on the voice coil mechanism. This actuator is designed for with the range of travel of ±1.5mm and the force range of 25N along with an offloading capability to reduce the power consumption. The prototype actuator is undergoing different tests at Indian Institute of Astrophysics (IIA), Bangalore. The tracking rate of 324nm/s is achieved with the tracking error of 22.5 nm RMS.
ZERODUR strength modeling with Weibull statistical distributions
The decisive influence on breakage strength of brittle materials such as the low expansion glass ceramic ZERODUR is the surface condition. For polished or etched surfaces it is essential if micro cracks are present and how deep they are. Ground surfaces have many micro cracks caused by the generation process. Here only the depths of the micro cracks are relevant. In any case presence and depths of micro cracks are statistical by nature.

The Weibull distribution is the model used traditionally for the representation of such data sets. It is based on the weakest link ansatz. The use of the two or three parameter Weibull distribution for data representation and reliability prediction depends on the underlying crack generation mechanisms. Before choosing the model for a specific evaluation, some checks should be done. Is there only one mechanism present or is it to be expected that an additional mechanism might contribute deviating results? For ground surfaces the main mechanism is the diamond grains’ action on the surface. However, grains breaking from their bonding might be moved by the tool across the surface introducing a slightly deeper crack. It is not to be expected that these scratches follow the same statistical distribution as the grinding process. Hence, their description with the same distribution parameters is not adequate. Before including them a dedicated discussion should be performed.

If there is additional information available influencing the selection of the model, for example the existence of a maximum crack depth, this should be taken into account also. Micro cracks introduced by small diamond grains on tools working with limited forces cannot be arbitrarily deep. For data obtained with such surfaces the existence of a threshold breakage stress should be part of the hypothesis. This leads to the use of the three parameter Weibull distribution. A differentiation based on the data set alone without preexisting information is possible but requires a large data set. With only 20 specimens per sample such differentiation is not possible. This requires 100 specimens per set, the more the better.

The validity of the statistical evaluation methods is discussed with several examples. These considerations are of special importance because of their consequences on the prognosis methods and results. Especially the use of the two parameter Weibull distribution for high strength surfaces has led to non-realistic results. Extrapolation down to low acceptable probability of failure covers a wide range without data points existing and is mainly influenced by the slope determined by the high strength specimens. In the past this misconception has prevented the use of brittle materials for stress loads, which they could have endured easily.
Development and final design of FAME active array
FAME (Freeform Active Mirror Experiment - part of the FP7 OPTICON/FP7 development programme) intends to demonstrate the huge potential of active mirrors and freeform optical surfaces. Freeform active surfaces can help to address the new challenges of next generation astronomical instruments, which are bigger, more complex and have tighter specifications than their predecessors.

The FAME design consists of a pre-formed, deformable thin mirror sheet with an active support system. The thin face sheet provides a close to final surface shape with very high surface quality. The active array provides the support, and through actuation, the control to achieve final surface shape accuracy.

In this paper the development path, trade-offs and demonstrator design of the FAME active array is presented. The key step in the development process of the active array is the design of the mechanical structure and especially the optimization of the actuation node positions, where the actuator force is transmitted to the thin mirror sheet. This is crucial for the final performance of the mirror where the aim is to achieve an accurate surface shape, with low residual (high order) errors using the minimum number of actuators. These activities are based on the coupling of optical and mechanical engineering, using analytical and numerical methods, which results in an active array with optimized node positions and surface shape.
Stressed mirror annular polishing for scale-down TMT primary segments
A new type Stressed Mirror Polishing method using annular polishing machine is developed in NIAOT. It provides good efficiency for the massive production of off-axis segments for the extremely large telescope because 3 or more pieces of segment can be polished simultaneously on a AP machine. With an annular polishing machine with 3.6m diameter, two scale-down TMT segments have been polished. Both 2 segments are Φ1100mm in diameter, with the vertex radius of curvature of 60m and aspheric constant K=-1.000953. The off-axis distances (OAD) are 8m and 12m respectively. After SMAP process, the acceptable surface accuracy can be reached, which is 1.12μm/0.23μm of PV/RMS value for the segment with 8m OAD, and 1.22 μm/0.26 μm for another one.
Research on key technical issues of contact method for large diameter off-axis aspheric segments
ZiBo Jiang, XinNan Li, YongJun Liang, et al.
The research works are summarized for Φ1.1m off-axis aspheric segments which are scaled-down TMT segments polished by NIAOT using SMAP (Stressed Mirror Annular Polishing) method in the previous phase and testing preparations for 1.45m mirrors. The detailed introduction is given on result of errors analysis in contact detection testing of segments. In the second part, the selected basis of sampling number for contact-type detector arrays and the terms number of Zernike polynomials we need are studied. And the situations on orthogonality destruction of Zernike polynomials in discrete points sampling case and spectral analysis for each order Spherical aberrations in continuous sampling are introduced.
Optical Fabrication
Technique for diamond machining large ZnSe grisms for the Rapid Infrared/Imager Spectrograph (RIMAS)
Paul J. Kuzmenko, Steve L. Little, Alexander S. Kutyrev, et al.
The Rapid Infrared Imager/Spectrograph (RIMAS) is an instrument designed to observe gamma ray burst afterglows following initial detection by the SWIFT satellite. Operating in the near infrared between 0.9 and 2.4 μm, it has capabilities for both low resolution (R~25) and moderate resolution (R~4000) spectroscopy. Two zinc selenide (ZnSe) grisms provide dispersion in the moderate resolution mode: one covers the Y and J bands and the other covers the H and K. Each has a clear aperture of 44 mm. The YJ grism has a blaze angle of 49.9° with a 40 μm groove spacing. The HK grism is blazed at 43.1° with a 50 μm grooves spacing.

Previous fabrication of ZnSe grisms on the Precision Engineering Research Lathe (PERL II) at LLNL has demonstrated the importance of surface preparation, tool and fixture design, tight thermal control, and backup power sources for the machine. The biggest challenges in machining the RIMAS grisms are the large grooved area, which indicates long machining time, and the relatively steep blaze angle, which means that the grism wavefront error is much more sensitive to lathe metrology errors. Mitigating techniques are described.
Polishing techniques for MEGARA pupil elements optics
R. Izazaga, E. Carrasco, D. Aguirre, et al.
MEGARA (Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía) is the new integral-field and multi-object optical spectrograph for the 10.4m Gran Telescopio Canarias.. It will offer RFWHM ~6,000, 12,000 and 18,700 for the low- , mid- and high-resolution, respectively in the wavelength range 3650-9700Å. .The dispersive elements are volume phase holographic (VPH) gratings, sandwiched between two flat Fused Silica windows of high optical precision in large apertures. The design, based in VPHs in combination with Ohara PBM2Y prisms allows to keep the collimator and camera angle fixed. Seventy three optical elements are being built in Mexico at INAOE and CIO. For the low resolution modes, the VPHs windows specifications in irregularity is 1 fringe in 210mm x 170mm and 0.5 fringe in 190mm x 160mm. for a window thickness of 25 mm. For the medium and high resolution modes the irregularity specification is 2 fringes in 220mm x 180mm and 1 fringe in 205mm x 160mm, for a window thickness of 20mm. In this work we present a description of the polishing techniques developed at INAOE optical workshop to fabricate the 36 Fused Silica windows and 24 PBM2Y prisms that allows us to achieve such demanding specifications. We include the processes of mounting, cutting, blocking, polishing and testing.
A large size ion beam figuring system for 1.2m astronomical telescopes fabrication
Xuhui Xie, Bing Yang, Lin Zhou, et al.
An ion beam figuring system (KDIBF2000) for final figuring of large size optics has been designed and built by National University of Defense Technology in China. It can figure optics up to the maximum dimensions of 2.0m×2.0m×0.4m with five axes of servo-motion used to control ion source movement. For operational facility, there are two vacuum chambers with main work chamber and a small supplementary chamber isolated by a flapper valve. The main chamber has two work zones, which can meantime hold a large optics with Φ1.5m and a small optics with 0.4m. The small optics can be transferred through supplementary chamber with a moving vehicle. By this way, it is very convenient and economical to gain the material removal function and check the system’s process performance. Now, this system has been gone into running to figure large SiC off-axis aspheric optics. Next step, a 1.2m SiC aspheric primary mirror will be figure by this system.
Manufacturing of super-polished large aspheric/freeform optics
Several next generation astronomical telescopes or large optical systems utilize aspheric/freeform optics for creating a segmented optical system. Multiple mirrors can be combined to form a larger optical surface or used as a single surface to avoid obscurations. In this paper, we demonstrate a specific case of the Daniel K. Inouye Solar Telescope (DKIST). This optic is a 4.2 m in diameter off-axis primary mirror using ZERODUR thin substrate, and has been successfully completed in the Optical Engineering and Fabrication Facility (OEFF) at the University of Arizona, in 2016. As the telescope looks at the brightest object in the sky, our own Sun, the primary mirror surface quality meets extreme specifications covering a wide range of spatial frequency errors. In manufacturing the DKIST mirror, metrology systems have been studied, developed and applied to measure low-to-mid-to-high spatial frequency surface shape information in the 4.2 m super-polished optical surface. In this paper, measurements from these systems are converted to Power Spectral Density (PSD) plots and combined in the spatial frequency domain. Results cover 5 orders of magnitude in spatial frequencies and meet or exceed specifications for this large aspheric mirror. Precision manufacturing of the super-polished DKIST mirror enables a new level of solar science.
Freeform and advanced optics for ELT instrumentation
In this paper we share some recent work performed at REOSC in the domain of advanced optics for space and that is also directly applicable to astronomical instrumentation, e.g. for the Extremely Large Telescopes (ELT), the construction of which has already started. We present firstly the results of some design investigations performed on Three Mirror Anastigmat (TMA) imaging optics when using freeform optical surfaces clearly showing gain in performance (WFE, distortion, … ) or compactness of the optics. We separate smart freeform from more aggressive freeform offering increased level of gain in design performances.

Secondly we present our development in freeform and direct off-axis high performance optical manufacturing capabilities and the industrialization efforts conducted in the frame of the European Extremely Large Telescope (E-ELT) primary mirror segments.

A third subject is the demonstration of an extreme freeform surface manufacturing with the prototyping of a huge 500 mm aperture, 90° deviation angle, F/2.5 high output NA Off Axis Parabola (OAP), a unique achievement aimed to confirm the viability of potential new design opportunities involving such type of extreme optics.

Finally we present in this paper our technology development on polishing layer for SiC material, named R-SiC, a polishing layer that reduces costs, risks and schedule for advanced SiC optics manufacturing for Vis and IR applications.
ZERODUR thermo-mechanical modelling and advanced dilatometry for the ELT generation
Ralf Jedamzik, Clemens Kunisch, Thomas Westerhoff
Large amounts of low thermal expansion material are required for the upcoming ELT projects. The main mirror is designed using several hundreds of hexagonal 1.4 m sized mirror blanks. The M2 and M3 are monolithic 4 m class mirror blanks. The mirror blank material needs to fulfill tight requirements regarding CTE specification and homogeneity.

Additionally the mirror blanks need to be dimensionally stable for more than 30 years. In particular, stress effects due to the changes in the environment shall not entail shape variation of more than 0.5 μm PV within 30 years.

In 2010 SCHOTT developed a physically based model to describe the thermal and mechanical long time behavior of ZERODUR. The model enables simulation of the long time behavior of ZERODUR mirror blanks under realistic mechanical and thermal constraints. This presentation shows FEM simulation results on the long time behavior of the ELT M1, M2 and M3 mirror blanks under different loading conditions. Additionally the model results will be compared to an already 15 years lasting long time measurement of a ZERODUR sample at the German federal physical standardization institute (PTB).

In recent years SCHOTT pushed the push rod dilatometer measurement technology to its limit. With the new Advanced Dilatometer CTE measurement accuracies of +- 3 ppb/K and reproducibilities of better 1 ppb/K have been achieved. The new Advanced Dilatometer exhibits excellent long time stability.
Fused silica challenges in sensitive space applications
Space bound as well as earthbound spectroscopy of extra-terrestrial objects finds its challenge in light sources with low intensities. High transmission for every optical element along the light path requires optical materials with outstanding performance to enable the measurement of even a one-photon event. Using the Lunar Laser Ranging Project and the LIGO and VIRGO Gravitational Wave Detectors as examples, the influence of the optical properties of fused silica will be described. The Visible and Infrared Surveillance Telescope for Astronomy (VISTA) points out the material behavior in the NIR regime, where the chemical composition of optical materials changes the performance. Special fibers are often used in combination with optical elements as light guides to the spectroscopic application. In an extended spectral range between 350 and 2,200 nm Heraeus developed STU fiber preforms dedicated for broad band spectroscopy in astronomy. STU fibers in the broad spectral range as well as SSU fibers for UV transmission (180 – 400 nm) show also high gamma radiation resistance which allows space applications.
Manufacturing aspheric mirrors made of zero thermal expansion cordierite ceramics using Magnetorheological Finishing (MRF)
NEXCERATM cordierite ceramics, which have ultra-low thermal expansion properties, are perfect candidate materials to be used for light-weight satellite mirrors that are used for geostationary earth observation and for mirrors used in ground-based astronomical metrology. To manufacture the high precision aspheric shapes required, the deterministic aspherization and figure correction capabilities of Magnetorheological Finishing (MRF) are tested. First, a material compatibility test is performed to determine the best method for achieving the lowest surface roughness of RMS ~0.8nm on plano surfaces made of NEXCERATM ceramics. Secondly, we will use MRF to perform high precision figure correction and to induce a hyperbolic shape into a conventionally polished 100mm diameter sphere.
Silicon carbide main structure for EUCLID NISP instrument in final development
T. Pamplona, J.-L. Gimenez, A. Febvre, et al.
In the scope of EUCLID spatial mission, NISP instrument requires high positioning accuracy and high dimensional stability to achieve the required optical performances. LAM is in charge of the development of the instrument main structure which is based on silicon carbide material technology and allows the accurate positioning and maintain of the optomechanical concept sub-systems. This article presents the main steps of this development. It describes the challenging design of this mechanical concept. The associated finite element model, demonstrating the thermomechanical strength of the structure, is presented. Spatial environment vibrations tests performed on the hardware are explained and detailed: requirements, instrumentation and test methodology with the introduction of notching. Finally, the correlation study between finite element analyses and tests is exposed.
Large Optics Manufacturing
Fabrication and testing of 4.2m off-axis aspheric primary mirror of Daniel K. Inouye Solar Telescope
Daniel K. Inouye Solar Telescope (formerly known as Advanced Technology Solar Telescope) will be the largest optical solar telescope ever built to provide greatly improved image, spatial and spectral resolution and to collect sufficient light flux of Sun. To meet the requirements of the telescope the design adopted a 4m aperture off-axis parabolic primary mirror with challenging specifications of the surface quality including the surface figure, irregularity and BRDF. The mirror has been completed at the College of Optical Sciences in the University of Arizona and it meets every aspect of requirement with margin. In fact this mirror may be the smoothest large mirror ever made.

This paper presents the detail fabrication process and metrology applied to the mirror from the grinding to finish, that include extremely stable hydraulic support, IR and Visible deflectometry, Interferometry and Computer Controlled fabrication process developed at the University of Arizona.
New and improved technology for manufacture of GMT primary mirror segments
Dae Wook Kim, James H. Burge, Jonathan M. Davis, et al.
The Giant Magellan Telescope (GMT) primary mirror consists of seven 8.4 m light-weight honeycomb mirrors that are being manufactured at the Richard F. Caris Mirror Lab (RFCML), University of Arizona. In order to manufacture the largest and most aspheric astronomical mirrors various high precision fabrication technologies have been developed, researched and implemented at the RFCML. The unique 8.4 m (in mirror diameter) capacity fabrication facilities are fully equipped with large optical generator (LOG), large polishing machine (LPM), stressed lap, rigid conformal lap (RC lap) and their process simulation/optimization intelligence called MATRIX. While the core capability and key manufacturing technologies have been well demonstrated by completing the first GMT off-axis segment, there have been significant hardware and software level improvements in order to improve and enhance the GMT primary mirror manufacturing efficiency. The new and improved manufacturing technology plays a key role to realize GMT, the next generation extremely large telescope enabling new science and discoveries, with high fabrication efficiency and confidence.
Polishing and testing of the 3.4 m diameter f/1.5 primary mirror of the INO telescope
Tapio Korhonen, Perttu Keinänen, Mikko Pasanen, et al.
Polishing and testing methods used in the manufacture of the 3.4 m primary mirror of the Iranian National Observatory (INO) telescope are described and the test results of the finished mirror are presented. Mirror lapping and polishing was performed using several rectangular non-rotating tools arranged in a linear array across the mirror radius. Each tool is equipped with two computer controlled force actuators for regulating the surface pressure and removal efficiency during the lapping and polishing operations. The same tool system was used from the lapping phase to the end of the final polishing. The principal optical test method was the interferometric Hartmann test with the aid of a two component null lens in the mirror center of curvature. Mirror measurements were made also with pentaprism test to verify its correct conic constant. The mirror was finished to extremely good surface accuracy and smoothness.
Modern technologies of fabrication and testing of large convex secondary mirrors
Modern large telescopes such as TAO, LSST, TMT and EELT require 0.9m-4m monolithic convex secondary mirrors. The fabrication and testing of these large convex secondary mirrors of astronomical telescopes is getting challenging as the aperture of the mirror is getting bigger. The biggest challenge to fabricate these large convex aspheric mirrors is to measure the surface figure to a few nanometers, while maintaining the testing and fabrication cycle to be efficient to minimize the downtime. For the last a couple of decades there was huge advancement in the metrology and fabrication of large aspheric secondary mirrors. College of Optical Sciences in the University Arizona developed a full fabrication and metrology process with extremely high accuracy and efficiency for manufacturing the large convex secondary mirrors.

In this paper modern metrology systems including Swing-Arm Optical Coordinate Measuring System (SOCMM) which is comparable to Interferometry and a Sub-aperture stitching interferometry scalable to a several meters have been presented. Also a Computer Controlled Fabrication Process which produces extremely fine surface figure and finish has been demonstrated. These most recent development has been applied to the fabrication and testing of 0.9m aspheric convex secondary mirror for the Tokyo Atacama Observatory’s 6.5m telescope and the result has been presented.
Advanced Mirror Technology Development (AMTD) project: overview and year four accomplishments
The Advanced Mirror Technology Development (AMTD) project is in Phase 2 of a multiyear effort initiated in Fiscal Year (FY) 2012, to mature toward the next Technology Readiness Level (TRL) critical technologies required to enable 4-m-or-larger monolithic or segmented ultraviolet, optical, and infrared (UVOIR) space telescope primary-mirror assemblies for general astrophysics and ultra-high-contrast observations of exoplanets. Key hardware accomplishments of 2015/16 are the successful low-temperature fusion of a 1.5-meter diameter ULE mirror that is a 1/3rd scale model of a 4-meter mirror and the initiation of polishing of a 1.2-meter Extreme-Lightweight Zerodur mirror. Critical to AMTD’s success is an integrated team of scientists, systems engineers, and technologists; and a science-driven systems engineering approach.
Thin glass shells for AO: from plano to off-axis aspherics
Emmanuelle Harel, Alain Anretar, Jean-Pierre Antelme, et al.
Reosc has been working on thin glass shells for many years and was recently selected by ESO for the production of the E-ELT M4 mirror thin glass shells. Previously Reosc also produced the aspheric thin shell for the VLT-M2 AO Facility. Based on this experience we will discuss how off axis thin glass shells can be made for the next generation AO systems like the GMT one.
Status of mirror segment production for the Giant Magellan Telescope
H. M. Martin, J. H. Burge, J. M. Davis, et al.
The Richard F. Caris Mirror Lab at the University of Arizona is responsible for production of the eight 8.4 m segments for the primary mirror of the Giant Magellan Telescope, including one spare off-axis segment. We report on the successful casting of Segment 4, the center segment. Prior to generating the optical surface of Segment 2, we carried out a major upgrade of our 8.4 m Large Optical Generator. The upgrade includes new hardware and software to improve accuracy, safety, reliability and ease of use. We are currently carrying out an upgrade of our 8.4 m polishing machine that includes improved orbital polishing capabilities. We added and modified several components of the optical tests during the manufacture of Segment 1, and we have continued to improve the systems in preparation for Segments 2-8. We completed two projects that were prior commitments before GMT Segment 2: casting and polishing the combined primary and tertiary mirrors for the LSST, and casting and generating a 6.5 m mirror for the Tokyo Atacama Observatory.
Manufacture and final tests of the LSST monolithic primary/tertiary mirror
H. M. Martin, J. R. P. Angel, G. Z. Angeli, et al.
The LSST M1/M3 combines an 8.4 m primary mirror and a 5.1 m tertiary mirror on one glass substrate. The combined mirror was completed at the Richard F. Caris Mirror Lab at the University of Arizona in October 2014. Interferometric measurements show that both mirrors have surface accuracy better than 20 nm rms over their clear apertures, in nearsimultaneous tests, and that both mirrors meet their stringent structure function specifications. Acceptance tests showed that the radii of curvature, conic constants, and alignment of the 2 optical axes are within the specified tolerances. The mirror figures are obtained by combining the lab measurements with a model of the telescope’s active optics system that uses the 156 support actuators to bend the glass substrate. This correction affects both mirror surfaces simultaneously. We showed that both mirrors have excellent figures and meet their specifications with a single bending of the substrate and correction forces that are well within the allowed magnitude. The interferometers do not resolve some small surface features with high slope errors. We used a new instrument based on deflectometry to measure many of these features with sub-millimeter spatial resolution, and nanometer accuracy for small features, over 12.5 cm apertures. Mirror Lab and LSST staff created synthetic models of both mirrors by combining the interferometric maps and the small highresolution maps, and used these to show the impact of the small features on images is acceptably small.
Test and Metrology I
Large aperture freeform VIS telescope with smart alignment approach
Matthias Beier, Wilko Fuhlrott, Johannes Hartung, et al.
The development of smart alignment and integration strategies for imaging mirror systems to be used within astronomical instrumentation are especially important with regard to the increasing impact of non-rotationally symmetric optics. In the present work, well-known assembly approaches preferentially applied in the course of infrared instrumentation are transferred to visible applications and are verified during the integration of an anamorphic imaging telescope breadboard. The four mirror imaging system is based on a modular concept using mechanically fixed arrangements of each two freeform surfaces, generated by servo assisted diamond machining and corrected using Magnetorheological Finishing as a figuring and smoothing step. Surface testing include optical CGH interferometry as well as tactile profilometry and is conducted with respect to diamond milled fiducials at the mirror bodies. A strict compliance of surface referencing during all significant fabrication steps allow for an easy integration and direct measurement of the system's wave aberration after initial assembly. The achievable imaging performance, as well as influences of the tight tolerance budget and mid-spatial frequency errors, are discussed and experimentally evaluated.
Effects of thermal inhomogeneity on 4m class mirror substrates
Ralf Jedamzik, Clemens Kunisch, Thomas Westerhoff
The new ground based telescope generation is moving to a next stage of performance and resolution. Mirror substrate material properties tolerance and homogeneity are getting into focus. The coefficient of thermal expansion (CTE) homogeneity is even more important than the absolute CTE. The error in shape of a mirror, even one of ZERODUR, is affected by changes in temperature, and by gradients in temperature. Front to back gradients will change the radius of curvature R that in turn will change the focus. Some systems rely on passive athermalization and do not have means to focus. Similarly changes in soak temperature will result in surface changes to the extent there is a non-zero coefficient of thermal expansion. When there are in-homogeneities in CTE, the mirror will react accordingly. Results of numerical experiments are presented discussing the impact of CTE in-homogeneities on the optical performance of 4 m class mirror substrates. Latest improvements in 4 m class ZERODUR CTE homogeneity and the thermal expansion metrology are presented as well.
Unmanned aerial vehicles in astronomy
In this work we discuss some options for using Unmanned Aerial Vehicles (UAVs) for daylight alignment activities and maintenance of optical telescopes, relating them to a small numbers of parameters, and tracing which could be the schemes, requirements and benefits for employing them both at the stage of erection and maintenance. UAVs can easily reach the auto-collimation points of optical components of the next class of Extremely Large Telescopes. They can be equipped with tools for the measurement of the co-phasing, scattering, and reflectivity of segmented mirrors or environmental parameters like C2n and C2T to characterize the seeing during both the day and the night.
Coordinate metrology of a primary surface composite panel from the Large Millimeter Telescope
David M. Gale, Maribel Lucero Álvarez, Lizeth Cabrera Cuevas, et al.
The Large Millimeter Telescope (LMT) is a single-dish fully-steerable radio telescope presently operating with a 32.5 m parabolic primary reflector, in the process of extension to 50 m. The project is managed by the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE) in México, and the University of Massachusetts Amherst, USA. A laminated surface panel from the LMT primary reflector has been subjected to a surface measurement assay at Mexico’s National Metrology Center (CENAM). Data obtained using a coordinate measuring machine and laser tracker owned by CENAM is compared with measurements using an identical model laser tracker and the photogrammetry technique, the latter systems owned and operated by the LMT. All measurements were performed within the controlled metrology environment at CENAM. The measurement exercise is intended to prepare the groundwork for converting this spare surface panel into a calibrated work-piece. The establishment of a calibrated work-piece provides quality assurance for metrology through measurement traceability. It also simplifies the evaluation of measurement uncertainty for coordinate metrology procedures used by the LMT project during reflector surface qualification.
Error analysis of back focal length measurement technique for long focus large-scaled lenses
Ivan N. Sudarikov, Magomed A. Abdulkadyrov, Aleksandr N. Ignatov, et al.
Interference method of testing the long-focus focusing lenses allows measuring such optical parameters as focal distances and distortions of wavefront passing through the lens. High measurement accuracy is achieved by means of a special laser measurement system (laser tracker) in autocollimation schematic based on dynamic interferometer. Main sources of instrument error were found out and their relation to the measurement schematic parameters was defined. The article shows that the method allows achieving total instrument measurement error of the 0.003% of the value being measured.
Test and Metrology II
Evaluation of novel approach to deflectometry for high accuracy optics
A deflectometrical facility was developed at Italian National Institute for Astrophysics-OAB to characterize free-form optics with shape errors within few microns rms.

Deflectometry is an interesting technique because it allows the fast characterization of free-form optics. The capabilities of deflectometry in measuring medium-high frequencies are well known, but the low frequencies error characterization is more challenging. Our facility design foresees an innovative approach based on the acquisition of multiple direct images to enhance the performance on the challenging low frequencies range.

This contribution presents the error-budget analysis of the measuring method and a study of the configuration tolerances required to allow the use of deflectometry in the realization of optical components suitable for astronomical projects with a requirement of high accuracy for the optics. As test examples we took into account mirrors for the E-ELT telescope.
Laboratory and field testing results of the LMT/GTM primary surface actuators
David R. Smith, Kamal Souccar, Gabriela Montalvo, et al.
With the final installation of the two outermost rings of the primary surface of the Large Millimeter Telescope/ Gran Telescopio Milimétrico (LMT/GTM), the project is also upgrading the primary surface actuators. There are commercial actuators that can approach the required operational accuracy and stroke, but the combination of the size and load requirements ultimately required a customized design. The new actuators fit within the volume constraints imposed by the tighter interior angles in the outer rings and are designed to support the operational and survival loading conditions even for the largest surface segments. Laboratory testing confirmed that the actuators should meet the precision, repeatability, load, and lifetime requirements.

However, the LMT/GTM is at a particularly difficult site for electromechanical systems. The high altitude has the usual effect of reducing cooling effectiveness for the drives and motors, and the ambient temperature hovers near freezing. Since there is a significant amount of precipitation during some times of the year, there are frequent freeze/thaw cycles. The constant formation and either sublimation or melting of ice, along with the associated high humidity, has been a challenge for the environmental protection of many devices at the LMT/GTM. Because there are a total of 720 primary surface actuators in the system, it is particularly important that the actuators, their local drive control boxes, and their cable connections be able to meet its specifications even under the site conditions.

To confirm the suitability of the actuators, the LMT/GTM procured an initial set of sixteen actuators for testing at the site. After laboratory testing, the actuators were installed into the outer two rings of the telescope and cycled during the early winter months of the 2015–16 scientific observing season. Because of the continuing installation activities in these two rings, they are not illuminated by the receivers, so field testing under actual operational conditions could be conducted without affecting the ongoing scientific observations. This paper presents the characterized performance of the actuators before and after testing, as well as a report on their environmental robustness.
Optical tests of the Si immersed grating demonstrator for METIS
Immersed gratings offer several advantages over conventional gratings: more compact spectrograph designs, and by using standard semiconductor industry techniques, higher diffraction-efficiency and lower stray-light can be achieved. We present the optical tests of the silicon immersed grating demonstrator for the Mid-infrared E-ELT Imager and Spectrograph, METIS. We detail the interferometric tests that were done to measure the wavefront-error and present the results of the throughput and stray-light measurements. We also elaborate on the challenges encountered and lessons learnt during the immersed grating demonstrator test campaign that helped us to improve the fabrication processes of the grating patterning on the wafer.
Technologies for Cryogenic Instruments
Optical performance analysis and test results of the EUCLID near-infrared spectro-photometer
C. Gal, H. Thiele, E. Gubbini, et al.
The Near Infrared Spectrometer and Photometer (NISP) of the EUCLID satellite project encompasses high precision large lens mounts of 168 mm diameter that are operated at cryogenic temperatures down to 135K. The four lenses of the optical system are made of different materials: SUPRASIL 30001, CaF2, and S-FTM16, which are mounted in a separate lens mount design using glue connections. Each lens assembly has its individual mechanical interface to the structure, the so called lens barrel. Exhaustive structural and thermal investigations have determined lens surface deformations and lens position changes that are introduced by various environmental loads, such as thermal-, mechanical-, interface-, and gravity loads, as well as mechanical stress of the lenses due to glue shrinkage during curing. All these impacts change the lens optical behaviours under real operational conditions of the optical assembly, which are thoroughly investigated in the optical performance assessment activity. Especially, great effort has been made for the simulation of interface tolerances. Due to the complexity of all mechanical interfaces (baffle, lens mounts, housing, telescope structure, etc.) statistical simulation is conducted applying Monte Carlo method. From the result of the statistical simulation 3 representative cases are selected for the optical performance assessment, which have 95% confidence level of the lens surface deformation. In the context of the evaluation procedure the surface form error of all EUCLID lenses as well as the RMS WFE at the focal plane is assessed, and results are compared with the nominal performance of the system, as well as with interferometrically measured results achieved during the interface– and gravity release test campaign. The performance of the lens holder design in terms of glue shrinkage effects, gravity release and interface tolerances is verified by an adapted test facility including an interferometer based optical metrology system. Finally, the measured values are compared with the analytical results, which show great confidence and hence proves validation of the analytical model. The paper presents the optical performance analysis results and the measured performance of the EUCLID high precision large cryogenic lens mounts. The achievements are presented on behalf of the EUCLID consortium.
GMTIFS: the adaptive optics beam steering mirror for the GMT integral-field spectrograph
J. Davies, G. Bloxham, R. Boz, et al.
To achieve the high adaptive optics sky coverage necessary to allow the GMT Integral-Field Spectrograph (GMTIFS) to access key scientific targets, the on-instrument adaptive-optics wavefront-sensing (OIWFS) system must patrol the full 180 arcsecond diameter guide field passed to the instrument. The OIWFS uses a diffraction limited guide star as the fundamental pointing reference for the instrument. During an observation the offset between the science target and the guide star will change due to sources such as flexure, differential refraction and non-sidereal tracking rates. GMTIFS uses a beam steering mirror to set the initial offset between science target and guide star and also to correct for changes in offset. In order to reduce image motion from beam steering errors to those comparable to the AO system in the most stringent case, the beam steering mirror is set a requirement of less than 1 milliarcsecond RMS. This corresponds to a dynamic range for both actuators and sensors of better than 1/180,000.

The GMTIFS beam steering mirror uses piezo-walk actuators and a combination of eddy current sensors and interferometric sensors to achieve this dynamic range and control. While the sensors are rated for cryogenic operation, the actuators are not. We report on the results of prototype testing of single actuators, with the sensors, on the bench and in a cryogenic environment. Specific failures of the system are explained and suspected reasons for them. A modified test jig is used to investigate the option of heating the actuator and we report the improved results. In addition to individual component testing, we built and tested a complete beam steering mirror assembly. Testing was conducted with a point source microscope, however controlling environmental conditions to less than 1 micron was challenging. The assembly testing investigated acquisition accuracy and if there was any un-sensed hysteresis in the system. Finally we present the revised beam steering mirror design based on the outcomes and lessons learnt from this prototyping.
Development of superconducting voice coil motor of a cold chopper for MICHI
Kiyoshi Mori, Takashi Miyata, Mitsuhiko Honda, et al.
A cold chopper is a key device for next generation mid-infrared instruments such as TMT/MICHI. It should achieve fast and accurate position switching with a large chopping throw at cryogenic temperature. To satisfy the requirements, voice coil motors using superconducting MgB2 wire have been developed. We have made a first prototype of the VCM and carried out its performance measurements such as a transition temperature, transfer functions, and power dissipation in the laboratory. The results are almost consistent with the expectations and the calculations, but some show significant inconsistency. We have also made a next prototype which is small to fit the size of the MICHI chopper. This will be installed to a developing mid-infrared instrument MIMIZUKU and used for actual observations.
Characterization of the actuator of EMIR configurable slit unit
A. Mato Martínez, M. Núñez Cagigal, M. Barreto Cabrera, et al.
EMIR1,2 (Espectrógrafo Multiobjeto Infra-Rojo) is a wide field multi-object spectrograph already installed in the Nasmyth focus of GTC (Gran Telescopio Canarias). It operates in the near-infrared (NIR), in the wavelength range from 0.9 μm to 2.5 μm and it will include several mechanism working in cryogenic conditions.

A key component of EMIR is the CSU (Configurable Slit Unit), which is a robotic cryo-mechanism used to generate a multi-slit configuration and a long slit on EMIR focal plane when working in spectroscopic mode. The system has 110 sliding bars which can be configured at cryogenic working temperature to create up to 55 slits with a high position accuracy and repeatability. The movement of the bars is performed by an actuator which allows reaching a relatively high speed for the coarse movement and controllable steps up to 2 microns for the fine positioning. This subsystem has been designed and manufactured by the Dutch company Janssen Precision Engineering (JPE) and the Spanish company NTE-SENER. Afterwards, it was thoroughly verified at the IAC (Instituto de Astrofísica de Canarias) facilities.

In this paper, the CSU will be briefly described. One of the more important parts of the CSU is the actuators, which move the bars by means of a stick-slip effect. A set of tests designed for characterizing and improving the robustness and performance of the actuators will be presented. Finally, an overview of the current CSU performance will be presented.
A cryogenic 'set-and-forget' deformable mirror
Robin Trines, Huub Janssen, Sander Paalvast, et al.
This paper discusses the development, realization and initial characterization of a demonstrator for a cryogenic 'set and forget' deformable mirror. Many optical and cryogenic infrared instruments on modern very and extremely large telescopes aim at diffraction-limited performance and require total wave front errors in the order of 50 nanometers or less. At the same time, their complex optical functionality requires either a large number of spherical mirrors or several complex free-form mirrors. Due to manufacturing and alignment tolerances, each mirror contributes static aberrations to the wave front. Many of these aberrations are not known in the design phase and can only be measured once the system has been assembled. A 'set-and-forget' deformable mirror can be used to compensate for these aberrations, making it especially interesting for systems with complex free-form mirrors or cryogenic systems where access to iterative realignment is very difficult or time consuming.

The mirror with an optical diameter of 200 mm is designed to correct wave front aberrations of up to 2 μm root-mean square (rms). The shape of the wave front is approximated by the first 15 Zernike modes. Finite element analysis of the mirror shows a theoretically possible reduction of the wave front error from 2 μm to 53 nm rms. To produce the desired shapes, the mirror surface is controlled by 19 identical actuator modules at the back of the mirror.

The actuator modules use commercially available Piezo-Knob actuators with a high technology readiness level (TRL). These provide nanometer resolution at cryogenic temperatures combined with high positional stability, and allow for the system to be powered off once the desired shape is obtained. The stiff design provides a high resonance frequency (>200 Hz) to suppress external disturbances.

A full-size demonstrator of the deformable mirror containing 6 actuators and 13 dummy actuators is realized and characterized. Measurement results show that the actuators can provide sufficient stroke to correct the 2 μm rms WFE. The resolution of the actuator influence functions is found to be 0.24 nm rms or better depending on the position of the actuator within the grid. Superposition of the actuator influence functions shows that a 2 μm rms WFE can be accurately corrected with a 38 nm fitting error. Due to the manufacturing method of the demonstrator an artificially large print-through error of 182 nm is observed. The main cause of this print-through error has been identified and will be reduced in future design iterations. After these design changes the system is expected to have a total residual error of less than 70 nm and offer diffraction limited performance (λ14) for wavelengths of 1 μm and above.
Telescope Structures and Domes
Local seeing determination by thermal-CFD analysis to optimize the European Solar Telescope image quality
The European Solar Telescope, EST, ([1], [2]) is a 4-meter solar telescope to be built in the Canary Islands in the near future. In order to select the best configuration for the EST telescope facilities, thermal and CFD analyses have been carried out to evaluate the seeing degradation produced by the telescope environment. The aim of this study is to calculate the values of optical parameters in different configurations and to find out which one causes the lowest image quality degradation. Starting from the determination of seeing degradation along the optical path by CFD techniques, several configurations have been compared making it possible to decide the future development line for the EST.
Initial development of high-accuracy CFRP panel for DATE5 antenna
Yuan Qian, Zheng Lou, Xufeng Hao, et al.
DATE5 antenna, which is a 5m telescope for terahertz exploration, will be sited at Dome A, Antarctica. It is necessary to keep high surface accuracy of the primary reflector panels so that high observing efficiency can be achieved. In antenna field, carbon fiber reinforced composite (CFRP) sandwich panels are widely used as these panels are light in weight, high in strength, low in thermal expansion, and cheap in mass fabrication. In DATE5 project, CFRP panels are important panel candidates. In the design study phase, a CFRP prototype panel of 1-meter size is initially developed for the verification purpose. This paper introduces the material arrangement in the sandwich panel, measured performance of this testing sandwich structure samples, and together with the panel forming process. For anti-icing in the South Pole region, a special CFRP heating film is embedded in the front skin of sandwich panel. The properties of some types of basic building materials are tested. Base on the results, the deformation of prototype panel with different sandwich structures and skin layers are simulated and a best structural concept is selected. The panel mold used is a high accuracy one with a surface rms error of 1.4 μm. Prototype panels are replicated from the mold. Room temperature curing resin is used to reduce the thermal deformation in the resin transfer process. In the curing, vacuum negative pressure technology is also used to increase the volume content of carbon fiber. After the measurement of the three coordinate measure machine (CMM), a prototype CFRP panel of 5.1 μm rms surface error is developed initially.
Completely open-foldable domes remaining cool in sunshine
Robert H. Hammerschlag, Sander Deelen, Pieter W. Hoogendoorn, et al.
These open-foldable very light-weight domes, based on very strong textile membranes highly tensioned between steel bows, are designed for bad-weather protection and maintenance of instruments for astronomical, meteorological and civil-engineering measurements and have extremely high wind stability. The domes of the GREGOR telescope and the Dutch Open Telescope are the two existing prototypes. Improvements were developed with all parts light-colored to remain cool in solar light. The new specially made connection parts (eyes) between the textile parts are made from white-colored PETP, a very strong and UV-stable synthetic, and have a better geometrical shape giving higher stability. The rubber seal tubes on top of the dome were of black-colored chloride rubber CR (neoprene), strong and UV stable, but very warm in sunlight. New UV-stable EPDM rubber tubes were produced in natural light color. To get this rubber stiff enough to give good sealing, a black-colored stiff EPDM rubber is put inside the light-colored one. Tests were performed and the forces necessary for compression of the rubber tubes were measured. An inside black tube with a circa 1.3 times larger compression force than the original black tubes was applied. The assembling of the black tubes into the light-colored tubes was successfully applied at the DOT and GREGOR domes.
New isostatic mounting concept for a space born Three Mirror Anastigmat (TMA) on the Meteosat Third Generation Infrared Sounder Instrument (MTG-IRS)‎
Maximilian Freudling, Jesko Klammer, Gregory Lousberg, et al.
A novel isostatic mounting concept for a space born TMA of the Meteosat Third Generation Infrared Sounder is presented. The telescope is based on a light-weight all-aluminium design. The mounting concept accommodates the telescope onto a Carbon-Fiber-Reinforced Polymer (CRFP) structure. This design copes with the high CTE mismatch without introducing high stresses into the telescope structure. Furthermore a Line of Sight stability of a few microrads under geostationary orbit conditions is provided. The design operates with full performance at a temperature 20K below the temperature of the CFRP structure and 20K below the integration temperature. The mounting will sustain launch loads of 47g. This paper will provide the design of the Back Telescope Assembly (BTA) isostatic mounting and will summarise the consolidated technical baseline reached following a successful Preliminary Design Review (PDR).
Analytical optimization and test validation of the submicron dimensional stability of the CHEOPS space telescope's CFRP structure
L. Blecha, D. Zindel, H. Cottard, et al.
The CHEOPS (CHaracterising ExOPlanet Satellite), which is an ESA mission developed in cooperation with Switzerland and a number of other member-states, is the first one dedicated to search for transits by means of ultrahigh precision photometry on bright stars already known to host planets. The optical design is based on a Ritchey-Chretien style telescope to provide a de-focussed image of the target stars.

The telescope’s mirrors M1, M2 as well as the focal plane detector are supported by a thermally controlled CFRP structure suspended on isostatic mounts. The dimensional stability of the structural system supporting the optics is a key requirement as it directly impacts the instrument’s accuracy. The M1 and M2 mirrors are supported by a tubular CFRP telescope design which has been optimized by analyses down to carbon fibre layer level with the support of extensive sample test results for model correlation and accurate dimensional stability predictions. This sample characterization test campaign has been conducted on samples with different carbon fibre layups (orientation and stack sequence) to measure accurately the Coefficient of Thermal Expansion (CTE) over a wide temperature range extending from -80°C to +80°C. Using the correlated Finite Element Model, the fibre orientation layup that minimized the relative displacement between the M1 and M2 mirrors, including the consideration of the thermo-elastic contributions of the isostatic mounts on the overall stability of this optical system, has been identified and selected for the baseline design of the CHEOPS Structure.

A dedicated Structural and Thermal Model (STM2), which was then refurbished to a PFM, was manufactured and tested with an ad hoc setup to verify the overall structural stability of the optical train assembly [2]. The relative distance between M1 and M2 was measured under thermal vacuum conditions using laser interferometer techniques. Thermal cycling tests were initially conducted to eliminate and characterize settling effects. Then, the structure’s stability was measured at three stabilised operational temperatures: -5, -10 and -15°C. The thermally induced M1-M2 misalignment on the optical axis was measured to be between -0.156 and -0.168 micron/°C. Relative mirror tilt and lateral centre shifts were also measured. The obtained focal distance, tilt and centre shift stability between mirrors M1 and M2 were all compliant with the system level requirements such that both an STM and PFM model of the CHEOPS CFRP Structure were successfully qualified and delivered in due time for integration on the spacecraft.
Active Instruments (Active Structures, Active Optics)
Developments in active optics for space instruments: an ESA perspective
An increasing need for higher resolution for both Science and Earth Observation applications demands a bigger entrance aperture of future optical payloads, leading to large primary mirrors, either monolithic or deployable. Correcting issues linked to manufacturing, integration, launch and use of large light-weighted optics (and associated structures), Active Optics constitutes an enabling technology for future large space instruments. This paper presents the current status of technological developments at ESA in this very promising field.
FAME: freeform active mirror experiment
FAME is a four-year project and part of the OPTICON/FP7 program that is aimed at providing a breakthrough component for future compact, wide field, high resolution imagers or spectrographs, based on both Freeform technology, and the flexibility and versatility of active systems.

Due to the opening of a new parameter space in optical design, Freeform Optics are a revolution in imaging systems for a broad range of applications from high tech cameras to astronomy, via earth observation systems, drones and defense. Freeform mirrors are defined by a non-rotational symmetry of the surface shape, and the fact that the surface shape cannot be simply described by conicoids extensions, or off-axis conicoids. An extreme freeform surface is a significantly challenging optical surface, especially for UV/VIS/NIR diffraction limited instruments.

The aim of the FAME effort is to use an extreme freeform mirror with standard optics in order to propose an integrated system solution for use in future instruments. The work done so far concentrated on identification of compact, fast, widefield optical designs working in the visible, with diffraction limited performance; optimization of the number of required actuators and their layout; the design of an active array to manipulate the face sheet, as well as the actuator design.

In this paper we present the status of the demonstrator development, with focus on the different building blocks: an extreme freeform thin face sheet, the active array, a highly controllable thermal actuator array, and the metrology and control system.
Novel and efficient ADC concept for BlackGEM telescope
Rik ter Horst, Jan Kragt, Dirk Lesman, et al.
Ground based telescopes suffer from Atmospheric Dispersion that can be compensated for with an Atmospheric Dispersion Corrector (ADC). In the BlackGEM array of 650 mm diameter telescopes, the ADC is fully integrated in the three-lens field corrector and requires lateral displacement of only one lens for a full correction of the Atmospheric Dispersion. This concept results in a very compact and efficient ADC design without the need for any additional optical components. This paper describes the optical trade-offs, optical design and optimization, as well as the mechanical design and implementation of this novel ADC solution.
Multilayer active shell mirrors for space telescopes
John Steeves, Kathryn Jackson, Sergio Pellegrino, et al.
A novel active mirror technology based on carbon fiber reinforced polymer (CFRP) substrates and replication techniques has been developed. Multiple additional layers are implemented into the design serving various functions. Nanolaminate metal films are used to provide a high quality reflective front surface. A backing layer of thin active material is implemented to provide the surface-parallel actuation scheme. Printed electronics are used to create a custom electrode pattern and flexible routing layer. Mirrors of this design are thin (< 1.0 mm), lightweight (2.7 kg/m2), and have large actuation capabilities. These capabilities, along with the associated manufacturing processes, represent a significant change in design compared to traditional optics. Such mirrors could be used as lightweight primaries for small CubeSat-based telescopes or as meter-class segments for future large aperture observatories. Multiple mirrors can be produced under identical conditions enabling a substantial reduction in manufacturing cost and complexity.

An overview of the mirror design and manufacturing processes is presented. Predictions on the actuation performance have been made through finite element simulations demonstrating correctabilities on the order of 250-300× for astigmatic modes with only 41 independent actuators. A description of the custom metrology system used to characterize the active mirrors is also presented. The system is based on a Reverse Hartmann test and can accommodate extremely large deviations in mirror figure (> 100 μm PV) down to sub-micron precision. The system has been validated against several traditional techniques including photogrammetry and interferometry. The mirror performance has been characterized using this system, as well as closed-loop figure correction experiments on 150 mm dia. prototypes. The mirrors have demonstrated post-correction figure accuracies of 200 nm RMS (two dead actuators limiting performance).
Long-term stable active mount for reflective optics
C. Reinlein, A. Brady, C. Damm, et al.
We report on the development of an active mount with an orthogonal actuator matrix offering a stable shape optimization for gratings or mirrors. We introduce the actuator distribution and calculate the accessible Zernike polynomials from their actuator influence function. Experimental tests show the capability of the device to compensate for aberrations of grating substrates as we report measurements of a 110x105 mm2 and 220x210 mm2 device With these devices, we evaluate the position depending aberrations, long-term stability shape results, and temperature-induced shape variations. Therewith we will discuss potential applications in space telescopes and Earth-based facilities where long-term stability is mandatory.
Optical Fibers and Positioners I
Multicore fibre technology: the road to multimode photonics
For the past forty years, optical fibres have found widespread use in ground-based and space-based instruments. In most applications, these fibres are used in conjunction with conventional optics to transport light. But photonics offers a huge range of optical manipulations beyond light transport that were rarely exploited before 2001. The fundamental obstacle to the broader use of photonics is the difficulty of achieving photonic action in a multimode fibre. The first step towards a general solution was the invention of the photonic lantern1 in 2004 and the delivery of high-efficiency devices (< 1 dB loss) five years on2. Multicore fibres (MCF), used in conjunction with lanterns, are now enabling an even bigger leap towards multimode photonics. Until recently, the single-moded cores in MCFs were not sufficiently uniform to achieve telecom (SMF-28) performance. Now that high-quality MCFs have been realized, we turn our attention to printing complex functions (e.g. Bragg gratings for OH suppression) into their N cores. Our first work in this direction used a Mach-Zehnder interferometer (near-field phase mask) but this approach was only adequate for N=7 MCFs as measured by the grating uniformity3. We have now built a Sagnac interferometer that gives a three-fold increase in the depth of field sufficient to print across N ≥ 127 cores. We achieved first light this year with our 500mW Sabre FRED laser. These are sophisticated and complex interferometers. We report on our progress to date and summarize our first-year goals which include multimode OH suppression fibres for the Anglo-Australian Telescope/PRAXIS instrument and the Discovery Channel Telescope/MOHSIS instrument under development at the University of Maryland.
A microlens-array based pupil slicer and double scrambler for MAROON-X
We report on the design and construction of a microlens-array (MLA)-based pupil slicer and double scrambler for MAROON-X, a new fiber-fed, red-optical, high-precision radial-velocity spectrograph for one of the twin 6.5m Magellan Telescopes in Chile. We have constructed a 3X slicer based on a single cylindrical MLA and show that geometric efficiencies of ≥85% can be achieved, limited by the fill factor and optical surface quality of the MLA. We present here the final design of the 3x pupil slicer and double scrambler for MAROON-X, based on a dual MLA design with (a)spherical lenslets. We also discuss the techniques used to create a pseudo-slit of rectangular core fibers with low FRD levels.
Fiber mode scrambler experiments for the Subaru Infrared Doppler Instrument (IRD)
M. Ishizuka, T. Kotani, J. Nishikawa, et al.
We report the results of fiber mode scrambler experiments for the Infra-Red Doppler instrument (IRD) on the Subaru 8.2-m telescope. IRD is a near infrared, high-precision radial velocity (RV) instrument to search for exoplanets around M dwarfs. It is a fiber-fed, high-resolution (R~70000) spectrograph with an Echelle grating and a state-of-the art laser frequency comb. Expected precision of RV measurements is 1m/s. To achieve 1m/s accuracy, we must reduce modal noise, which is intensity instability of light at the end of multimode fibers. Modal noise is caused by interference of finite number of propagating modes of light. This noise can cause false RV signals, which reduce the accuracy of RV measurements. A mode scrambler is a mechanism to reduce modal noise. However, the best mode scrambler system at near infrared wavelengths is still unknown. Thus, we tested many kinds of mode scramblers, various length fibers, a double scrambler, and octagonal fibers, as static scramblers. We also tested dynamic scramblers, which make output uniform by moving optical fibers dynamically. We report the effects of these mode scramblers.
12.5-GHz-spaced laser frequency comb covering Y, J, and H bands for infrared Doppler instrument
T. Kokubo, T. Mori, T. Kurokawa, et al.
In order to detect Earth-like planets around nearby red dwarfs (in particular late-M stars), it is crucial to conduct precise radial velocity measurements at near-infrared wavelengths where these stars emit most of the light. We have been developing the Infrared Doppler (IRD) spectrograph which is a high dispersion spectrograph for the Subaru telescope. To achieve 1m/s RV measurement precision, we have developed a direct generation of laser frequency comb (LFC) that uses high-repetition-rate pump pulse synthesized by a line-by-line pulse-shaping technique. Our LFC generator has some advantages including simple and easy frequency stabilization, all fiber-optic configuration, and broadband calibration by the precise frequency shift of all modes in the LFC. We have successfully generated a 12.5-GHz-spaced comb spanning over 700 nm from 1040 to 1750 nm. The frequency stability was measured by optically heterodyning the comb with an acetylene-stabilized laser at 1542 nm as a reference light. The LFC showed a frequency stability of less than 0.2 MHz and an almost constant spectrum profile for 6 days. The original LFC that has just produced from highly nonlinear fibers needs some optical processing including spectrum shaping, depolarization, and a mode scramble in a multi-mode fiber before it is input into a spectrograph for the calibration. We have investigated the optical processing of the LFC which is necessary for the precise spectrograph calibration. Keywords: laser frequency comb, infrared, spectrograph, Doppler shift
Design of real-time measurement for optical fiber positioning based on FPGA
Huajie Han, Yonggang Gu, Guocheng Liu, et al.
In order to track the stellar objects in fiber spectroscopic telescopes, measuring the fiber positions is necessary, which is currently unsolved due to the large number of data. In this paper, we propose a novel measure method based on the Field Programmable Gate Arrays (FPGA). Firstly, the fiber spots are obtained from the complicated original images by preprocessing including the median filtering technique to remove the image noise and the correlation operation technique to enhance the fiber spot characteristic. Then, a novel fast connected component labeling technique is employed to acquire the accurate fiber positions. The design of the image processing system is realized by a five-stage pipeline technique based on the FPGA, thus the image processing speed is greatly improved and meanwhile the real-time calculation of the fiber positions is fulfilled.
Optical Fibers and Positioners II
Optimal non-circular fiber geometries for image scrambling in high-resolution spectrographs
Julian Stürmer, Christian Schwab, Stephan Grimm, et al.
Optical fibers are a key component for high-resolution spectrographs to attain high precision in radial velocity measurements. We present a custom fiber with a novel core geometry - a 'D'-shape. From a theoretical standpoint, such a fiber should provide superior scrambling and modal noise mitigation, since unlike the commonly used circular and polygonal fiber cross sections, it shows chaotic scrambling. We report on the fabrication process of a test fiber and compare the optical properties, scrambling performance, and modal noise behaviour of the D-fiber with those of common polygonal fibers.
Communication architecture system for fiber positioning of DESI spectrograph
Karim Kaci, Guillermo Glez-de-Rivera, Fernando Lopez-Colino, et al.
This paper presents a design proposal for controlling the five thousand fiber positioners within the focal plate of the DESI instrument. Each of these positioners is a robot which allows positioning its optic fiber with a resolution within the range of few microns. The high number and density of these robots poses a challenge for handling the communication from a central control device to each of these five thousand. Furthermore, an additional restriction applies as the required time to communicate to every robot of its position must be smaller than a second. Additionally. a low energy consumption profile is also desired.

Both wireless and wired communication protocols have been evaluated, proposing single-technology-based architectures and hybrid ones (a combination of them). Among the wireless solutions, ZigBee and CyFi have been considered. Using simulation tools these wireless protocols have been discarded as they do not allow an efficient communication. The studied wired protocols comprise I2C, CAN and Ethernet.

The best solution found is a hybrid multilayer architecture combining both Ethernet and I2C. A 100 Mbps Ethernet based network is used to communicate the central control unit with ten management boards. Each of these boards is a low-cost, low-power embedded device that manages a thirty six degrees sector of the sensing plate. Each of these boards receives the positioning data for five hundred robots and communicate with each one through a fast mode plus I2C bus. This proposal allows to communicate the positioning information for all five thousand robots in 350 ms total.
TAIPAN instrument fibre positioner and Starbug robots: engineering overview
TAIPAN will conduct a stellar and galaxy survey of the Southern sky. The TAIPAN positioner is being developed as a prototype for the MANIFEST instrument on the GMT. The design for TAIPAN incorporates 150 optical fibres (with an upgrade path to 300) situated within independently controlled robotic positioners known as Starbugs. Starbugs allow precise parallel positioning of individual fibres, thus significantly reducing instrument configuration time and increasing the amount of observing time. Presented is an engineering overview of the UKST upgrade of the completely new Instrument Spider Assembly utilized to support the Starbug Fibre Positioning Robot and current status of the Starbug itself.
Pupil slicer design for the NASA-NSF extreme precision Doppler spectrograph concept WISDOM
Mark Egan, Gábor Fűrész, Robert Simcoe
The WIYN Spectrograph for Doppler Monitoring (WISDOM) was a concept responding to NASA's solicitation for an extreme precision radial velocity instrument for the 3.5 meter WIYN telescope on Kitt Peak in Arizona. In order to meet the spectral resolution requirement of R = 110,000 while maintaining good throughput and a manageable beam diameter, the front end design of the instrument employed a pupil slicing technique wherein a collimated beam is sliced and fed to six separate fibers. This paper presents the optical and mechanical design of the pupil slicer subassembly, a unique method of dealing with thermally induced defocus error, and the methods and results of aligning a prototype.
Post-inscription tuning of multicore fiber Bragg gratings
Emma Y. Lindley, Seong-sik Min, Sergio G. Leon-Saval, et al.
Fiber Bragg gratings are used in astronomy for their ability to suppress narrow atmospheric emission lines of temporally varying brightness before the light is dispersed. These gratings can only operate in a single-mode fiber as the suppressed wavelength depends on mode velocity in the core. Recent experiments with fibers containing multiple single-moded cores have demonstrated the potential for inscribing identical gratings across all cores in a single pass. We have already improved the uniformity of gratings in 7-core fibers via modifications to the writing process; further progress can be achieved by tuning the gratings of the outer and inner cores relative to one another. Our eventual goal is to make the entire fiber suppress one wavelength to a depth of 30 dB or greater. By coating the fiber in a heat-conductive material with a high expansion coefficient, we can examine the effects of temperature and strain on the spectral response of each core. In this paper we present methods and results from experiments concerning the post-write tuning of gratings in multicore fibers.
Multi Object Spectroscopy
First results of tests on the WEAVE fibres
Frédéric Sayède, Youssef Younes, Gilles Fasola, et al.
WEAVE is a new wide-field spectroscopy facility proposed for the prime focus of the 4.2m William Herschel Telescope. The facility comprises a new 2-degree field of view prime focus corrector with a 1000-multiplex fibre positioner, a small number of individually deployable integral field units, and a large single integral field unit. The IFUs (Integral Field Units) and the MOS (Multi Object Spectrograph) fibres can be used to feed a dual-beam spectrograph that will provide full coverage of the majority of the visible spectrum in a single exposure at a spectral resolution of ~5000 or modest wavelength coverage in both arms at a resolution ~20000. The instrument is expected to be on-sky by the first quarter of 2018 to provide spectroscopic sampling of the fainter end of the Gaia astrometric catalogue, chemical labeling of stars to V~17, and dedicated follow up of substantial numbers of sources from the medium deep LOFAR surveys. After a brief description of the Fibre System, we describe the fibre test bench, its calibration, and some test results. We have to verify 1920 fibres from the MOS bundles and 740 fibres from the mini-IFU bundles with the test bench. In particular, we present the Focal Ratio Degradation of a cable.
Echidna Mark II: one giant leap for 'tilting spine' fibre positioning technology
The Australian Astronomical Observatory's 'tilting spine' fibre positioning technology has been redeveloped to provide superior performance in a smaller package. The new design offers demonstrated closed-loop positioning errors of <2.8 μm RMS in only five moves (~10 s excluding metrology overheads) and an improved capacity for open-loop tracking during observations. Tilt-induced throughput losses have been halved by lengthening spines while maintaining excellent accuracy. New low-voltage multilayer piezo actuator technology has reduced a spine's peak drive amplitude from ~150V to <10V, simplifying the control electronics design, reducing the system's overall size, and improving modularity. Every spine is now a truly independent unit with a dedicated drive circuit and no restrictions on the timing or direction of fibre motion.
The Potsdam MRS spectrograph: heritage of MUSE and the impact of cross-innovation in the process of technology transfer
B. Moralejo, M. M. Roth, P. Godefroy, et al.
After having demonstrated that an IFU, attached to a microscope rather than to a telescope, is capable of differentiating complex organic tissue with spatially resolved Raman spectroscopy, we have launched a clinical validation program that utilizes a novel optimized fiber-coupled multi-channel spectrograph whose layout is based on the modular MUSE spectrograph concept. The new design features a telecentric input and has an extended blue performance, but otherwise maintains the properties of high throughput and excellent image quality over an octave of wavelength coverage with modest spectral resolution. We present the opto-mechanical layout and details of its optical performance.
TAIPAN fibre feed and spectrograph: engineering overview
Nicholas F. Staszak, Jon Lawrence, Ross Zhelem, et al.
TAIPAN will conduct a stellar and galaxy survey of the Southern sky. The TAIPAN positioner is being developed as a prototype for the MANIFEST instrument on the GMT. The TAIPAN Spectrograph is an AAO designed all-refractive 2-arm design that delivers a spectral resolution of R>2000 over the wavelength range 370-870 nm. It is fed by a custom fibre cable from the TAIPAN Starbugs positioner. The design for TAIPAN incorporates 150 optical fibres (with an upgrade path to 300). Presented is an engineering overview of the UKST Fibre Cable design used to support Starbugs, the custom slit design, and the overall design and build plan for the TAIPAN Spectrograph.
Slit Spectroscopy and Image Slicers
Fabrication of a wide-field NIR integral field unit for SWIMS using ultra-precision cutting
Yutaro Kitagawa, Yutaka Yamagata, Shin-ya Morita, et al.
We describe overview of fabrication methods and measurement results of test fabrications of optical surfaces for an integral field unit (IFU) for Simultaneous color Wide-field Infrared Multi-object Spectrograph, SWIMS, which is a first-generation instrument for the University of Tokyo Atacama Observatory 6.5-m telescope. SWIMS-IFU provides entire near-infrared spectrum from 0.9 to 2.5 μm simultaneously covering wider field of view of 17" × 13" compared with current near-infrared IFUs. We investigate an ultra-precision cutting technique to monolithically fabricate optical surfaces of IFU optics such as an image slicer. Using 4- or 5-axis ultra precision machine we compare the milling process and shaper cutting process to find the best way of fabrication of image slicers. The measurement results show that the surface roughness almost satisfies our requirement in both of two methods. Moreover, we also obtain ideal surface form in the shaper cutting process. This method will be adopted to other mirror arrays (i.e. pupil mirror and slit mirror, and such monolithic fabrications will also help us to considerably reduce alignment procedure of each optical elements.
Stop-less Lyot coronagraph for exoplanet characterization: first on-sky validation in VLT/SPHERE
The VLT/SPHERE instrument includes a unique long-slit spectroscopy (LSS) mode coupled with Lyot coronagraphy dedicated to the spectral characterization of directly imaged giant exoplanets. The performance of this mode is limited by its non-optimal coronagraph, but in a previous work we demonstrated that it could be significantly improved at small inner-working angles using the stop-less Lyot coronagraph (SLLC). A prototype of the SLLC was installed in VLT/SPHERE in 2014 during the reintegration of the instrument in Paranal, and it was extensively tested in 2015 to characterize its performance. The performance is tested in both imaging and spectroscopy using data acquired on the internal source of SPHERE. In imaging, we obtain a raw contrast gain of a factor 10 at 0.3" with the SLLC. We also demonstrate that no Lyot stop is required to reach the full performance, which validates the SLLC concept. Comparison with a realistic simulation model shows that we are currently limited by the internal phase aberrations of SPHERE. In spectroscopy, we obtain a gain of 1 mag in a limited range of angular separations. Simulations show that although the main limitation comes from phase errors, the performance in the non-SLLC case is very close to the ultimate limit of the LSS mode. We present the very first on-sky data with the SLLC, which appear extremely promising for the future scientific exploitation of an apodized LSS mode in SPHERE. Finally, we explore a new possibility for the speckle subtraction in the LSS mode that could significantly improve the data analysis with respect to methods based on spectral differences.
Development of an efficient photonic device for the reformatting of celestial light
D. G. MacLachlan, R. J. Harris, I. Gris-Sánchez, et al.
The advent of 30 m class Extremely Large Telescopes will require spectrographs of unprecedented spectral resolution in order to meet ambitious science goals, such as detecting Earth-like exoplanets via the radial velocity technique. The consequent increase in the size of the spectrograph makes it challenging to ensure their optimal environmental stabilization and precise spectral calibration. The multimode optical fibers used to transport light from the telescope focal plane to the separately housed environmentally stabilized spectrograph introduces modal noise. This phenomena manifests as variations in the light pattern at the output of the fiber as the input coupling and/or fiber position changes which degrades the spectrograph line profile, reducing the instrument precision. The photonic lantern is a guided wave transition that efficiently couples a multimode point spread function into an array of single modes. If arranged in a linear array at the input of the spectrograph these single modes can in principle provide a diffraction-limited mode noise free spectra in the dispersion axis. In this paper we describe the fabrication and throughput performance of the hybrid reformatter. This device combines the proven low-loss performance of a multicore fiber-based photonic lantern with an ultrafast laser inscribed three-dimensional waveguide interconnect that performs the reformatting function to a diffraction-limited pseudo-slit. The device provided an in laboratory throughput of 65 ± 2% at 1550 ± 20 nm and an on-sky throughput of 53 ± 4% at 1530 ± 80 nm using the CANARY adaptive optics system at the William Herschel Telescope.
Modal noise characterisation of a hybrid reformatter
Izabela Spaleniak, David G. MacLachlan, Itandehui Gris-Sánchez, et al.
This paper reports on the modal noise characterisation of a hybrid reformatter. The device consists of a multicore-fibre photonic lantern and an ultrafast laser-inscribed slit reformatter. It operates around 1550 nm and supports 92 modes. Photonic lanterns transform a multimode signal into an array of single-mode signals, and thus combine the high coupling efficiency of multimode fibres with the diffraction-limited performance of single-mode fibres. This paper presents experimental measurements of the device point spread function properties under different coupling conditions, and its throughput behaviour at high spectral resolution. The device demonstrates excellent scrambling but its point spread function is not completely stable. Mode field diameter and mode bary-centre position at the device output vary as the multicore fibre is agitated due to the fabrication imperfections.
A rubidium traced white-light etalon calibrator for MAROON-X
We report on the construction and testing of a vacuum-gap Fabry-Perot etalon calibrator for high precision radial velocity spectrographs. The etalon is referenced against hyper fine transitions of rubidium to provide a precise wavelength calibrator for MAROON-X, a new fiber-fed, red-optical, high-precision radial-velocity spectrograph currently under construction for one of the twin 6.5m Magellan Telescopes in Chile. We demonstrate a turnkey system, ready to be installed at any current and next generation radial velocity spectrograph that requires calibration over a wide spectral band-pass. Uncertainties in the position of one etalon line are at the 10 cm s-1 level in individual measurements taken at 4 Hz. Our long-term stability is mainly limited by aging effects of the spacer material Zerodur, which imprints a 12 cm s-1 daily drift. However, as the etalon position is traced by the rubidium reference with a precision of <3 cm s-1 for integration times longer than 10s, we can fully account for this effect at the RV data reduction level.
Coatings, Filters and Gratings I
New grating concepts in the NIR and SWIR spectral band for high resolution earth-observation spectrometers
T. Flügel-Paul, G. Kalkowski, T. Benkenstein, et al.
We report about our latest achievements to realize the diffraction gratings during the development activities for a future Earth observation high resolution spectrometer studied by ESA. The gratings are manufactured by electron beam lithography on fused silica substrates. The optical performance is considerably increased by applying a dedicated high refractive index coating to the grating structure using atomic layer deposition (ALD). Thus, we were able to achieve diffraction efficiencies larger than 75% averaged over both linear polarizations states, i.e. TE and TM. At the same time, the polarization sensitivity is well below 10% in both cases. Finally, the diffraction gratings for the SWIR-1 spectral channel were bonded on a massive prism substrate in order to realize a GRISM element. This process was achieved by direct fused silica bonding performed under atmospheric pressure within special mechanical equipment designed and constructed particularly for this purpose.
Characterizing the cross dispersion reflection gratings of CRIRES+
Roman Follert, Dieter Taubert, Jörg Hollandt, et al.
The CRIRES+ project attempts to upgrade the CRIRES instrument into a cross dispersed Echelle spectrograph with a simultaneous recording of 8-10 diffraction orders. In order to transform the CRIRES spectrograph into a cross-dispersing instrument, a set of six reflection gratings, each one optimized for one of the wavelength bands CRIRES+ will operate in (YJHKLM), will be used as cross dispersion elements in CRIRES+. Due to the upgrade nature of the project, the choice of gratings depends on the fixed geometry of the instrument. Thus, custom made gratings would be required to achieve the ambitious design goals. Custom made gratings have the disadvantage, though, that they come at an extraordinary price and with lead times of more than 12 months. To mitigate this, a set of off-the-shelf gratings was obtained which had grating parameters very close to the ones being identified as optimal. To ensure that the rigorous specifications for CRIRES+ will be fulfilled, the CRIRES+ team started a collaboration with the Physikalisch-Technische Bundesanstalt Berlin (PTB) to characterize gratings underconditions similar to the operating conditions in CRIRES+ (angle of incidence, wavelength range).

The respective test setup was designed in collaboration between PTB and the CRIRES+ consortium. The PTB provided optical radiation sources and calibrated detectors for each wavelength range. With this setup, it is possible to measure the absolute efficiency of the gratings both wavelength dependent and polarization state dependent in a wavelength range from 0.9 μm to 6 μm.
Final design and choices for EUCLID NISP grism
ESA Euclid mission is designed to map the geometry of the dark Universe. The NISP (Near Infrared Spectro- Photometer) is one of its two instruments dedicated to NIR with two main observing modes: the photometric mode and the spectroscopic mode, for the acquisition of slitless dispersed images using four low resolution grisms: three "red" grisms for 1250-1850nm, and one "blue" grism for 920-1300nm. The NISP grisms are complex optical components that combine four main functions: a grism done by the grating on the prism hypotenuse, a spectral filter done by a multilayer filter deposited on the first surface of the prism, a focus function done by a curved surface and a spectral wavefront correction done by the grating with curved grooves. This specific grating is made thanks to a new technic developed with SILIOS Technologies to manufacture a resin-free grating. The optical component is glued onto a mechanical ring, designed to survive to 60g DLL and to keep optical performance at 130K. The design and manufacturing of these components represent an important challenge to obtain the best performances with very constraining requirements. We will present the performance obtained on scale-1 prototypes of the filter, the grating and the mount manufactured to validate the final design choices and used to make the necessary trade-off during the development phase. All the prototypes have shown very good optical performances and have withstood vibrations and vacuum cryogenic tests that confirm the feasibility of NISP grisms and prepare the next phase for the procurement and tests of NISP grism flight models.
Coatings, Filters and Gratings II
Advances in far-ultraviolet reflective and transmissive coatings for space applications
Luis Rodríguez-de Marcos, José A. Aznárez, José A. Méndez, et al.
Exploitation of far ultraviolet (FUV, 100-200 nm) observations extends to most areas of modern astronomy, from detailed observations of Solar System objects, the interstellar medium, exoplanets, stars and galaxies, to studies of crucial cosmological relevance. Despite several developments in recent decades, yet many observations are not possible due to technical limitations, of which one of the most important is the lack of optical coatings with high throughput. Development and optimization of such efficient FUV coatings have been identified in several roadmap reports as a key goal for future missions. The success of this development will ultimately improve the performance of nowadays feasible optical instruments and will enable new scientific imaging capabilities.

GOLD’s research is devoted to developing novel coatings with enhanced performance for space optics. Several deposition systems are available for the deposition of multilayer coatings. A deposition system was developed to deposit FUV coatings to satisfy space requirements. It consists of a 75-cm-diameter deposition chamber pumped with a cryo-pump and placed in an ISO-6 clean room. This chamber is available for deposition by evaporation of top-requirement coatings such as Al/ MgF2 mirrors or (Al/MgF2)n multilayer coatings for transmittance filters. A plan to add an Ion-Beam-Sputtering system in this chamber is under way.

In this and other chambers at GOLD the following FUV coatings can be prepared:

Transmittance filters based on (Al/MgF2)n multilayer coatings. These filters can be designed to have a peak at the FUV spectral line or band of interest and a high peak-to-visible transmittance ratio. Filters can be designed with a peak transmittance at a wavelength as short as 120 nm and with a transmittance in the visible smaller than 10-5.

Narrowband reflective coatings peaked close to H Lyman β (102.6 nm) with a reflectance at H Lyman α (121.6 nm) two orders of magnitude below the one at 102.6 nm. Other potential spectral lines at which these coatings could be peaked are the OVI doublet (103.2, 103.8 nm).

Narrowband reflective mirrors based on (MgF2/LaF3)n multilayers peaked at a wavelength as short as 120 nm. Target wavelengths include lines of high interest for space observations, such as H Lyman α (121.6 nm), OI (130.4 and 135.6 nm), CIV (154.8, 155.1 nm), among others.

Coating-based linear polarizers tuned at H Lyman α (121.6 nm) both based on reflectance or on transmittance. Reflective polarizers present a high efficiency. Transmissive polarizers have a more modest peak performance compared to reflective polarizers; however, they involve spectral filtering properties to reject the long FUV and even more the near UV to the IR, which turn them competitive compared to reflective polarizers.

In this communication we present a summary of our research on the above FUV coatings developed at GOLD.
Use of plasma enhanced ALD to construct efficient interference filters for astronomy in the FUV
Paul A. Scowen, Robert Nemanich, Brianna Eller, et al.
Over the past few years the advent of atomic layer deposition (ALD) technology has opened new capabilities to the field of coatings deposition for use in optical elements. At the same time, there have been major advances in both optical designs and detector technologies that can provide orders of magnitude improvement in throughput in the far ultraviolet (FUV) and near ultraviolet (NUV) passbands. Recent review work has shown that a veritable revolution is about to happen in astronomical diagnostic work for targets ranging from protostellar and protoplanetary systems, to the intergalactic medium that feeds gas supplies for galactic star formation, and supernovae and hot gas from star forming regions that determine galaxy formation feedback. These diagnostics are rooted in access to a forest of emission and absorption lines in the ultraviolet (UV)[1], and all that prevents this advance is the lack of throughput in such systems, even in space-based conditions. We outline an approach to use a range of materials to implement stable optical layers suitable for protective overcoats with high UV reflectivity and unprecedented uniformity, and use that capability to leverage innovative ultraviolet/optical filter construction to enable astronomical science. These materials will be deposited in a multilayer format over a metal base to produce a stable construct. Specifically, we will employ the use of PEALD (plasma-enhanced atomic layer deposition) methods for the deposition and construction of reflective layers that can be used to construct unprecedented filter designs for use in the ultraviolet.
Update on UCO's advanced coating lab development of silver-based mirror coatings
Andrew C. Phillips, David M. Fryauf, Nobuhiko P. Kobayashi, et al.
We present progress in efforts underway at the University of California Observatories to develop high performance durable silver-based mirror coatings for telescope and instruments. Silver-based coatings are extremely prone to tarnish and/or corrosion, and successful coatings depend not only on the materials used but also the deposition processes employed. Our physical vapor deposition (PVD) chamber allows both sputtering and ion-assisted e-beam depositions for head-to-head comparison of deposition processes, and we present results of these comparisons. In this paper, we review the problem and discuss our recent activities and findings. We discuss a systematic study to determine which oxides, nitrides and fluorides provide the best protection in environmental tests. We present initial results into the effects of stress in our specific thin films, and thee effects of stress on mirror coating durability. We also discuss studies using Atomic Layer Deposition (ALD) over-coating of Ag, and we describe a large ALD research chamber currently under construction that will demonstrate ALD processes on larger substrates (70 cm diameter).
Advanced astronomical filter design: challenges, strategy, and results to meet current and future requirements
S. Reichel, R. Biertümpfel, U. Brauneck, et al.
The Observatorio Astrofisico de Javalambre in Spain will conduct an all-sky astronomical survey using multi-bands, where optical filters are needed. These filters are narrow bandpass steep edge filters (FWHM = 14.5 nm) in a spectrum between 390 to 920 nm with 10.0 nm steps. In order to fulfill the demanding requirements for final scientific image quality and transmitted wavefront error a new white-light Shack Hartmann sensor and difficult refractive index measurements of the sub-assembly were needed. In addition due to the spectral requirements the design and manufacturing of the filters were pushed at its technological limit.
Advanced optical coatings for astronomical instrumentation
Fabien Pradal, Hervé Leplan, Hervé Vayssade, et al.
Recently Safran Reosc worked and progressed on various thin film technology for:

    Large mirrors with low stress and stable coatings.
    Large lens elements with strong curvature and precise layer specifications.
    Large filters with high spectral response uniformity specifications.
    IR coatings with low stress and excellent resistance to cryogenic environment for NIR to LWIR domains.
    Pixelated coatings.
Results will be presented and discussed on the basis of several examples.
Coronography and High Contrast Imaging
Mathematical and computational modeling of a ferrofluid deformable mirror for high-contrast imaging
Aaron J. Lemmer, Ian M. Griffiths, Tyler D. Groff, et al.
Deformable mirrors (DMs) are an enabling and mission-critical technology in any coronagraphic instrument designed to directly image exoplanets. A new ferro fluid deformable mirror technology for high-contrast imaging is currently under development at Princeton, featuring a flexible optical surface manipulated by the local electromagnetic and global hydraulic actuation of a reservoir of ferro fluid. The ferro fluid DM is designed to prioritize high optical surface quality, high-precision/low-stroke actuation, and excellent low-spatial-frequency performance - capabilities that meet the unique demands of high-contrast coronagraphy in a space-based platform. To this end, the ferro-fluid medium continuously supports the DM face sheet, a configuration that eliminates actuator print-through (or, quilting) by decoupling the nominal surface figure from the geometry of the actuator array. The global pressure control allows independent focus actuation. In this paper we describe an analytical model for the quasi-static deformation response of the DM face sheet to both magnetic and pressure actuation. These modeling efforts serve to identify the key design parameters and quantify their contributions to the DM response, model the relationship between actuation commands and DM surface-profile response, and predict performance metrics such as achievable spatial resolution and stroke precision for specific actuator configurations. Our theoretical approach addresses the complexity of the boundary conditions associated with mechanical mounting of the face sheet, and makes use of asymptotic approximations by leveraging the three distinct length scales in the problem - namely, the low-stroke (~nm) actuation, face sheet thickness (~mm), and mirror diameter (cm). In addition to describing the theoretical treatment, we report the progress of computational multi physics simulations which will be useful in improving the model fidelity and in drawing conclusions to improve the design.
Apodized vortex coronagraph designs for segmented aperture telescopes
Current state-of-the-art high contrast imaging instruments take advantage of a number of elegant coronagraph designs to suppress starlight and image nearby faint objects, such as exoplanets and circumstellar disks. The ideal performance and complexity of the optical systems depends strongly on the shape of the telescope aperture. Unfortunately, large primary mirrors tend to be segmented and have various obstructions, which limit the performance of most conventional coronagraph designs. We present a new family of vortex coronagraphs with numerically-optimized gray-scale apodizers that provide the sensitivity needed to directly image faint exoplanets with large, segmented aperture telescopes, including the Thirty Meter Telescope (TMT) as well as potential next-generation space telescopes.
Digital adaptive coronagraphy using SLMs: promising prospects of a novel approach, including high-contrast imaging of multiple stars systems
Jonas Kühn, Polychronis Patapis
We introduce a new technological framework for high-contrast coronagraphy, namely digital adaptive coronagraphy (DAC) using spatial light modulators (SLMs), taking advantage of recent advances in this technology. We present proof-of-principle experimental results in the visible, using a transmissive twisted nematic liquid crystal SLM display to show that SLMs can be successfully implemented as focal-plane phase-mask coronagraphs (4QPM, 8OPM,...), and that the technology is essentially in place to address realistic instrumental configurations. We explore a specific application where SLM-based adaptive coronagraphy might be particularly competitive, which is direct imaging of multiple stars systems, by simultaneously nulling multiple point sources in the field. Using a simple approach to compute a brightness-weighted synthetized coronagraphic phase map, we show that in the case of binaries the contrast gain over using a regular phase map can exceed 4 stellar magnitudes for a 1:1 binaries down to separation as close as 1 λ/D. Thanks to video-rate update frequency of the SLM, the technique is in principle compatible with sky rotation in the case of altitude-azimuth telescope mounts, and can address multiple target configurations with no actual mechanical or hardware change.
Precision optical edges for a starshade external occulter
John Steeves, Stefan Martin, David Webb, et al.
The use of a starshade is one technique to perform high contrast imaging with space-based telescopes. The primary function of a starshade is to suppress light from a target star in order to image its orbiting planets. In order to provide the proper apodization function the edges of the starshade must follow a precise in-plane profile. However of equal importance is the issue of light from our own sun scattering off of the edges and entering the telescope. A method to alleviate this problem is to make the edges extremely sharp (< 1 μm terminal radius) such that the area available for scattering is minimized. The combination of these two requirements, along with the need to integrate the edges into a 30-40 m dia. deployable structure, present a number of significant engineering challenges. Substrate etching techniques are used to obtain both the intended profile as well as the edge sharpness. Current efforts implement an isotropic etching process on thin metal substrates. This paper discusses the progress towards producing a sharp optical edge at the coupon level. Samples have been characterized using scanning electron microscopy as well as a custom testbed to assess their scattered-light performance.
Poster Session: Coatings, Filters and Gratings
Revisiting static modulation in pyramid wavefront sensing
L. Marafatto, R. Ragazzoni, D. Vassallo, et al.
The Pyramid Sensor (PS) is based on the Focault knife-edge test, yielding then, in geometrical approximation, only the sign of the wavefront slope. To provide linear measurements of the wavefront slopes the PS relies on a technique known as modulation, which also plays a central role to improve the linear range of the pyramid WFS, very small in the nonmodulated case. In the main PS using modulation so far, this task is achieved by moving optical components in the WFS, increasing the complexity of the system. An attractive idea to simplify the optical and mechanical design of a pyramid WFS is to work without any dynamic modulation.

This concept was only merely described and functionally tested in the framework of MAD, and subsequently, with a holographic diffuser. The latter produce a sort of random distribution of the light coming out from the pupil plane, leading to sort of inefficient modulation, as most of the rays are focused in the central region of the light diffused by such device. The bi-dimensional original grating is, in contrast, producing a well defined deterministic distribution of the light onto a specifically shaped pattern. A crude option has been already discussed as a possibility, and it is here generalized to holographic plates leading to various distribution of lights, including a circle whose diameter would match the required modulation pattern, or more cost effective approaches like the one of a square pattern. These holographic diffusers would exhibit also zero-th and high order patterns and the actual size of the equivalent modulation would be linearly wavelength dependent, leading to colour effects that requires a careful handling in order to properly choose the right amount of equivalent modulation.
J-Black: a stray light coating for optical and infrared systems
Patrick Waddell, David S. Black
A new stray light coating, called J-Black, has been developed for NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA). The coating is a layered composition of Nextel-Suede 3101 primers and top coats and silicon carbide grit. J-Black has been applied to large areas of the SOFIA airborne telescope and is currently operating within the open cavity environment of the Boeing 747. Over a series of discrete filter bands, from 0.4 to 21 microns, J-Black optical and infrared reflectivity performance is compared with other available coatings. Measured total reflectance values are less than 2% at the longest wavelengths, including at high incidence angles. Detailed surface structure characteristics are also compared via electron and ion microscopy. Environmental tests applicable for aerospace applications are presented, as well as the detailed steps required to apply the coating.
Atomic Layer Deposited (ALD) coatings for future astronomical telescopes: recent developments
Christopher Samuel Moore, John Hennessy, April D. Jewell, et al.
Atomic Layer Deposition (ALD) can create conformal, near stoichiometric and pinhole free transmissive metal fluoride coatings to protect reflective aluminum films. Spectral performance of astronomical mirror coatings strongly affect the science capabilities of astronomical satellite missions. We are utilizing ALD to create a transmissive overcoat to protect aluminum film mirrors from oxidation with the goal of achieving high reflectance (> 80%) from the UV (~100 nm) to the IR (~2,000 nm). This paper summarizes the recent developments of ALD aluminum fluoride (AlF3) coatings on Al. Reflectance measurements of aluminum mirrors protected by ALD AlF3 and future applications are discussed. These measurements demonstrate that Al + ALD AlF3, even with an interfacial oxide layer of a few nanometers, can provide higher reflectance than Al protected by traditional physical vapor deposited MgF2 without an oxide layer, below ~115 nm.
Astronomical large Ge immersion grating by Canon
Takashi Sukegawa, Takeshi Suzuki, Tsuyoshi Kitamura
Immersion grating is a powerful optical device for thee infrared high-resolution spectroscope. Germanium (GGe) is the best material for a mid-infrared immersion grating because of Ge has very large reflective index (n=4.0). On the other hands, there is no practical Ge immersion grating under 5umm use. It was very difficult for a fragile IR crystal to manufacture a diffraction grating precisely. Our original free-forming machine has accuracy of a few nano-meter in positioning and stability. We already fabricated the large CdZnTe immersion grating. (Sukegawa et al. (2012), Ikeda et al. (2015)) Wee are developing Ge immersion grating that can be a good solution for high-resolution infrared spectroscopy with the large ground-based/space telescopes. We succeeded practical Ge immersion grating with the grooved area off 75mm (ruled direction) x 119mm (grove width) and the blaze angle of 75 degrees. Our astronomical large Ge immersion grating has the grooved area of 155mm (ruled direction) x 41mmm (groove width) and groove pitch off 91.74um. We also report optical performance of astronomical large Ge immersion grating with a metal coating on the diffraction surface.
The legacy of filter design and how that has extended into current choices for advanced astronomical filter
SCHOTT was one of the interference filters inventors starting around 1935. Based on this legacy optical bandpass filters were design, manufactured, and integrated into optical instruments in satellites. In addition a special blocking coating was developed reducing cross talk and ghost. For ground based telescopes steep-edge narrow bandpass filters with low transmitted wavefront error and about 100 mm x 100 mm size were manufactured pushing the filter design and the technology to its limits. The reached results for design and measurements will be shown on an H-alpha filter.
Mid-infrared transmission gratings in chalcogenide glass manufactured using ultrafast laser inscription
David Lee, David G. MacLachlan, Helen L. Butcher, et al.
Ultrafast laser inscription is a versatile manufacturing technique which can be used to modify the refractive index of various glasses on a microscopic scale. This enables the production of a number of photonic devices such as waveguides, beam-splitters, photonic lanterns, and diffraction gratings. In this paper, we report on the use of ultrafast laser inscription to fabricate volume phase transmission gratings in mid-infrared transmitting chalcogenide glass.

We describe the optimisation of the laser inscription process parameters enhancing grating performances via the combination of spectrally resolved grating transmission measurements and theoretical analysis models. The first order diffraction efficiency of the gratings was measured at mid-infrared wavelengths (3-5 μm), and found to exceed 60% at the Littrow blaze wavelength, compared to a substrate external transmittance of 67%. This impressive result implies the diffraction efficiency should exceed 90% for a grating substrate treated with an anti-reflection coating. There is excellent agreement between the modelled grating efficiency and the measured data, and from a least squares fit to the measured data the refractive index modulation achieved during the inscription process is inferred. These encouraging initial results demonstrate that ultrafast laser inscription of chalcogenide glass may provide a potential new and alternative technology for the manufacture of astronomical diffraction gratings for use at near-infrared and mid-infrared wavelengths.
Thin-film optical pass band filters based on new photo-lithographic process for CaSSIS FPA detector on Exomars TGO mission: development, integration, and test
L. Gambicorti, D. Piazza, M. Gerber, et al.
A new technique based on photolithographic processes of thin-film optical pass band coatings on a monolithic substrate has been applied to the filters of the Focal Plane Assembly (FPA) of the Colour and Stereo Surface Imaging System (CaSSIS) that will fly onboard of the ExoMars Trace Gas Orbiter to be launched in March 2016 by ESA.

The FPA including is one of the spare components of the Simbio-Sys instrument of the Italian Space Agency (ASI) that will fly on ESA’s Bepi Colombo mission to Mercury. The detector, developed by Raytheon Vision Systems, is a 2kx2k hybrid Si-PIN array with a 10 μm pixel. The detector is housed within a block and has filters deposited directly on the entrance window. The window is a 1 mm thick monolithic plate of fused silica. The Filter Strip Assembly (FSA) is produced by Optics Balzers Jena GmbH and integrated on the focal plane by Leonardo-Finmeccanica SpA (under TAS-I responsibility). It is based on dielectric multilayer interference coatings, 4 colour bands selected with average in-band transmission greater than 95 percent within wavelength range (400-1100 nm), giving multispectral images on the same detector and thus allows CaSSIS to operate in push-frame mode.

The Field of View (FOV) of each colour band on the detector is surrounded by a mask of low reflective chromium (LRC), which also provides with the straylight suppression required (an out-of-band transmission of less than 10-5/nm). The mask has been shown to deal effectively with cross-talk from multiple reflections between the detector surface and the filter.

This paper shows the manufacturing and optical properties of the FSA filters and the FPA preliminary on-ground calibration results.
Novel diffraction gratings for next generation spectrographs with high spectral dispersion
N. Ebizuka, T. Okamoto, T. Hosobata, et al.
As a transmission grating, a surface-relief (SR) grating with sawtooth shaped ridges and volume phase holographic (VPH) grating are widely used for instruments of astronomical observations. However the SR grating is difficult to achieve high diffraction efficiency at high angular dispersion, and the VPH grating has low diffraction efficiency in high diffraction orders. We propose novel gratings that solve these problems. We introduce the hybrid grism which combines a high refractive index prism with a replicated transmission grating, which has sawtooth shaped ridges of an acute apex angle. The birefringence VPH (B-VPH) grating which contains an anisotropic medium, such as a liquid crystal, achieves diffraction efficiency up to 100% at the first diffraction order for natural polarization and for circular polarization. The quasi-Bragg (QB) grating which consists of long rectangular mirrors aligned in parallel precisely, like a window blind, achieves diffraction efficiency of 60% or more in higher than the 4th diffraction order. The volume binary (VB) grating with narrow grooves also achieves diffraction efficiency of 60% or more in higher than the 6th diffraction order. The reflector facet transmission (RFT) grating which is a SR grating with sawtooth shaped ridges of an acute apex angle achieves diffraction efficiency up to 80% in higher than the 4th diffraction order.
Manufacturing and coating of optical components for the EnMAP hyperspectral imager
M. Schürmann, D. Gäbler, R. Schlegel, et al.
The optical system of the hyperspectral imager of the Environmental Mapping and Analysis Program (EnMAP) consists of a three-mirror anastigmat (TMA) and two independent spectrometers working in the VNIR and SWIR spectral range, respectively. The VNIR spectrometer includes a spherical NiP coated Al6061 mirror that has been ultra-precisely diamond turned and finally coated with protected silver as well as four curved fused silica (FS) and flint glass (SF6) prisms, respectively, each with broadband antireflection (AR) coating, while the backs of the two outer prisms are coated with a high-reflective coating. For AR coating, plasma ion assisted deposition (PIAD) has been used; the high-reflective enhanced Ag-coating on the backside has been deposited by magnetron sputtering. The SWIR spectrometer contains four plane and spherical gold-coated mirrors, respectively, and two curved FS prisms with a broadband antireflection coating. Details about the ultra-precise manufacturing of metal mirrors and prisms as well as their coating are presented in this work.
Performance characteristics of advanced volume phase holographic gratings for operation in the near infrared
Volume phase holographic (VPH) gratings are proven dispersing elements in astronomical spectrographs over the visible spectrum. VPH gratings have also been successfully deployed for use at cryogenic temperatures. Recent advances in production technology now permit the production of gratings for use in the near infrared up to 2450 nm at cryogenic conditions. This paper describes the requirements of VPH gratings for use in the H (wavelengths from 1500 nm to 1800 nm) and K (wavelengths from 1950 nm to 2450 nm) bands, gives the theoretical performances of diffraction efficiency for the production designs and presents the measured performances on the production gratings
Strategies for single-point diamond machining a large format germanium blazed immersion grating
R. C. Montesanti, S. L. Little, P. J. Kuzmenko, et al.
A large format germanium immersion grating was flycut with a single-point diamond tool on the Precision Engineering Research Lathe (PERL) at the Lawrence Livermore National Laboratory (LLNL) in November – December 2015. The grating, referred to as 002u, has an area of 59 mm x 67 mm (along-groove and cross-groove directions), line pitch of 88 line/mm, and blaze angle of 32 degree. Based on total groove length, the 002u grating is five times larger than the previous largest grating (ZnSe) cut on PERL, and forty-five times larger than the previous largest germanium grating cut on PERL. The key risks associated with cutting the 002u grating were tool wear and keeping the PERL machine running uninterrupted in a stable machining environment. This paper presents the strategies employed to mitigate these risks, introduces pre-machining of the as-etched grating substrate to produce a smooth, flat, damage-free surface into which the grooves are cut, and reports on trade-offs that drove decisions and experimental results.
Programmable CGH on photochromic material using DMD
Romain Alata, Giorgio Pariani, Frederic Zamkotsian, et al.
Computer Generated Holograms (CGHs) are useful for wavefront shaping and complex optics testing, including aspherical and free-form optics. Today, CGHs are recorded directly with a laser or intermediates masks but allows only recording binary CGHs; binary CGHs are efficient but can reconstruct only pixilated images. We propose to use a Digital Micro-mirror Device (DMD) for writing binary CGHs as well as grayscale CGHs, able to reconstruct fulfilled images. DMD is actually studied at LAM, for generating programmable slit masks in multi-object spectrographs. It is composed of 2048x1080 individually controllable micro-mirrors, with a pitch of 13.68 μm. This is a real-time reconfigurable mask, perfect for recording CGHs. A first setup has been developed for hologram recording, where the DMD is enlightened with a collimated beam and illuminates a photosensible plate through an Offner relay, with a magnification of 1:1. Our set up resolution is 2-3 μm, leading to a CGH resolution equal to the DMD micro mirror size. In order to write and erase CGHs during test procedure or on request, we use a photochromic plate called PUR-GD71-50-ST developed at Politecnico di Milano. It is opaque at rest, and becomes transparent when it is illuminated with visible light, between 500 and 700 nm; then it can be erased by a UV flash. We choose to code the CGHs in equally spaced levels, so called stepped CGH. We recorded up to 1000x1000 pixels CGHs with a contrast greater than 50, knowing that the material is able to reach an ultimate contrast of 1000. A second bench has also been developed, dedicated to the reconstruction of the recorded images with a 632.8nm He-Ne laser beam. Very faithful reconstructions have been obtained. Thanks to our recording and reconstruction set-ups, we have been able to successfully record binary and stepped CGHs, and reconstruct them with a high fidelity, revealing the potential of this method for generating programmable/rewritable stepped CGHs on photochromic materials.
Cooled optical filters for Q-band infrared astronomy (15-40 µm)
Gary J. Hawkins, Richard E. Sherwood, Karim Djotni, et al.
With a growing interest in mid- and far-infrared astronomy using cooled imaging and spectrometer instruments in highaltitude observatories and spaceflight telescopes, it is becoming increasingly important to characterise and assess the spectral performance of cooled multilayer filters across the Q-band atmospheric window. This region contains spectral features emitted by many astrophysical phenomena and objects fundamental to circumstellar and planetary formation theories. However extending interference filtering to isolate radiation at progressively longer wavelengths and improve photometric accuracy is an area of ongoing and challenging thin-film research. We have successfully fabricated cooled bandpass and edge filters with high durability for operation across the 15-30 μm Q-band region. In this paper we describe the rationale for selection of optical materials and properties of fabricated thin-film coatings for this region, together with FTIR spectral measurements and assessment of environmental durability.
A local attenuation filter for accurate photometry of near-infrared bright stars
I have developed a special ND filter (Local Attenuation Filter) for observing bright near-infrared stars. This filter is a 60mm diameter with a 4mm thickness, on which an attenuation (0.02% transparency) patch with an 8mm diameter is coated. This filter is expected to be installed near the focal plane of telescope, and the flux through this patch is attenuated. Using this filter, we can observe the attenuated bright star together with not affected field stars as reference for relative photometry. This filter has been installed to the IRSF 1.4m telescope and used for the monitoring of NIR bright stars, for example, η Car.
Characterization of an integrally wound tungsten and aluminum filament for physical vapor deposition
William Goble, Ricardo Ortiz
As part of the effort to increase the reliability of the MMT Observatory (MMTO) 6.5m Primary Mirror Coating System, the specified filament has changed from a configuration in which the aluminum charge is hand wound around a tungsten filament to a configuration in which the aluminum is integrally wound with the tungsten at the time of filament manufacture. In the MMTO configuration, this filament consists of the three strands of tungsten wire and one strand of aluminum wire. In preparation of a full system test utilizing two hundred filaments fired simultaneously, an extensive testing program was undertaken to characterize these filaments using a four filament configuration in the MMTO small coating chamber (0.5m) and then a forty filament configuration in the University of Arizona Steward Observatory coating chamber (2m). The testing using the smaller coating chambers has shown these filaments provide very predicable coatings from test to test, and with the proper heating profile, these filaments greatly reduce the likelihood of aluminum drips. The initial filament design was modified during the course of testing by shortening the unwound filament length to closer match the aluminum load required in the MMTO coating chamber. This change increased the aluminum deposition rates without increasing the power delivered of the filament power supplies (commercial welders). Filament power levels measured at the vacuum chamber feed throughs, currents, and deposition rates from multiple coating tests, including a full system test, are presented.
Improved silver mirror coating for ground and space-based astronomy
A durable, UV-enhanced, silver mirror coating has been developed by ZeCoat Corporation. The coating is highly reflective from 350-nm through the long IR, and durable in warm-humid environments on earth, as well as, high radiation environments in space. This paper presents polarized-angular reflectance data, as well as, average reflectance data before and after simulated space radiation exposure.
First results on narrow bandpass steep edge optical filters for the JST/T250 telescope instrumentation
The Observatorio Astrofisico de Javalambre in Spain will conduct an all-sky astronomical surveys using multi-bands, where optical filters are needed. 54 narrow bandpass (FWHM = 14.5 nm) filters will continuously populate the spectrum between 370 to 920 nm with 10.0 nm steps. Here results on 2 filters with center wavelength of 460 nm and 470 nm and blocking from 250 to 1050 nm with OD5 will be shown. The filters have a maximum transmission of larger than 85% and a transmitted wavefront error of better than λ/2 over an aperture >~ 100mm.
Photopolymer based VPHGs: from materials to sky results
A. Zanutta, E. Orselli, T. Fäcke, et al.
Volume Phase Holographic Gratings cover a relevant position as transmission dispersing elements in astronomical spectrographs and each astronomical observation could take advantage of specific dispersive elements with features tailored for achieving the best performances. The design and manufacturing of high efficiency and reliable VPHGs require photosensitive materials where it is possible to control both the refractive index modulation and the film thickness. By means of Bayfol® HX photopolymers, we designed and manufactured six VPHGs for astronomical instrumentation in a GRISM configuration. We demonstrated how photopolymers are reliable holographic materials for making astronomical VPHGs with performances comparable to those provided by VPHGs based on Dichromated Gelatins (DCGs), but with a much simpler production process.
Poster Session: Materials
Transmission and opto-mechanical performance of the liquid lens coupling in the Robert Stobie Spectrograph on SALT
Ockert J. Strydom, Éric Depagne, Darragh O'Donoghue, et al.
Liquid lens coupling provides excellent transmission efficiency when compared to multilayer coatings especially for applications where broadband transmission is required. However, long term reliability of liquid coupling is difficult to achieve. This is typically due to chemical compatibility issues affecting both the optical transmission and the integrity of the opto-mechanical support. As part of a recent service of the Robert Stobie Spectrograph on SALT we had the opportunity to study these problems further and in this paper we provide analysis of problems identified and some solutions to prevent them. We also present general guidelines which could aid future opto-mechanical designs for liquid coupling of lenses.
Large optical glass blanks for the ELT generation
Ralf Jedamzik, Uwe Petzold, Volker Dietrich, et al.
The upcoming extremely large telescope projects like the E-ELT, TMT or GMT telescopes require not only large amount of mirror blank substrates but have also sophisticated instrument setups. Common instrument components are atmospheric dispersion correctors that compensate for the varying atmospheric path length depending on the telescope inclination angle. These elements consist usually of optical glass blanks that have to be large due to the increased size of the focal beam of the extremely large telescopes.

SCHOTT has a long experience in producing and delivering large optical glass blanks for astronomical applications up to 1 m and in homogeneity grades up to H3 quality in the past.

The most common optical glass available in large formats is SCHOTT N-BK7. But other glass types like F2 or LLF1 can also be produced in formats up to 1 m. The extremely large telescope projects partly demand atmospheric dispersion components even in sizes beyond 1m up to a range of 1.5 m diameter. The production of such large homogeneous optical glass banks requires tight control of all process steps.

To cover this demand in the future SCHOTT initiated a research project to improve the large optical blank production process steps from melting to annealing and measurement. Large optical glass blanks are measured in several sub-apertures that cover the total clear aperture of the application. With SCHOTT's new stitching software it is now possible to combine individual sub-aperture measurements to a total homogeneity map of the blank. In this presentation first results will be demonstrated.
ULE design considerations for a 3m class light weighted mirror blank for E-ELT M5
It is expected that the next generation of large ground based astronomical telescopes will need large fast-steering/tip-tilt mirrors made of ultra-lightweight construction. These fast-steering mirrors are used to continuously correct for atmospheric disturbances and telescope vibrations. An example of this is the European Extremely Large Telescope (E-ELT) M5 lightweight mirror, which is part of the Tip-Tilt/Field-Stabilization Unit. The baseline design for the E-ELT M5 mirror, as presented in the E-ELT Construction Proposal, is a closed-back ULE mirror with a lightweight core using square core cells. Corning Incorporated (Corning) has a long history of manufacturing lightweight mirror blanks using ULE in a closed-back construction, going back to the 1960’s, and includes the Hubble Space Telescope primary mirror, Subaru Telescope secondary and tertiary mirrors, the Magellan I and II tertiary mirrors, and Kepler Space Telescope primary mirror, among many others. A parametric study of 1-meter class lightweight mirror designs showed that Corning’s capability to seal a continuous back sheet to a light-weighted core structure provides superior mirror rigidity, in a near-zero thermal expansion material, relative to other existing technologies in this design space. Corning has investigated the parametric performance of several design characteristics for a 3-meter class lightweight mirror blank for the E-ELT M5. Finite Element Analysis was performed on several design scenarios to obtain weight, areal density, and first Eigen frequency. This paper presents an overview of Corning ULE and lightweight mirror manufacturing capabilities, the parametric performance of design characteristics for 1-meter class and 3-meter class lightweight mirrors, as well as the manufacturing advantages and disadvantages of those characteristics.
High volume ULE segment production
It is expected that many of the next generation large ground based telescopes will utilize a segmented design for the primary mirror and, in some cases, the secondary mirror. Corning Incorporated (Corning) presents a process to manufacture segment mirror blanks from Corning ULE titania silicate glass in segment sizes ranging from 1.0 m to 1.8 m flat to flat. This paper will review ULE properties and describe the facilities, equipment, resources, and processes required to produce a few hundred to a few thousand mirror segment blanks for extremely large telescope (ELT) applications.
Smart telescope for astronomy
In this paper, we present the preliminary design of a smart telescope, i.e. an optomechanical device whose structure is able to monitor external loads (gravity, wind, thermal gradients, displacements caused by earthquake) and actively adapt to them in order to correct misalignments. To obtain that, the final solution will foresee the use of smart materials, or rather integrated smart structures containing sensors (such as fibre optics), and actuators (shape memory alloys or piezoelectric). Starting from the optical design, where the primary mirror is supposed to be in the class of 60cm diameter, with this work we illustrate the mechanical design philosophy. The basic idea is to conceive of a "low-performance" telescope from the stability point of view, in order to emphasize the environmental loads contributions, show that it is possible to correct them a posteriori, and generalize the results for more optimized structures (Serrurier-like). Therefore, it is shown the finite element model of a first naked version of the telescope (without smart structures), useful to know the displacements caused by predictable loads. In this first design phase, the secondary mirror re-centering is taken into account as a study case: to achieve the goal, Macro Fibre Composite piezoelectric actuators have been selected.
Advanced structural design for precision radial velocity instruments
The GMT-Consortium Large Earth Finder (G-CLEF) is an echelle spectrograph with precision radial velocity (PRV) capability that will be a first light instrument for the Giant Magellan Telescope (GMT). G-CLEF has a PRV precision goal of 40 cm/sec (10 cm/s for multiple measurements) to enable detection of Earth-like exoplanets in the habitable zones of sun-like stars1. This precision is a primary driver of G-CLEF’s structural design. Extreme stability is necessary to minimize image motions at the CCD detectors. Minute changes in temperature, pressure, and acceleration environments cause structural deformations, inducing image motions which degrade PRV precision. The instrument’s structural design will ensure that the PRV goal is achieved under the environments G-CLEF will be subjected to as installed on the GMT azimuth platform, including:
    Millikelvin (0.001 °K) thermal soaks and gradients
    10 millibar changes in ambient pressure
    Changes in acceleration due to instrument tip/tilt and telescope slewing
Carbon fiber/cyanate composite was selected for the optical bench structure in order to meet performance goals. Low coefficient of thermal expansion (CTE) and high stiffness-to-weight are key features of the composite optical bench design. Manufacturability and serviceability of the instrument are also drivers of the design.

In this paper, we discuss analyses leading to technical choices made to minimize G-CLEF’s sensitivity to changing environments. Finite element analysis (FEA) and image motion sensitivity studies were conducted to determine PRV performance under operational environments. We discuss the design of the optical bench structure to optimize stiffness-to-weight and minimize deformations due to inertial and pressure effects. We also discuss quasi-kinematic mounting of optical elements and assemblies, and optimization of these to ensure minimal image motion under thermal, pressure, and inertial loads expected during PRV observations.
ESPRESSO optical bench: from mind to reality
F. Tenegi, S. Santana, J. Gómez, et al.
ESPRESSO [1] is a high-resolution spectrograph under development for the VLT telescope. In general, the Optical Bench (OB) structure can be considered as a 3D one, conformed by welding thin plates of Structural Steel (St-52) with a nickelplated surface treatment, combined for getting maximum stiffness and minimum weight, that will be finally re-machined to get stringent geometrical and dimensional tolerances at I/Fs positions. TIG conventional welding procedure has been selected to minimize the cost and facilitate the own welding process. This solution follows the inheritance from HARPS [2] due to its success to achieve the required performance for the bench.

This paper contains an overview of the whole process of designing and manufacturing the Optical Bench of ESPRESSO, from the very first beginning with the specifications to the current status of the bench with its integration on the Spectrograph (including the Finite Element Models and the delivery of the final structure by the supplier) and lessons learned.
Poster Session: Optical Fabrication
Approaching perfection in the manufacturing of silicon immersion gratings
Benjamin T. Kidder, Cynthia B. Brooks, Michelle M. Grigas, et al.
Silicon immersion gratings make near-IR spectrographs compact and allow them to have continuous wavelength coverage over a large bandwidth. We have produced an exceptional silicon immersion grating that approaches optical perfection in terms of surface error. This grating has a peak-to-valley error of 79 nm over a 25 mm beam, which exceeds the 85 nm requirement to have λ/4 peak-to-valley error at the shortest wavelength where silicon immersion gratings can be used. In order to reduce the level of large-scale errors we have honed our contact printing method by optimizing our UV exposure system, introducing additional process checks and inspections and carefully evaluating large-scale errors in the gratings produced.
Towards freeform curved blazed gratings using diamond machining
Concave blazed gratings greatly simplify the architecture of spectrographs by reducing the number of optical components. The production of these gratings using diamond-machining offers practically no limits in the design of the grating substrate shape, with the possibility of making large sag freeform surfaces unlike the alternative and traditional method of holography and ion etching. In this paper, we report on the technological challenges and progress in the making of these curved blazed gratings using an ultra-high precision 5 axes Moore-Nanotech machine. We describe their implementation in an integral field unit prototype called IGIS (Integrated Grating Imaging Spectrograph) where freeform curved gratings are used as pupil mirrors. The goal is to develop the technologies for the production of the next generation of low-cost, compact, high performance integral field unit spectrometers.
Nonconventional ultra-precision manufacturing of ULE mirror surfaces using atmospheric reactive plasma jets
Thomas Arnold, Georg Böhm, Hendrik Paetzelt
In this paper we present a non-abrasive surface manufacturing technology suited for fast and efficient figuring of optical surfaces made of ULE (Corning Ultra Low Expansion) glass. Plasma Jet Machining (PJM) technology is based on an atmospheric chemical reactive plasma jet tool that locally interacts with the surface in order to remove material by chemical reactions forming volatile species. ULE has been proven to be suited for the PJM process. It has been found that the volume removal rate is approximately 25% higher than for fused silica and values up to 50mm3/min can be reached with our setup. Thus, figuring and figure error correction of large optics like mirror segments for earth based telescopes can be realized within a reasonable time. In the paper principles of the PJM process as well as ULE specific issues are discussed and machining results are presented.
Vibrating membrane mirror concept for adaptive optics
A novel vibrating membrane mirror (VMM) based on the mechanical concepts of vibrating membranes is proposed. This mirror has the capability of being a proper alternative for the traditional optical mirrors. A finite element model of membrane mirror is developed using ANSYS Workbench and its dynamic characteristics extracted to compare with main wavefront aberrations. The similarities between normal modes of a vibrating membrane and Zernike polynomials, which approximate mathematically distorted wavefront, are investigated and the degree of similarities is calculated using RMSE criteria and effective radius. To eliminate unwelcome vibrations of actuators, the excitation ring has been introduced.
A segmented subreflector with electroformed nickel laminated panels for the Large Millimeter Telescope
G. Valsecchi, R. Banham, G. Bianucci, et al.
The Large Millimeter Telescope (LMT) Alfonso Serrano is a 50 m diameter single-dish radio telescope optimized for astronomical observations at wavelengths of about a millimeter. Built and operated by the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE) in collaboration with the University of Massachusetts (UMASS), the telescope is located at the 4600 m summit of volcano Sierra Negra, Mexico. Anticipating the completion of the main reflector, currently operating over a 32 m subaperture, INAOE has contracted Media Lario for the design and manufacturing of a new 2.63 m subreflector that will enable higher efficiency astronomical observations with the entire main reflector surface. The new subreflector manufactured by Media Lario is segmented in 9 smaller panels, one central dome and eight identical petals, assembled and precisely aligned on a steel truss structure that will be connected to the hexapod mounted on the tetrapod head. Each panel was fabricated with Media Lario’s unique laminated technology consisting of front and rear Nickel skins, electroformed from precise molds and bonded to a lightweight Aluminum honeycomb core. The reflecting surface of each panel was given a thin galvanic Rhodium coating that ensures that the reflector survives the harsh environmental conditions at the summit of Sierra Negra during the 30 year lifetime of the telescope. Finally, the 2.63 m subreflector produced by Media Lario was qualified for typical cold night through hot day observation conditions with a maximum RMS error of 24.8 μm, which meets INAOE’s requirements.
Optimisation of grolishing freeform surfaces with rigid and semi-rigid tools
Guoyu Yu, Hsing-Yu Wu, David Walker, et al.
After the formal acceptance of our fabrication of E-ELT segments, we aim to further accelerate the mass production by introducing an intermediate grolishing procedure using industrial robots, reducing the total process time by this much faster and parallel link. In this paper, we have presented research outputs on tool design, tool path generation, study of mismatch between rigid, semi-rigid tool and aspheric surface. It is indicated that the generation of mid-spatial frequency is proportional to the grit size and misfit between work piece and tool surfaces. Using a Non-Newtonian material tool with a spindle speed of 30 rpm has successfully reduce the mid-spatial error. The optimization of process parameters involve the study the combination effects of the above factors. These optimized parameters will result in a lookup table for reference of given input surface quality. Future work may include the higher spindle speed for grolishing with non- Newtonian tool looking for potential applications regarding to form correction, higher removal rate and edge control.
Manufacturing methods of testing the large-sized optics at the stage of grinding, aspherical surface centering, and interface elements positioning before gluing
The article describes the method of testing the absolute profile of large-sized astronomical mirrors grinded aspherical surface and the method of test the aspherical surface decentering relative to the astronomical mirror geometrical center by means of a linear three-point spherometer, which is subsequently moved perpendicular to the direction from the optical surface center to the edge, as well as the method of positioning the interface elements being glued.
Studying the stability of Astrositall thermal and mechanical properties while manufacturing the astronomical and space mirrors
Magomed A. Abdulkadyrov, Aleksandr P. Semenov, Alexey P. Patrikeev, et al.
The technology of producing the astronomical and space mirrors from Astrositall material, including its properties and stability of these properties in the course of time, is described. The results of long-term material tests are presented. In particular, the method of grinding and polishing the off-axis segment of the mirror of very large telescope in the stress-strain state is considered.
Diamond fly cutting of aluminum thermal infrared flat mirrors for the OSIRIS-REx Thermal Emission Spectrometer (OTES) instrument
Christopher E. Groppi, Matthew Underhill, Zoltan Farkas, et al.
We present the fabrication and measurement of monolithic aluminum flat mirrors designed to operate in the thermal infrared for the OSIRIS-Rex Thermal Emission Spectrometer (OTES) space instrument. The mirrors were cut using a conventional fly cutter with a large radius diamond cutting tool on a high precision Kern Evo 3-axis CNC milling machine. The mirrors were measured to have less than 150 angstroms RMS surface error.
Combined fabrication technique for high-precision aspheric optical windows
Specifications made on optical components are becoming more and more stringent with the performance improvement of modern optical systems. These strict requirements not only involve low spatial frequency surface accuracy, mid-and-high spatial frequency surface errors, but also surface smoothness and so on. This presentation mainly focuses on the fabrication process for square aspheric window which combines accurate grinding, magnetorheological finishing (MRF) and smoothing polishing (SP). In order to remove the low spatial frequency surface errors and subsurface defects after accurate grinding, the deterministic polishing method MRF with high convergence and stable material removal rate is applied. Then the SP technology with pseudo-random path is adopted to eliminate the mid-and-high spatial frequency surface ripples and high slope errors which is the defect for MRF. Additionally, the coordinate measurement method and interferometry are combined in different phase. Acid-etched method and ion beam figuring (IBF) are also investigated on observing and reducing the subsurface defects. Actual fabrication result indicates that the combined fabrication technique can lead to high machining efficiency on manufaturing the high-precision and high-quality optical aspheric windows.
Process optimization of laser-based solderjet bumping for the mounting of optical components
Thomas Burkhardt, Marcel Hornaff, Andreas Kamm, et al.
Advanced optical systems of telescopes and scientific instrumentation require high accuracy mounting and joining of components. Applications for deep UV, under high energetic radiation, for vacuum operation, or assemblies subjected to environmental loads (e.g. humidity and temperature) require a replacement of organic adhesives or optical cement by a more robust bonding agent. Soldering allows the bonding of different materials with an inorganic filler material. We present the optimization of the laser-based Solderjet Bumping for the mounting of optical components and the parameters of the bonding process for fused silica and LAK9G15 (radiation resistant glass) with thermally matched metal mounts. The investigation covers the experimental determination and optimization of solder wetting to the respective base materials and the bond strengths achieved.
Every photon counts: improving low, mid, and high-spatial frequency errors on astronomical optics and materials with MRF
Chris Maloney, Jean Pierre Lormeau, Paul Dumas
Many astronomical sensing applications operate in low-light conditions; for these applications every photon counts. Controlling mid-spatial frequencies and surface roughness on astronomical optics are critical for mitigating scattering effects such as flare and energy loss. By improving these two frequency regimes higher contrast images can be collected with improved efficiency. Classically, Magnetorheological Finishing (MRF) has offered an optical fabrication technique to correct low order errors as well has quilting/print-through errors left over in light-weighted optics from conventional polishing techniques. MRF is a deterministic, sub-aperture polishing process that has been used to improve figure on an ever expanding assortment of optical geometries, such as planos, spheres, on and off axis aspheres, primary mirrors and freeform optics. Precision optics are routinely manufactured by this technology with sizes ranging from 5-2,000mm in diameter. MRF can be used for form corrections; turning a sphere into an asphere or free form, but more commonly for figure corrections achieving figure errors as low as 1nm RMS while using careful metrology setups.

Recent advancements in MRF technology have improved the polishing performance expected for astronomical optics in low, mid and high spatial frequency regimes. Deterministic figure correction with MRF is compatible with most materials, including some recent examples on Silicon Carbide and RSA905 Aluminum. MRF also has the ability to produce ‘perfectly-bad’ compensating surfaces, which may be used to compensate for measured or modeled optical deformation from sources such as gravity or mounting. In addition, recent advances in MRF technology allow for corrections of mid-spatial wavelengths as small as ~1mm simultaneously with form error correction. Efficient midspatial frequency corrections make use of optimized process conditions including raster polishing in combination with a small tool size. Furthermore, a novel MRF fluid, called C30, has been developed to finish surfaces to ultra-low roughness (ULR) and has been used as the low removal rate fluid required for fine figure correction of mid-spatial frequency errors. This novel MRF fluid is able to achieve <4Å RMS on Nickel-plated Aluminum and even <1.5Å RMS roughness on Silicon, Fused Silica and other materials. C30 fluid is best utilized within a fine figure correction process to target mid-spatial frequency errors as well as smooth surface roughness 'for free' all in one step.

In this paper we will discuss recent advancements in MRF technology and the ability to meet requirements for precision optics in low, mid and high spatial frequency regimes and how improved MRF performance addresses the need for achieving tight specifications required for astronomical optics.
Process improvements in the production of silicon immersion gratings
Cynthia B. Brooks, Benjamin Kidder, Michelle Grigas, et al.
We have explored a number of lithographic techniques and improvements to produce the resist lines that then define the grating groove edges of silicon immersion gratings. In addition to our lithographic process using contact printing with photomasks, which is our primary technique for the production of immersion gratings, we explored two alternative fabrication methods, direct-write electron beam and photo-lithography. We have investigated the application of antireflection (AR) coatings during our contact printing lithography method to reduce the effect of Fizeau fringes produced by the contact of the photomask on the photoresist surface. This AR coating reduces the amplitude of the periodic errors by a factor of 1.5. Electron beam (e-beam) patterning allows us to manufacture gratings that can be used in first order, with groove spacing down to 0.5 micrometer or smaller (2,000 grooves/mm), but could require significant e-beam write times of up to one week to pattern a full-sized grating. The University of Texas at Austin silicon diffractive optics group is working with Jet Propulsion Laboratory to develop an alternate e-beam method that employs chromium liftoff to reduce the write time by a factor of 10. We are working with the National Institute of Standards and Technology using laser writing to explore the possibility of creating very high quality gratings without the errors introduced during the contact-printing step. Both e-beam and laser patterning bypass the contact photolithography step and directly write the lines in photoresist on our silicon substrates, but require increased cost, time, and process complexity.
Aspherical mirrors for the Gamma-ray Cherenkov Telescope, a Schwarschild-Couder prototype proposed for the future Cherenkov Telescope Array
J. L. Dournaux, J. Gironnet, J. M. Huet, et al.
The Cherenkov Telescope Array (CTA) project, led by an international collaboration of institutes, aims to create the world's largest next generation Very High-Energy (VHE) gamma-ray telescope array, devoted to observations in a wide band of energy, from a few tens of GeV to more than 100 TeV. The Small-Sized Telescopes (SSTs) are dedicated to the highest energy range. Seventy SSTs are planned in the baseline array design with a required lifetime of about 30 years.

The GCT (Gamma-ray Cherenkov Telescope) is one of the prototypes proposed for CTA's SST sub-array. It is based on a Schwarzschild-Couder dual-mirror optical design. This configuration has the benefit of increasing the field-of-view and decreasing the masses of the telescope and of the camera. But, in spite of these many advantages, it was never implemented before in ground-based Cherenkov astronomy because of the aspherical and highly curved shape required for the mirrors.

The optical design of the GCT consists of a primary 4 meter diameter mirror, segmented in six aspherical petals, a secondary monolithic 2-meter mirror and a light camera. The reduced number of segments simplifies the alignment of the telescope but complicates the shape of the petals. This, combined with the strong curvature of the secondary mirror, strongly constrains the manufacturing process. The Observatoire de Paris implemented metallic lightweight mirrors for the primary and the secondary mirrors of GCT. This choice was made possible because of the relaxed requirements of optical Cherenkov telescopes compared to optical ones. Measurements on produced mirrors show that these ones can fulfill requirements in shape, PSF and reflectivity, with a clear competition between manufacturing cost and final performance.

This paper describes the design of these mirrors in the context of their characteristics and how design optimization was used to produce a lightweight design. The manufacturing process used for the prototype and planned for the large scale production is presented as well as the performance, in terms of geometric and optical properties, of the produced mirrors. The alignment procedure of the mirrors is also detailed. This technique is finally compared to other manufacturing techniques based on composite glass mirrors within the framework of GCT mirrors specificities.
Development of the fast steering secondary mirror assembly of GMT
The Giant Magellan Telescope (GMT) will be featured with two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). The FSM has an effective diameter of 3.2 m and built as seven 1.1 m diameter circular segments, which are conjugated 1:1 to the seven 8.4m segments of the primary. Each FSM segment contains a tip-tilt capability for fine co-alignment of the telescope sub-apertures and fast guiding to attenuate telescope wind shake and mount control jitter. This tip-tilt capability thus enhances performance of the telescope in the seeing limited observation mode. As the first stage of the FSM development, Phase 0 study was conducted to develop a program plan detailing the design and manufacturing process for the seven FSM segments. The FSM development plan has been matured through an internal review by the GMTO-KASI team in May 2016 and fully assessed by an external review in June 2016. In this paper, we present the technical aspects of the FSM development plan.
Results from bonding of the SALT primary mirror edge sensors
The Southern African Large Telescope has till recently operated without active closed loop control of its Primary Mirror. The reason for this was that there were no suitable edge sensor system available on the market. Recently a system became available and SALT form Fogale Nanotech. The system consist of a sensor, cables and control electronics. The system was still under development and SALT was responsible for the integration of the sensors before deployment on the Telescope. Several issues still had to be addressed. One of these issues was the integration of the sensors at an appropriate production rate. The sensors was supplied as flexible pc boards with different types making up the transmitters and receivers. These flexible boards were bonded to ClearCeram Z L-Brackets before the appropriate connectors were installed. This paper describes the process used to integrate and test the sensors.
Manufacturing process for the WEAVE prime focus corrector optics for the 4.2m William Hershel Telescope
Emilie Lhomé, Tibor Agócs, Don Carlos Abrams, et al.
In this paper, we detail the manufacturing process for the lenses that will constitute the new two-degree field-of-view Prime Focus Corrector (PFC) for the 4.2m William Herschel Telescope (WHT) optimised for the upcoming WEAVE Multi-Object Spectroscopy (MOS) facility. The corrector, including an Atmospheric Dispersion Corrector (ADC), is made of six large lenses, the largest being 1.1-meter diameter. We describe how the prescriptions of the optical design were translated into manufacturing specifications for the blanks and lenses. We explain how the as-built glass blank parameters were fed back into the optical design and how the specifications for the lenses were subsequently modified. We review the critical issues for the challenging manufacturing process and discuss the trade-offs that were necessary to deliver the lenses while maintaining the optimal optical performance. A short description of the lens optical testing is also presented. Finally, the subsequent manufacturing steps, including assembly, integration, and alignment are outlined.
Aspherization of off-axis high-asphericity mirrors with arbitrary external circuit by means of CNC machines
Technology with the use of programmable computer-controlled system and a set of special instruments, which makes possible aspherization of off-axis large-size optical elements of astronomical and space telescopes with deviation from the nearest sphere of more than 1 mm, was developed.
Euclid mirrors and test collimator: AMOS developments
Pierre Gloesener, Fabrice Wolfs, Marcel Cola, et al.
EUCLID is an optical/near-infrared survey mission to be launched in 2020 towards the L2 Lagrange point. It will aim at studying the dark universe and providing a better understanding of the origin of the accelerating expansion of the universe. Through the use of cosmological sounding, it will investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the universe and on the cosmic history of large structures formation.

The EUCLID payload module (PLM) consists of a 1.2 m-class telescope and will accommodate two instruments.

As a subcontractor of AIRBUS Defence and Space, AMOS is responsible for the manufacturing of the secondary and the third mirrors of the telescope as well as for the flat folding mirror set within the focal plane arrangement of EUCLID telescope, which incorporates dedicated filtering functions. AMOS produces in addition the 1.3 m-class test collimator for the on-ground validation of the EUCLID instrument.
Investigation of four classical groove patterns with three typical tool types for use in the rigid tool smoothing process
When a surface experiencing robotic processing to improve its optical performance (such as removing mid-spatial frequencies, localized grinding errors, and regional surface scratches), spindle speed, tool travel speed, pressure, slurry density as well as groove patterns are main factors to influence surface finishes. Based on the desired material removal rate, the Preston equation can provide optimized pressures and velocities between the tool and processed surface. Various groove patterns, however, can hardly predict by the equation because different patterns can cause unique tool deformation and pressure distribution, leading to determine unique smoothing result. In this paper, four typical groove patterns are studied: non-groove, grid grove, annular groove and radial groove with three typical tool types are evaluated by Finite Element Method (FEM) and statistics. Characteristics of these tools and groove patterns are presented in the end of this paper.
Poster Session: Test and Metrology
The influence of technological mounting of lightweight large size space astronomical mirrors into the shape of its reflecting surface during interferometric control
Oleg Vlasenko, Mikhail Sachkov
The technological mounting is necessary during the ground base tests to exclude the gravity influence into the lightweight large-size space astronomical mirrors (diameter of 1.7 m and more). We propose a method that allows to correct the influence of this mounting onto the mirror shape surface during an interferometric control. Here we present results based on primary mirror tests of the WSO-UV project.
MEGARA: large pupil element tests and performance
MEGARA is a third generation spectrograph for the Spanish 10.4m telescope (GTC) providing two observing modes: a large central Integral Field Unit (IFU), called the Large Compact Bundle (LCB), covering a FOV of 12.5 × 11.3 arcsec2, and a Multi-Object Spectrograph (MOS) with a FOV of 3.5 × 3.5 arcmin2. MEGARA will observe the whole visible range from 3650A to 10000A allowing different spectral resolutions (low, medium and high) with R = 6000, 11000 and 18000 respectively. The dispersive elements are placed at the spectrograph pupil position in the path of the collimated beam and they are composed of a set of volume phase hologram gratings (VPHs) sandwiched between two flat windows and coupled in addition to two prisms in the case of the medium- and high-resolution units. We will describe the tests and setups developed to check the requirements of all units, as well as the obtained performance at laboratory
Thermal testing results of an electroformed nickel secondary (M2) mirror
To support higher-frequency operation, the Large Millimeter Telescope/Gran Telescopio Milimetrico (or LMT/GTM) is replacing its existing monolithic aluminum secondary mirror (M2). The new mirror is a segmented design based on the same electroformed nickel reflector panel technology that is already in use for the primary reflector segments. While the new M2 is lighter and has better surface accuracy than the original mirror, the electroformed panels are more sensitive to high temperatures. During the design phase, concerns were raised over the level of temperature increase that could occur at M2 during daytime observations. Although the panel surface is designed to scatter visible light, the LMT primary mirror is large enough to cause substantial solar heating, even at significant angular separation from the Sun.

To address these concerns, the project conducted a series of field tests, within the constraint of having minimum impact on night time observations. The supplier sent two coupon samples of a reflector panel prepared identically to their proposed M2 surface. Temperature sensors were mounted on the samples and they were temporarily secured to the existing M2 mirror at different distances from the center. The goal was to obtain direct monitoring of the surface temperature under site thermal conditions and the concentration effects from the primary reflector. With the sensors installed, the telescope was then commanded to track the Sun with an elevation offset. Initially, elevation offsets from as far as 40 degrees to as close as 6 degrees were tested. The 6 degree separation test quickly passed the target maximum temperature and the telescope was returned to a safer separation. Based on these initial results, a second set of tests was performed using elevation separations from 30 degrees to 8 degrees.

To account for the variability of site conditions, the temperature data were analyzed using multiple metrics. These metrics included maximum temperature, final time average temperature, and an curve fit for heating/ cooling. The results indicate that a solar separation angle of 20 degrees should be suitable for full performance operation of the LMT/GTM. This separation not only is sufficient to avoid high temperatures at the mirror, but also provides time to respond to any emergency conditions that could occur (e.g., switching to a generator after a power failure) for observations that are ahead of the motion of the Sun. Additionally, even approaches of 10 to 15 degrees of angular separation on the sky may be achievable for longer wavelength observations, though these would likely be limited to positions that are behind the position of the Sun along its motion.
Mapping the Large Millimeter Telescope primary reflector using photogrammetry: a first comparison with 12 GHz holography
The Large Millimeter Telescope (LMT) makes extensive use of 12 GHz holography during maintenance periods to finetune the alignment of primary reflector segments to the best-fit design parabola. Tracker measurements have also been used for this task, however the technique is severely limited by environmental noise and large data collection times, on the order of many hours for a single map. In 2015 we started photogrammetry trials as a complimentary measurement technique. Photogrammetry can offer reduced mapping times compared with laser trackers, and like holography, allows maps to be made at arbitrary elevation angles. Depending on the placement of reflecting targets, the technique can also provide higher spatial resolution than currently achieved using our holography system.

Accurate photogrammetry requires a robust strategy for the incorporation of multiple camera stations, a task complicated by the size of the antenna, obstructions of the surface by the sub-reflector and tetrapod legs, and the practicability of using the site tower crane as a moving camera platform. Image scaling is also a major consideration, since photogrammetry lacks any inherent distance reference. Therefore appropriate scale bars must be fabricated and located within the camera field of view. Additional considerations relate to the size and placement of reflective targets, and the optimization of camera settings. In this paper we present some initial comparisons of laser tracker, holography and photogrammetry measurements taken in 2015, showing clearly the status of alignment for distinct zones of the currently operating 32.5 m primary collecting area.
Day-time local phasing of neighbouring segments of the E-ELT primary mirror, based simultaneous multi-λ shearing interferometry
S. Leveque, C. Falldorf, R. Klattenhoff, et al.
In the context of the segmented primary mirror of the E-ELT, the ability to measure the phasing state of neighbouring segments during day-time represents a way to mitigate the risk of potentially time consuming on-sky phasing after segment replacement. This paper presents the concept of a local phasing sensor based on simultaneous multi-wavelength shearing interferometry, as well as experimental results obtained on the ESO M1 test facility.
Low-cost measurement and monitoring system for cryogenic applications
Cryostats are closed chambers that hinder the monitoring of materials, structures or systems installed therein. This paper presents a webcam-based measurement and monitoring system, which can operate under vacuum and cryogenic conditions to be mainly used in astrophysical applications. The system can be configured in two different assemblies: wide field that can be used for mechanism monitoring and narrow field, especially useful in cryogenic precision measurements with a resolution up to 4 microns/pixel.
Three-dimensional metrology inside a vacuum chamber
Anne Costille, Florent Beaumont, Eric Prieto, et al.
Several three dimensional coordinates systems are proposed by companies to provide accurate measurement of mechanical parts in a volume. None of them are designed to perform the metrology of a system in a vacuum chamber. In the frame of the test of NISP instrument from ESA Euclid mission, the question was raised to perform a three dimensional measurement of different parts during the thermal test of NISP instrument into ERIOS chamber done at Laboratoire d’Astrophysique de Marseille (LAM). One of the main objectives of the test campaign will be the measurement of the focus position of NISP image plane with respect to the EUCLID object plane to ensure a good focalisation of NISP instrument after integration on the payload. A Metrology Verification System (MVS) has been proposed. Its goal is to provide at operational temperature the measurement of references frames set on a EUCLID telescope simulator and NISP, the knowledge of the coordinates of the object point source provided by the telescope simulator and the measurement of the angle between the telescope simulator optical axis and NISP optical axis. The MVS concept is based on the use of a laser tracker, outside the vacuum chamber, that measures reflectors inside the vacuum chamber through a curved window. We will present preliminary results that show the possibility to perform this type of measurements and the accuracy reached in this configuration. An analysis of the contributors to the measurement error budget of the MVS is proposed, based on the current knowledge of the MVS performance and constraints during the TB/TV tests.
The alidade temperature behaviour of TM65m antenna and its effects on pointing accuracy
Li Fu, Quan B. Ling, Xu G. Geng, et al.
Aimed at the alidade of TM65m antenna, the distributions of temperature field and effects of thermal deformations on pointing accuracy were analyzed based on thermometers and inclinometer. The alidade temperature and cross-elevation tilt were recorded for one year when the antenna was at different azimuth and elevation angles. And, the emphasis is on studying the data of sunny days in summer and winter during the antenna parked its home position (azimuth angle=155°, elevation angle=90°). The results show the maximum temperature difference between day and night is 14.6 °C in summer and is 27 °C in winter. In winter, the larger temperature difference, shorter sunshine time and later sunrise contribute to the temperature variation per unit time is larger. In addition, pointing offset was checked by scanning polestar for continuous 24 hours. From the results obtained so far, it seems that the elevation offset caused by the alidade temperature variation is more than 20 arcsec from 8:00 am to 10:00 am. The research results provide a base for the enhancement of pointing accuracy.
Hollow-cathode lamps as optical frequency standards: the influence of optical imaging on the line-strength ratios
Hollow cathode discharge lamps (HCLs) have been successfully used in recent years as calibration sources of optical astronomical spectrographs. The numerous narrow metal lines have stable wavelengths, which makes them well suited for m/s calibration accuracy of high-resolution spectrographs, while the buffer-gas lines are less stable and less useful. Accordingly, an important property is the metal-to-gas line-strength ratio (Rmetal/gas). Processes inside the lamp cause the light to be emitted from different regions between the cathode and the anode leaing to the emission of different beams with different values of Rmetal/gas. We used commercially- available HCLs to measure and characterize these beams with respect to their spatial distribution, their angle of propagation relative to the optical axis, and their values of Rmetal/gas. We conclude that a good imaging of an HCL into a fiber-fed spectrograph would consist of an aperture close to its front window in order to filter out the parts of the beam with low Rmetal/gas, and of a lens to collimate the important central beam. We show that Rmetal/gas can be further improved with only minor adjustments of the imaging parameters, and that the imaging scheme that yields the highest Rmetal/gas does not necessarily provide the highest flux.
Alignment of the ESPRESSO Coudé train on the ESO VLT
Gerardo Avila, Alexandre Cabral, Joao Pinto Coelho, et al.
ESPRESSO is a high resolution UV-vis spectrograph that will be placed in the combined Coudé laboratory of the ESO VLT. The instrument is in its assembly phase and the Coudé optics will start to be mounted at the telescope in the first quarter of 2016. This paper describes the optics of the train and the strategies for its alignment taking into account the main constraints: accessibility, mechanical, as per built optics, tolerances and tools.
VUV optical ground system equipment and its application to the ICON FUV flight grating characterization and selection
Pascal Blain, Richard Desselle, Isabelle Domken, et al.
ICON FUV is a two channel spectrographic imager that measures intensity and spatial distribution of oxygen (135.6 nm) and molecular nitrogen (157 nm) of the ionosphere. As those wavelengths are strongly absorbed by the atmosphere, the optical elements of the system have to be tested inside vacuum chambers. Prior to the instrument alignment and calibration, two 3600 gr/mm gratings were characterized. The primary focus is the measurement of the diffraction efficiencies; while the second objective is to select the best grating and to define which is the flight and the spare. A dedicated setup has been developed to assess the grating optical performances under vacuum. A 1 cm diameter collimated beam is generated using an off-axis parabola and a UV source at its focal point. The grating is placed at the center of two rotation stages collinearly aligned. One detector is placed on a rotating arm, deported from its rotation center. A PMT detector records diffracted light intensity with respect to its angular position and its wavelength. Angular incidence on the grating is tuned with the help of the second rotation stage. The grating efficiency homogeneity and scattering properties are measured through a Y-X scan.
Singular values behaviour optimization in the diagnosis of feed misalignments in radioastronomical reflectors
Amedeo Capozzoli, Claudio Curcio, Angelo Liseno, et al.
The communication presents an innovative method for the diagnosis of reflector antennas in radio astronomical applications. The approach is based on the optimization of the number and the distribution of the far field sampling points exploited to retrieve the antenna status in terms of feed misalignments, this to drastically reduce the time length of the measurement process and minimize the effects of variable environmental conditions and simplifying the tracking process of the source. The feed misplacement is modeled in terms of an aberration function of the aperture field. The relationship between the unknowns and the far field pattern samples is linearized thanks to a Principal Component Analysis. The number and the position of the field samples are then determined by optimizing the Singular Values behaviour of the relevant operator.
Characterizing the performance of cryogenic lens mounts for the HARMONI spectograph
Jamie R. Allen, Kieran O'Brien, James D. Lynn, et al.
Two different cryogenic lens mounts have been developed and tested for potential use in the HARMONI spectrograph cameras. Problems were encountered during initial tests whereby the lenses were cracking where they were adhered to the mount. This was found to be caused by the choice of adhesive and solved by changing to a silicone RTV glue. The cryogenic performance of the two lens mount designs was tested, with the baseline design seeing the lens move by 18 μm radially from warm to cold, which is just within the tightest tolerances from the optical design, as long as any misalignments in the mounting procedure can be removed when aligning the lenses in the camera barrel. The alternative design was found have much worse performance with the lens moving by 40 μm due to fragile flexures and so is no longer being considered. A mounting procedure for spherical lenses has also been developed which is capable of peak to valley alignments of 10 μm axially, and 5 μm radially.
Poster Session: Telescope Structures and Domes
Tolerancing of a carbon fiber reinforced polymer metering tube structure of a high-resolution space-borne telescope
Mustafa Ekinci
High resolution space borne telescopes require dimensionally stable structures to meet very stringent optical requirements. Furthermore, high resolution space borne telescope structures need to have high stiffness and be lightweight in order to survive launch loads. Carbon fiber reinforced polymers (CFRP) are lightweight and have tailorable mechanical properties like stiffness and coefficient of thermal expansion. However, mechanical properties are highly dependent on manufacturing processes and manufacturing precision. Moreover CFRP tend to absorb moisture which affects dimensional stability of the structure in the vacuum environment. In order to get specified properties out of manufacturing, tolerances need to be defined very accurately.

In this paper, behavior of CFRP metering tube structure of a high resolution space borne camera is investigated for ply orientation, fiber and void content deviations which may arise from manufacturing errors and limitations. A computer code is generated to determine laminate properties of stacked up uni-directional (UD) laminae using classical laminate theory with fiber and matrix properties obtained from suppliers and literature. After defining laminate stackup, many samples are virtually created with ply orientations, volumetric fiber and void content that randomly deviates in a tolerance range which will be used in manufacturing. Normal distribution, standard deviation and mean values are presented for elasticity modulus, coefficient of thermal expansion (CTE), coefficient of moisture expansion (CME) and thermal conductivity in axial and transverse directions of quasi-isotropic stackups and other stackups which have properties presented in literature.
TCS and peripheral robotization and upgrade on the ESO 1-meter telescope at La Silla Observatory
S. Ropert, V. Suc, A. Jordán, et al.
In this work we describe the robotization and upgrade of the ESO 1m telescope located at La Silla Observatory. The ESO 1m telescope was the first telescope installed in La Silla, in 1966. It now hosts as a main instrument the FIber Dual EchellE Optical Spectrograph (FIDEOS), a high resolution spectrograph designed for precise Radial Velocity (RV) measurements on bright stars. In order to meet this project's requirements, the Telescope Control System (TCS) and some of its mechanical peripherals needed to be upgraded. The TCS was also upgraded into a modern and robust software running on a group of single board computers interacting together as a network with the CoolObs TCS developed by ObsTech. One of the particularities of the CoolObs TCS is that it allows to fuse the input signals of 2 encoders per axis in order to achieve high precision and resolution of the tracking with moderate cost encoders. One encoder is installed on axis at the telescope and the other on axis at the motor. The TCS was also integrated with the FIDEOS instrument system so that all the system can be controlled through the same remote user interface. Our modern TCS unit allows the user to run observations remotely through a secured internet web interface, minimizing the need of an on-site observer and opening a new age in robotic astronomy for the ESO 1m telescope.
Error compensation research on the focal plane attitude measurement instrument
Hongfei Zhou, Feifan Zhang, Chao Zhai, et al.
The surface accuracy of astronomical telescope focal plate is a key indicator to precision stellar observation. Combined with the six DOF parallel focal plane attitude measurement instrument that had been already designed, space attitude error compensation of the attitude measurement instrument for the focal plane was studied in order to measure the deformation and surface shape of the focal plane in different space attitude accurately.
Research and implementation of the integrated cooling system for focal plate
Jianping Wang, Lixuan Cheng, Jiaru Chu, et al.
With the rapid development of multi-objective astronomical survey telescope technology, the heat of focal plate which high-density optical fiber positioners were mounted in has become the key factor of system precision. The new integrated cooling system designed multi curved composite grooves on the surface of focal plate for forced convection was proposed. Meanwhile, the manufacturing process, sealing structure and heat dissipation performance of the system were analyzed and tested with detail in the paper. The experimental results suggested that the new integrated cooling system of focal plate has a fast response speed and good heat dissipation performance.
Performance of the Gamma-ray Cherenkov Telescope structure: a dual-mirror telescope prototype proposed for the future Cherenkov Telescope Array
J. L. Dournaux, J. P. Amans, L. Dangeon, et al.
The Cherenkov Telescope Array (CTA) project aims to create the next generation Very High-Energy (VHE) gamma-ray telescope array. It will be devoted to the observation of gamma rays from 20 GeV to above 100 TeV. Because of this wide energy band, three classes of telescopes, associated with different energy ranges and different mirror sizes, are defined. The Small Size Telescopes (SSTs) are associated with the highest energy range. Seventy of these telescopes are foreseen on the Southern site of the CTA. The large number of telescopes constrains their mechanical structure because easy maintenance and reduced cost per telescope are needed. Moreover, of course, the design shall fulfill the required performance and lifetime in the environment conditions of the site.

The Observatoire de Paris started design studies in 2011 of the mechanical structure of the GCT (Gamma-ray Cherenkov Telescope), a four-meter prototype telescope for the SSTs of CTA, from optical and preliminary mechanical designs made by the University of Durham. At the end of 2014 these studies finally resulted in a lightweight (~8 tons) and stiff design. This structure was based on the dual-mirror Schwarzschild-Couder (SC) optical design, which is an interesting and innovative alternative to the one-mirror Davies-Cotton design commonly used in ground-based Cherenkov astronomy. The benefits of such a design are many since it enables a compact structure, lightweight camera and a good angular resolution across the entire field-of-view. The mechanical structure was assembled on the Meudon site of the Observatoire de Paris in spring 2015. The secondary mirror, panels of the primary mirror and the Telescope Control System were successfully implemented afterwards leading now to a fully operational telescope.

This paper focuses on the mechanics of the telescope prototype. It describes the mechanical structure and presents its performance identified from computations or direct measurements. Upgrades of the design in the context of the preproduction and the large scale CTA production are also discussed.
Development of a 0.5m clear aperture Cassegrain type collimator telescope
Mustafa Ekinci, Özgür Selimoğlu
Collimator is an optical instrument used to evaluate performance of high precision instruments, especially space-born high resolution telescopes. Optical quality of the collimator telescope needs to be better than the instrument to be measured. This requirement leads collimator telescope to be a very precise instrument with high quality mirrors and a stable structure to keep it operational under specified conditions. In order to achieve precision requirements and to ensure repeatability of the mounts for polishing and metrology, opto-mechanical principles are applied to mirror mounts. Finite Element Method is utilized to simulate gravity effects, integration errors and temperature variations. Finite element analyses results of deformed optical surfaces are imported to optical domain by using Zernike polynomials to evaluate the design against specified WFE requirements. Both mirrors are aspheric and made from Zerodur for its stability and near zero CTE, M1 is further light-weighted. Optical quality measurements of the mirrors are achieved by using custom made CGHs on an interferometric test setup. Spider of the Cassegrain collimator telescope has a flexural adjustment mechanism driven by precise micrometers to overcome tilt errors originating from finite stiffness of the structure and integration errors. Collimator telescope is assembled and alignment methods are proposed.
Poster Session: Optical Fibers and Positioners
On the origin and removal of interference patterns in coated multimode fibres
Yazmin Padilla Michel, Silvio Pulwer, Pouneh Saffari, et al.
In this study, we present the experimental investigations on interference patterns, such as those already reported in VIMOS-IFU, and up to now no appropriate explanation has been presented. These interference patterns are produced in multimode fibres coated with acrylate or polyimide, which is the preferred coating material for the fibres used in IFUs. Our experiments show that, under specific conditions, cladding modes interact with the coating and produce interference. Our results show that the conditions at which the fibre is held during data acquisition has an impact in the output spectrum. Altering the positioning conditions of the fibre leads to the changes into the interference pattern, therefore, fibres should be carefully manipulated in order to minimise this potential problem and improve the performance of these instruments. Finally we present a simple way of predicting and modelling this interference produced from the visible to the near infrared spectra. This model can be included in the data reduction pipeline in order to remove the interference patterns.

These results should be of interest for the optimisation of the data reduction pipelines of instruments using optical fibres. Considering these results will benefit innovations and developments of high performance fibre systems.
Effects of fiber manipulation methods on optical fiber properties
Optical fibers are routinely used to couple high-resolution spectrographs to modern telescopes, enabling important advantages in areas such as the search for extrasolar planets using spectroscopic radial velocity measurements of candidate stars. Optical fibers partially scramble the input illumination, and this feature enables a fiber feed to provide more uniform illumination to the spectrograph optics, thereby reducing systematic errors in radial velocity measurements. However fibers suffer from focal ratio degradation (FRD), a spreading of the beam at the output of the fiber with respect to that at the fiber input, which results in losses in throughput and resolution. Modal noise, a measurement uncertainty caused by inherent fiber properties and evident as a varying spatial intensity at the fiber exit plane, reduces the signal to noise ratio in the data. Devices such as double scramblers are often used to improve scrambling, and better fiber end preparation can mitigate FRD. Many instruments agitate the fiber during an observation to reduce modal noise, and stretching the fiber during use has been shown to offer a greater reduction in that noise. But effects of agitation and stretching on fiber parameters such as total transmission and focal ratio degradation have not been adequately studied. In this paper we present measurements of transmission loss and focal ratio degradation for both agitated and stretched fibers.
Influence of misalignment on output of astronomical large-core fibers of multi-object fiber spectroscopic telescopes
Weimin Sun, Jing Wang, Qi Yan, et al.
Large-core fibers are wildly used in astronomical applications. For multi-object fiber spectroscopic telescopes, for example the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), the aiming accuracy is important. The fibers in these telescopes have two ends; one input end is in the focal plane to collect the star image while another output end is inside a spectrometer. If the input end doesn't exactly match the image of the star, the emitting spot from the output end could not be a central-peak circular spot. If the misalignment is serious, the output would be a ring. The ring spot would lead to wrong spectrum analysis result. To obtain the relationship of the input position and the output spot, we designed a scanning experiment. We chose a single-mode fiber for 650nm, which core diameter is 4 μm, to scan a large-core astronomical fiber FBP320385415, which is used in LAMOST. The experimental results show that if the input point is 122μm away the center of the fiber, the output spot will be a ring spot instead of the supposed circular pattern. The nearer of the input position is to the edge of fiber, the closer the output spot to a ring. This experimental result is important for designing and optimizing the fiber-end adjusting devices of multi-object fiber spectroscopic telescopes. It can also apply for various fields of astronomical spectroscopy observation.
Back-illuminate fiber system research for multi-object fiber spectroscopic telescope
In the telescope observation, the position of fiber will highly influence the spectra efficient input in the fiber to the spectrograph. When the fibers were back illuminated on the spectra end, they would export light on the positioner end, so the CCD cameras could capture the photo of fiber tip position covered the focal plane, calculates the precise position information by light centroid method and feeds back to control system. A set of fiber back illuminated system was developed which combined to the low revolution spectro instruments in LAMOST. It could provide uniform light output to the fibers, meet the requirements for the CCD camera measurement. The paper was introduced the back illuminated system design and different test for the light resource. After optimization, the effect illuminated system could compare with the integrating sphere, meet the conditions of fiber position measurement.Using parallel controlled fiber positioner as the spectroscopic receiver is an efficiency observation system for spectra survey, has been used in LAMOST recently, and will be proposed in CFHT and rebuilt telescope Mayall. In the telescope observation, the position of fiber will highly influence the spectra efficient input in the fiber to the spectrograph. When the fibers were back illuminated on the spectra end, they would export light on the positioner end, so the CCD cameras could capture the photo of fiber tip position covered the focal plane, calculates the precise position information by light centroid method and feeds back to control system. After many years on these research, the back illuminated fiber measurement was the best method to acquire the precision position of fibers. In LAMOST, a set of fiber back illuminated system was developed which combined to the low revolution spectro instruments in LAMOST. It could provide uniform light output to the fibers, meet the requirements for the CCD camera measurement and was controlled by high-level observation system which could shut down during the telescope observation. The paper was introduced the back illuminated system design and different test for the light resource. After optimization, the effect illuminated system could compare the integrating sphere, meet the conditions of fiber position measurement.
A compact optical fiber positioner
In this paper, a compact optical fiber positioner is proposed, which is especially suitable for small scale and high density optical fiber positioning. Based on the positioning principle of double rotation, positioner’s center shaft depends on planetary gear drive principle, meshing with the fixed annular gear central motor gear driving device to rotate, and the eccentric shaft rotated driving by a coaxial eccentric motor, both center and the eccentric shaft are supported by a rolling bearings; center and eccentric shaft are both designed with electrical zero as a reference point, and both of them have position-limiting capability to ensure the safety of fiber positioning; both eccentric and center shaft are designed to eliminating clearance with spring structure, and can eliminate the influence of gear gap; both eccentric and center motor and their driving circuit can be installed in the positioner’s body, and a favorable heat sink have designed, the heat bring by positioning operation can be effectively transmit to design a focal plane unit through the aluminum component, on sleeve cooling spiral airway have designed, when positioning, the cooling air flow is inlet into install hole on the focal plate, the cooling air flow can effectively take away the positioning’s heat, to eliminate the impact of the focus seeing. By measuring position device’s sample results show that: the unit accuracy reached 0.01mm, can meet the needs of fiber positioning.
Research of subdivision driving technology for brushless DC motors in optical fiber positioning
Yi Kan, Yonggang Gu, Ye Zhu, et al.
In fiber spectroscopic telescopes, optical fiber positioning units are used to position thousands of fibers on the focal plane quickly and precisely. Stepper motors are used in existing units, however, it has some inherent deficiencies, such as serious heating and low efficiency. In this work, the universally adopted subdivision driving technology for stepper motors is transplanted to brushless DC motors. It keeps the advantages of stepper motors such as high positioning accuracy and resolution, while overcomes the disadvantages mentioned above. Thus, this research mainly focuses on develop a novel subdivision driving technology for brushless DC motor. By the proving of experiments of online debug and subdivision speed and position, the proposed brushless DC motor subdivision technology can achieve the expected functions.
Pupil Scrambling Integral Field Unit (PSI) for the Robert Stobie Spectrograph on SALT
We propose to build an integral field unit (IFU) for the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT). This IFU (PSI) will employ the scrambling properties of fibers to address fundamental problems in achieving photon-limited sky subtraction due to variations in pupil illumination during observations. PSI will be fully encapsulated with a compact folding scheme in a standard long slit mask—far thinner than any previous fiber-based implementation. The IFU will cover 14 x 24 arcsec on sky, achieving spectral resolution R ≡ λ/δλ up to 6200 and photon-limited sky subtraction for studies of faint extended gas around galaxies beyond the reach of current 4m-class instruments. It will incorporate new-technology octagonal shaped fiber cores coupled by telecentric, pupil-imaging lenslets to fully optimize pupil scrambling and provide 100% integral sky coverage.
Characterizing octagonal and rectangular fibers for MAROON-X
Adam P. Sutherland, Julian Stuermer, Katrina R. Miller, et al.
We report on the scrambling performance and focal-ratio-degradation (FRD) of various octagonal and rectangular fibers considered for MAROON-X. Our measurements demonstrate the detrimental effect of thin claddings on the FRD of octagonal and rectangular fibers and that stress induced at the connectors can further increase the FRD. We find that fibers with a thick, round cladding show low FRD. We further demonstrate that the scrambling behavior of non-circular fibers is often complex and introduce a new metric to fully capture non-linear scrambling performance, leading to much lower scrambling gain values than are typically reported in the literature (≤1000 compared to 10,000 or more). We find that scrambling gain measurements for small-core, non-circular fibers are often speckle dominated if the fiber is not agitated.
A new miniaturized fiber positioning node for LAMOST
Liang Guo, Yonggang Gu, Yuran Shen, et al.
To distribute more fiber positioning nodes on the LAMOST focal plate, two steps are proposed to miniaturize the fiber positioning node in this paper. The first step is to miniaturize the mechanical device of the fiber positioning node. The second step is to redesign the entire wireless driving board using smaller and performance-higher devices. As a result, the size of the new miniaturized fiber positioning node has to be reduced by above 40% and the dense of fiber positioning nodes on focal plate increases by 20% at least.
A new modular guidance system for the Southern African Large Telescope
Following successful commissioning of the SALT Fiber Instrument Feed guidance system, the concept was developed further to re-design the guidance probe currently supporting observations with the Robert Stobie Spectrograph. Major advances of the new system include a compact, modular and line-replaceable design, high optical efficiency, a doubleprobe positioning system allowing both translation and rotation guidance corrections as well as closed-loop focus feedback.

The mechanical and optical designs, the control system architecture and performance aspects of the system are presented. The probe‘s integration with the greater telescope software control system is also discussed.
Design, development, and performance of the fibres of MOONS
The Multi-Object Optical and Near-infrared Spectrograph (MOONS) will exploit the full 500 square arcmin field of view offered by the Nasmyth focus of the Very Large Telescope and will be equipped with two identical triple arm cryogenic spectrographs covering the wavelength range 0.64μm-1.8μm, with a multiplex capability of over 1000 fibres. Each spectrograph will produce spectra for 500 targets simultaneously, each with its own dedicated sky fibre for optimal sky subtraction. The system will have both a medium resolution (R~4000-6000) mode and a high resolution (R~20000) mode.

The fibres are used to pick off each sub field of 1" and are used to transport the light from the instrument focal plane to the two spectrographs. Each fibre has a microlens to focus the beam into the fibre at a relative fast focal ratio of F/3.65 to reduce the Focal Ratio Degradation (FRD).
PEPSI-feed: linking PEPSI to the Vatican Advanced Technology Telescope using a 450m long fibre
D. P. Sablowski, M. Weber, M. Woche, et al.
Limited observing time at large telescopes equipped with the most powerful spectrographs makes it almost impossible to gain long and well-sampled time-series observations. Ditto, high-time-resolution observations of bright targets with high signal-to-noise are rare. By pulling an optical fibre of 450m length from the Vatican Advanced Technology Telescope (VATT) to the Large Binocular Telescope (LBT) to connect the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) to the VATT, allows for ultra-high resolution time-series measurements of bright targets. This article presents the fibre-link in detail from the technical point-of-view, demonstrates its performance from first observations, and sketches current applications.
Design of multi-motor distributed control system for optical fibers positioning based on CAN bus
Tiancheng Li, Yonggang Gu, Liang Guo, et al.
In fiber spectroscopic telescopes, we propose a method of wire communication to control the fiber positioning nodes which are driven by two stepping motors. Taking the integration CAN control chip as main controller, the hardware of CAN intelligent node and adapter is designed to be the bottom hardware platform for the multi-motor distributed control system. The results proved that the system has good real-time performance and stability, and is able to realize not only CAN bus communication but also PC supervisory control, fundamentally meet the requirements of real-time precise position measurement of the optical fibers.
High numerical aperture multimode fibers for prime focus use
Kaiyuan Zhang, Jessica R. Zheng, Will Saunders
Modern telescopes typically have prime focus speeds too fast for direct use with standard numerical aperture (NA=0.22±0.02) silica-cored fibers. Specifically, the current design for the proposed Maunakea Spectroscopic Explorer (MSE) telescope is ~f/2, requiring fibers with NA>0.25. Micro foreoptics can be used to slow the beam, as used on the prime focus spectrograph (PFS) on Subaru, but this adds cost and complexity, and increases losses. An attractive alternative is offered by high NA pure silica-cored fibers, which can be used directly at f/2, and which are now available from multiple vendors. We present throughput and focal ratio degradation measurements on two samples of these high NA fibers. It is found that the measured attenuation losses are comparable with the best available standard NA fibers. The fibers were also tested for focal ratio degradation, and the fiber from CeramOptec was found to have acceptable FRD, representng additional collimator losses ~1%. The near field performance of the high NA fiber is also investigated and these high NA fibers exhibit very good scrambling performance; we saw no evidence for significant output near-field variations for varying input beam angles or position in a 50m fiber.
A 24mm diameter fibre positioner for spectroscopic surveys
Philipp Hörler, Luzius Kronig, Jean-Paul Kneib, et al.
One of the big research topics in modern cosmology is the mystery of dark Energy. To unveil the secret, cosmologists want to measure precisely the evolution of large scale structures in the universe. One way of doing so is to measure the 3D location of a high number of galaxies. By measuring the redshift of a galaxy, it is possible to find its distance. In order to measure a high number of galaxies in a practical amount of time, we need to observe multiple objects in parallel. Instead of a spectrograph, thousands of optical fibres are placed in the focal plane of a telescope. They will transmit the light of many objects to a spectrograph. Each fibre has to be positioned to several μm precision in the focal plane of a telescope for each exposure. Each fibre is positioned by a 2-axis fibre positioner. In this paper such a fibre positioner with 24-mm diameter is presented. It is driven by two brushless DC motors in combination with a backlash free gearbox. The positioner has an optimal central fibre path and improved angular alignment. The fibre runs through the centre of the positioner and is only bent at the top to reach its target position. In this way, the flexion and torsion of the fibre are minimal. In addition to the high positioning accuracy, the design is optimized to allow a minimal tilt error of the fibre. This is demonstrated using a novel optical tilt measurement system.
Poster Session: Technology for Spectroscopy
MEGARA: high-precision alignment system for gluing fibers and microlenses
MEGARA is the multi-object medium-resolution spectrograph for the GTC 10m telescope. MEGARA offers two observing modes, the LCB mode, a large central IFU; and a MOS mode composed by 92 robotic positioners carrying 7 fibers minibundles. Microlens are required to fit the GTC f/17 to the f/3 at the fiber entrance, where pupil image is oversized to have a fiber-to-fiber flux variation better than 10%. This tight requirement imposed manufacturing tolerances for the different components and required the development of a gluing station to provide a centering precision better than 5μm. We present the overview of the optical bundles, the gluing station and the final performance obtained during the integration and tests.
Shock and vibration testing of digital micromirror devices (DMDs) for space-based applications
Dmitry Vorobiev, Anton Travinsky, Alan D. Raisanen, et al.
Digital micromirror devices (DMDs) are a mature commercial technology, with several potential applications in space-based instruments. In particular, DMDs are currently the only practical alternative to microshutter arrays as slit mask generators for space-based multi-object spectrometers (MOS). A DMD is an array of micromirrors which can be addressed individually and tilted into one of two states (+/- 12 w.r.t. the device plane), which makes it a very versatile binary light modulator. These devices are widely utilized in a variety of optical systems, especially projectors. Recently, the use of DMDs for ground-based multi-object spectrometers has been demonstrated. The compact size and small weight of DMDs makes them especially attractive for a space- based MOS, where the only current alternative is an array of microshutters. DMDs were originally designed for visible range applications; therefore the protective glass window they are supplied with does not have sufficient throughput in the UV or IR and has to be replaced. In this work, we describe the procedure by which we replaced the standard window with UV-grade fused silica, sapphire and magnesium fluoride. We performed initial shock and vibrational tests to evaluate the mechanical robustness of the re-windowed devices, to investigate the ability of these devices to survive launch conditions. We performed residual gas analysis to study the outgassing properties of the new DMDs and evaluate the ability of the new seals to protect the device. The tested devices show near-hermetic seals before and after the mechanical testing.
Development of an optical device (Field Stacker) for achieving accurate photometry in ground-based mid-infrared observations
Strong time variation of atmospheric transmittance is a crucial problem for monitoring observations at mid- infrared wavelengths from the ground. To overcome this problem, a new device called "Field Stacker" has been developed. It is an optical device to combine two discrete fields in the telescope FoV into a single field and feed it in the camera. It enables us to observe a science target and a reference star simultaneously, and improve the photometric accuracy dramatically based on real-time calibration. To practically achieve highly accurate photometry, the tilt of the mirrors in the Field Stacker should be accurately adjusted. Acceptable error of the misalignment of each pick-up mirror is estimated to be < 0.0085 deg from a simple geometric calculation. The actual tilt error measured in the laboratory almost met this requirement. Spatial variation of the water vapor in the atmosphere is another concern for the accurate photometry. Assuming a simple model of the atmospheric structure, the spatial variation was estimated from time variation of infrared background radiation. The estimated variation of the water vapor was 0.00036 mm within the telescope FoV (Φ25 arc- minutes), suggesting that it does not significantly affect the photometric accuracy even at 31 and 37 μm. Number density of reference stars was examined based on all-sky infrared catalogues to estimate the availability of the Field Stacker. The estimated availabilities at 9 and 18 μm were 99.8% and 58.8%, respectively.
Multi-resolution waveguide image slicer for the PEPSI instrument
Erik Beckert, Klaus G. Strassmeier, Manfred Woche, et al.
A waveguide image slicer with resolutions up to 270.000 (planned: 300.000) for the fiber fed PEPSI echelle spectrograph at the LBT and single waveguide thicknesses of down to 70 μm has been manufactured and tested. The waveguides were macroscopically prepared, stacked up to an order of seven and thinned back to square stack cross sections. A high filling ratio was achieved by realizing homogenous adhesive gaps of 3.6 μm, using index matching adhesives for TIR within the waveguides. The image slicer stacks are used in immersion mode and are miniaturized to enable implementation in a set of 2x8. The overall efficiency is between 92 % and 96 %.
Opto-mechanical design of an image slicer for the GRIS spectrograph at GREGOR
N. Vega Reyes, M. A. Esteves, J. Sánchez-Capuchino, et al.
An image slicer has been proposed for the Integral Field Spectrograph [1] of the 4-m European Solar Telescope (EST) [2] The image slicer for EST is called MuSICa (Multi-Slit Image slicer based on collimator-Camera) [3] and it is a telecentric system with diffraction limited optical quality offering the possibility to obtain high resolution Integral Field Solar Spectroscopy or Spectro-polarimetry by coupling a polarimeter after the generated slit (or slits). Considering the technical complexity of the proposed Integral Field Unit (IFU), a prototype has been designed for the GRIS spectrograph at GREGOR telescope at Teide Observatory (Tenerife), composed by the optical elements of the image slicer itself, a scanning system (to cover a larger field of view with sequential adjacent measurements) and an appropriate re-imaging system. All these subsystems are placed in a bench, specially designed to facilitate their alignment, integration and verification, and their easy installation in front of the spectrograph. This communication describes the opto-mechanical solution adopted to upgrade GRIS while ensuring repeatability between the observational modes, IFU and long-slit. Results from several tests which have been performed to validate the opto-mechanical prototypes are also presented.
Performance estimates for spectrographs using photonic reformatters
Robert J. Harris, Lucas Labadie, Ulrike Lemke, et al.
Using a photonic reformatter to eliminate the effects of conventional modal noise could greatly improve the stability of a high resolution spectrograph. However the regimes where this advantage becomes clear are not yet defined. Here we will look at where modal noise becomes a problem in conventional high resolution spectroscopy and what impact photonic spectrographs could have. We will theoretically derive achievable radial velocity measurements to compare photonic instruments and conventional ones. We will discuss the theoretical and experimental investigations that will need to be undertaken to optimize and prove the photonic reformatting concept.
Collimating slicer for optical integral field spectroscopy
Florence Laurent, François Hénault
Integral Field Spectroscopy (IFS) is a technique that gives simultaneously the spectrum of each spatial sampling element of a given field. It is a powerful tool which rearranges the data cube represented by two spatial dimensions defining the field and the spectral decomposition (x, y, λ) in a detector plane. In IFS, the "spatial" unit reorganizes the field, the "spectral" unit is being composed of a classical spectrograph. For the spatial unit, three main techniques – microlens array, microlens array associated with fibres and image slicer – are used in astronomical instrumentations.

The development of a Collimating Slicer is to propose a new type of optical integral field spectroscopy which should be more compact. The main idea is to combine the image slicer with the collimator of the spectrograph mixing the "spatial" and "spectral" units. The traditional combination of slicer, pupil and slit elements and spectrograph collimator is replaced by a new one composed of a slicer and spectrograph collimator only. After testing few configurations, this new system looks very promising for low resolution spectrographs.

In this paper, the state of art of integral field spectroscopy using image slicers will be described. The new system based onto the development of a Collimating Slicer for optical integral field spectroscopy will be depicted. First system analysis results and future improvements will be discussed.
Spectral slicing for METIS: an efficient alternative to cross-dispersion
Image slicing integral field units were developed to provide spatially resolved spectroscopy over a two dimensional field of view. Spectral slicing applies similar design principles to provide an alternative to cross-dispersion. Key benefits include more efficient use of detector space and greater flexibility in selecting the wavelength ranges within each band. We will describe the design of a deployable spectral slicing mode as part of the METIS LM-band high resolution spectrometer.
A trial production of a large format image slicer unit for a possible future mid-infrared instrument on the TMT
Itsuki Sakon, Yuji Ikeda, Hiroyuki Nakagawa, et al.
We have carried out a trial production of the large-format (n=11) image slicer unit for a possible future mid-infrared instrument on the TMT aiming to verify its technical feasibility. The key elements in our trial production are the monolithic large-format slice mirrors and the monolithic large-format pupil mirrors. The results of our trial production of those key elements based on the ultra high-precision cutting techniques and the assembly of the large-format image slicer unit are presented in this paper.
Measurements of the reflectance, contrast ratio, and scattering properties of digital micromirror devices (DMDs)
Dmitry Vorobiev, Anton Travinsky, Manuel A. Quijada, et al.
Digital micromirror devices (DMDs) are micro-electro- mechanical systems, originally developed to display images in projector systems. A DMD in the focal plane of an imaging system can be used as a reprogrammable slit mask of a multi-object spectrometer (MOS) by tilting some of the mirrors towards the spectrometer and tilting the rest of the mirrors away, thereby rejecting the unwanted light (due to the background and foreground objects). A DMD-based MOS can generate new, arbitrary slit patterns in seconds, which significantly reduces the overhead time during astronomical observations. Critically, DMD-based slit masks are extremely lightweight, compact and mechanically robust, which makes them attractive for use in space-based telescopes. As part of a larger effort to investigate the use of DMDs in space telescopes (sponsored by a NASA Strategic Astrophysics Technologies grant), we characterized the optical performance of Texas Instruments DMDs to determine their suitability for use in multi-object spectrometers. The performance of a DMD-based MOS is significantly affected by its optical throughput (reflectance), contrast ratio (the ability of the DMD to reject unwanted light) and scattering properties (which could lead to crosstalk and reduced signal-to-noise ratio in the spectrometer). We measured and quantified the throughput and contrast ratio of a Texas Instruments DMD in several configurations (which emulate the operation of a typical DMD-based MOS) and investigated the scattering properties of the individual DMD mirrors. In this work we present the results of our analysis, describe the performance of a typical DMD- based MOS and discuss the practical limitations of these instruments (such as maximum density of sources and expected signal-to- noise ratio).
Optical evaluation of digital micromirror devices (DMDs) with UV-grade fused silica, sapphire, and magnesium fluoride windows and long-term reflectance of bare devices
Manuel A. Quijada, Anton Travinsky, Dmitry Vorobiev, et al.
Digital micromirror devices (DMDs) are commercial micro-electromechanical systems, consisting of millions of mirrors which can be individually addressed and tilted into one of two states (±12°). These devices were developed to create binary patterns in video projectors, in the visible range. Commercially available DMDs are hermetically sealed and extremely reliable. Recently, DMDs have been identified as an alternative to microshutter arrays for space-based multi-object spectrometers (MOS). Specifically, the MOS at the heart of the proposed Galactic Evolution Spectroscopic Explorer (GESE) uses the DMD as a reprogrammable slit mask. Unfortunately, the protective borosilicate windows limit the use of DMDs in the UV and IR regimes, where the glass has insufficient throughput. In this work, we present our efforts to replace standard DMD windows with custom windows made from UV-grade fused silica, low-absorption optical sapphire (LAOS) and magnesium fluoride (MgF2). We present transmission measurements of the antireflection coated windows and the reflectance of bare (window removed) DMDs. Furthermore, we investigated the long-term stability of the DMD reflectance and experiments for coating DMD active area with a layer of pure aluminum (Al) to boost reflectance performance in the UV spectral range (200−400 nm).
The effects of heavy ion radiation on digital micromirror device performance
Anton Travinsky, Dmitry Vorobiev, Zoran Ninkov, et al.
There is a pressing need in the astronomical community for space-suitable multi-object spectrometers (MOSs). Several digital micromirror device (DMD)-based prototype MOSs have been developed for ground-based observatories; however, their main use will come with deployment on a space based mission. Therefore, performance of DMDs under exoatmospheric radiation needs to be evaluated. In our previous work we demonstrated that DMDs are tolerant to heavy ion irradiation in general and calculated upset rate of 4.3 micromirrors in 24 hours in orbit for 1-megapixel device. The goal of this additional experiment was to acquire more data and therefore increase the accuracy of the predicted in-orbit micromirror upset rate. Similar to the previous experiment, for this testing 0.7 XGA DMDs were re-windowed with 2 μm thick pellicle and tested under accelerated heavy-ion radiation (with control electronics shielded from radiation) with a focus on detection of single-event upsets (SEUs). We concentrated on ions with low levels of linear energy transfer (LET) 1.8 – 13 MeV•cm2•mg-1 to cover the most critical range of the Weibull curve for those devices. As during the previous experiment, we observed and documented non-destructive heavy ion-induced micromirror state changes. All SEUs were always cleared with a soft reset (that is, sending a new pattern to the device). The DMDs we tested did not experience single-event induced permanent damage or functional changes that required a hard reset (power cycle), even at high ion fluences. Based on the data obtained in the experiments we predict micromirror in-orbit upset rate of 5.6 micromirrors in 24 hours in-orbit for the tested devices. This suggests that the heavy-ion induced SEU rate burden for a DMD-based instrument will be manageable when exposed to solar particle fluxes and cosmic rays in orbit.
On-sky performance evaluation and calibration of a polarization-sensitive focal plane array
Dmitry Vorobiev, Zoran Ninkov, Neal Brock, et al.
The advent of pixelated micropolarizer arrays (MPAs) has facilitated the development of polarization-sensitive focal plane arrays (FPAs) based on charge-coupled devices (CCDs) and active pixel sensors (APSs), which are otherwise only able to measure the intensity of light. Polarization sensors based on MPAs are extremely compact, light-weight, mechanically robust devices with no moving parts, capable of measuring the degree and angle of polarization of light in a single snapshot. Furthermore, micropolarizer arrays based on wire grid polarizers (so called micro-grid polarizers) offer extremely broadband performance, across the optical and infrared regimes. These devices have potential for a wide array of commercial and research applications, where measurements of polarization can provide critical information, but where conventional polarimeters could be practically implemented. To date, the most successful commercial applications of these devices are 4D Technology's PhaseCam laser interferometers and PolarCam imaging polarimeters. Recently, MPA-based polarimeters have been identified as a potential solution for space-based telescopes, where the small size, snapshot capability and low power consumption (offered by these devices) are extremely desirable. In this work, we investigated the performance of MPA-based polarimeters designed for astronomical polarimetry using the Rochester Institute of Technology Polarization Imaging Camera (RITPIC). We deployed RITPIC on the 0.9 meter SMARTS telescope at the Cerro Tololo Inter-American Observatory and observed a variety of astronomical objects (calibration stars, variable stars, reflection nebulae and planetary nebulae). We use our observations to develop calibration procedures that are unique to these devices and provide an estimate for polarimetric precision that is achievable.
Poster Session: Technologies for Cryogenic Instruments
How to achieve ultra-clean detectors and cryostats at astronomical instruments: measures to avoid contamination and dust on CCD detectors
ESO developed in its detector laboratory a complete routine to achieve ultra-clean detectors with lasting effect with special materials and surface treatments. All components of the detector cryostats are washed in ultrasonic baths, then baked to its maximum temperature in vacuum ovens. As final step plasma cleaning is used of individual and integrated systems. All handlings and the complete integrations are done in the clean room before its integration the detectors are dust cleaned with new methods e.g.: vapor cleaning. At observatory operation the detectors can be monitored by new methods (e.g.: pseudo FF dust evaluation, UV QE test) as a long term contamination control. The always unavoidable moisture in the ready installed instrument can even be cured by UV flashing in dry synthetic air without removing anything from the telescope. Such ESO provides ultra-clean detectors and instruments, which also do not degrade even after years of operation at their telescope sites.
Development of a cryogenic FTIR system for measuring very small attenuation coefficients of infrared materials
Sayumi Kaji, Yuki Sarugaku, Yuji Ikeda, et al.
We have been working on a long-term project for developing a variety of infrared immersion gratings for near- to mid-infrared wavelengths. The transmittance of material is essential to realize high-efficiency immersion gratings for astronomical applications. For a typical grating, the attenuation coefficient αatt must be <0.01 cm−1 for the absolute diffraction efficiency of >70%. However, as there are few reports of αatt < 0.01 cm−1 for infrared optical materials in the literatures, we performed high-accuracy measurements of αatt for a variety of infrared materials applicable to immersion gratings. We have already reported αatt at room temperature for single-crystal Si, single-crystal Ge, CVD-ZnS, CVDZnSe, and high-resistivity single-crystal CdZnTe (Ikeda et al. 2009[7], Kaji et al. 2014[10], and Sarugaku et al. 2016[9]). Next, we proceeded with the measurements of αatt at cryogenic temperatures of 20–80 K range, which is the typical operational temperatures of infrared instruments, and for which the shifts of the band gap and/or the sharpness of the lattice absorption lines from the corresponding room temperature values are expected. Thus, we developed a new cryogenic FTIR system that enables high-accuracy measurements at cryogenic temperatures. The system has a mechanism with which two sample cells and a reference cell can be easily and quickly switched without any vacuum leak or temperature change. Our preliminary measurement of Ge using this cryogenic FTIR system found that both the cut-on and cut-off wavelengths shift to the shorter (from 2.0 to 1.7 μm) and longer (from 10.6 to 10.9 μm) wavelengths, respectively, when the temperature is decreased from room temperature to the cryogenic temperature (<28 K). We plan to complete cryogenic measurements for a variety of infrared materials by the end of 2016.
Gluing interface qualification test results and gluing process ‎development of the EUCLID near-infrared spectro-photometer optical ‎assembly
A. Mottaghibonab, H. Thiele, E. Gubbini, et al.
The Near Infrared Spectro-Photometer Optical assembly (NIOA) of EUCLID satellite requires high precision large lens holders with different lens materials, shapes and diameters. The aspherical lenses are glued into their separate CTE matched lens holder. The gluing of the lenses in their holder with 2K epoxy is selected as bonding process to minimize the stress in the lenses to achieve the required surface form error (SFE) performance (32nm) and lens position stability (±10μm) due to glue shrinkage. Adhesive shrinkage stress occurs during the glue curing at room temperature and operation in cryogenic temperatures, which might overstress the lens, cause performance loss, lens breakage or failure of the gluing interface.

The selection of the suitable glue and required bonding parameters, design and qualification of the gluing interface, development and verification of the gluing process was a great challenge because of the low TRL and heritage of the bonding technology. The different material combinations (CaF2 to SS316L, LF5G15 and S-FTM16 to Titanium, SUPRASIL3001 to Invar M93), large diameter (168mm) and thin edge of the lenses, cryogenic nonoperational temperature (100K) and high performance accuracy of the lenses were the main design driver of the development. The different coefficients of thermal expansion (CTE) between lens and lens holder produce large local mechanical stress. As hygroscopic crystal calcium fluoride (CaF2) is very sensitive to moisture therefore an additional surface treatment of the gluing area is necessary.

Extensive tests e.g glue handling and single lap shear tests are performed to select the suitable adhesive. Interface connection tests are performed to verify the feasibility of selected design (double pad design), injection channel, the roughness and treatment of the metal and lens interfaces, glue thickness, glue pad diameter and the gluing process. CTE and dynamic measurements of the glue, thermal cycling, damp- heat, connection shear and tension tests with all material combinations at RT and 100K are carried out to qualify the gluing interface. The gluing interface of the glued lenses in their mounts is also qualified with thermal cycling, 3D coordinate measurements before and after environmental tests, Polarimetry and vibration test of the lens assemblies.

A multi-function double pad gluing tool and lens mounting tool is designed, manufactured and verified to meet the lens positioning and alignment performance of the lens in the holder which provides the possibility to glue lenses, filters, mirrors with different diameters, shapes and thickness with ±10μm accuracy in plane, out of plane and ±10 arcsec in tip/tilt with respect to the lens holder interface. The paper presents the glue interface qualification results, the qualification/verification methods, the developed ground support equipment and the gluing process of the EUCLID high precision large cryogenic lens mounts. Test results achieved in the test campaign demonstrate the suitability of the selected adhesive, glue pad design, interface parameters and the processes for the precise gluing of the lenses in lens holders for all lenses. The qualification models of the NIOA are successfully glued and qualified. The developed process can also be used for other glass materials e.g. MaF2 and optical black coated metallic surfaces.
Final design of the grism cryogenic mount for the Euclid-NISP mission
The grism mount design for the Euclid-NISP mission was studied to maintain optical performances and alignment at cryogenic temperature, and to survive to launch vibrations. An Invar mount with strong weight-relief bonded to the Silica grism through tangential blades has been designed. In spring 2015 we proceeded to thermal cycling and vibration tests to successfully qualify the Grism Engineering Model in the Euclid space environment. Thanks to detailed Finite Element analyses, we correlated simulations and tests. Now that phase C began, we are manufacturing the Engineering and Qualification Model and the four Flight Models. Thus, random coupled analyses of the grisms on the complete wheel assembly and impact of interface preloads on the grism behavior have also been studied.
CARMENES-NIR channel spectrograph cooling system AIV: thermo-mechanical performance of the instrument
S. Becerril, E. Mirabet, J. L. Lizon, et al.
CARMENES is the new high-resolution high-stability spectrograph built for the 3.5m telescope at the Calar Alto Observatory (CAHA, Almería, Spain) by a consortium formed by German and Spanish institutions. This instrument is composed by two separated spectrographs: VIS channel (550-1050 nm) and NIR channel (950- 1700 nm). The NIR-channel spectrograph's responsible is the Instituto de Astrofísica de Andalucía (IAACSIC). It has been manufactured, assembled, integrated and verified in the last two years, delivered in fall 2015 and commissioned in December 2015.

One of the most challenging systems in this cryogenic channel involves the Cooling System. Due to the highly demanding requirements applicable in terms of stability, this system arises as one of the core systems to provide outstanding stability to the channel. Really at the edge of the state-of-the-art, the Cooling System is able to provide to the cold mass (~1 Ton) better thermal stability than few hundredths of degree within 24 hours (goal: 0.01K/day).

The present paper describes the Assembly, Integration and Verification phase (AIV) of the CARMENES-NIR channel Cooling System implemented at IAA-CSIC and later installation at CAHA 3.5m Telescope, thus the most relevant highlights being shown in terms of thermal performance.

The CARMENES NIR-channel Cooling System has been implemented by the IAA-CSIC through very fruitful collaboration and involvement of the ESO (European Southern Observatory) cryo-vacuum department with Jean-Louis Lizon as its head and main collaborator. The present work sets an important trend in terms of cryogenic systems for future E-ELT (European Extremely Large Telescope) large-dimensioned instrumentation in astrophysics.
New cryogenic temperature monitor: PLT-HPT-32
Teodora Aleida Viera Curbelo, Sergio Gonzáles Martín-Fernández, R. Hoyland, et al.
The PLT-HPT-32, a new cryogenic temperature monitor, has been developed by the Institute of Astrophysics of the Canary Islands (IAC) and an external engineering company (Sergio González Martín-Fernandez). The PLT-HPT-32 temperature monitor offers precision measurement in a wide range of cryogenic and higher-temperature applications with the ability to easily monitor up to 32 sensor channels. It provides better measurement performance in applications where researchers need to ensure accuracy and precision in their low cryogenic temperature monitoring.

The PLT-HPT-32 supports PTC RTDs such as platinum sensors, and diodes such as the Lake Shore DT-670 Series. Used with silicon diodes, it provides accurate measurements in cryo-cooler applications from 16 K to above room temperature. The resolution of the measurement is less than 0.1K. Measurements can be displayed in voltage units or Kelvin units. For it, two different tables can be used. One can be programmed by the user, and the other one corresponds to Lake Shore DT670 sensor that comes standard.

There are two modes of measuring, the instantaneous mode and averaged mode. In this moment, all channels must work in the same mode but in the near future it expected to be used in blocks of eight channels. The instantaneous mode takes three seconds to read all channels. The averaged mode takes one minute to average twenty samples in all channels. Alarm thresholds can be configured independently for each input. The alarm events, come from the first eight channels, can activate the unit’s relay outputs for hard-wired triggering of other systems or audible annunciators. Activate relays on high, low, or both alarms for any input.

For local monitoring, "Stand-Alone Mode", the front panel of the PLT-HPT-32 features a bright liquid crystal display with an LED backlight that shows up to 32 readings simultaneously. Plus, monitoring can be done over a network "Remote Control Mode". Using the Ethernet port on the PLT-HPT-32, you can keep an eye on temperatures, log measurement and configured remotely via a Networked local PC or even remotely over a TCP/IP Internet connection from anywhere.
GMTIFS: cryogenic rotary mechanisms for the GMT Integral-Field Spectrograph
John Hart, Brady Espeland, Gabe Bloxham, et al.
A representative range of the rotary mechanisms proposed for use in GMTIFS is described. All are driven by cryogenically rated stepper motors. For each mechanism, angular position is measured by means of eddy current sensors arranged to function as a resolver. These measure the linear displacement of a decentered aluminum alloy target in two orthogonal directions, from which angular position is determined as a function of the displacement ratio. Resolver function and performance is described. For each mechanism, the mechanical design is described and the adequacy of positioning repeatability assessed. Options for improvement are discussed.
Developing a long duration 3He fridge for the LSPE-SWIPE instrument
Gabriele Coppi, Paolo de Bernardis, Andrew J. May, et al.
A 3He sorption cooler design for the Short-Wavelength Instrument for the Polarization Explorer (SWIPE) of the Large-Scale Polarization Explorer (LSPE) balloon-borne experiment is described. The aim of this experiment is the detection of the primordial B-mode polarisation component of the Cosmic Microwave Background. The SWIPE instrument will use Transition-Edge Sensors that are designed to work at temperature of almost 300 mK. Therefore, a 3He sorption cooler has been specifically designed that can reach this temperature with a heat load of up to 25 μW. The fridge is compact in order to be housed inside the SWIPE cryostat and operate vertically. It has been designed to have a cycle duration of at least 7 days. In order to meet these specifications, the fridge will be charged with 0.75 moles of 3He.
Sorption-cooled continuous miniature dilution refrigeration for astrophysical applications
Andrew J. May, Paolo G. Calisse, Gabriele Coppi, et al.
A progress report is provided on the development of a tiltable continuous miniature dilution refrigerator and associated 3He/4He sorption coolers. These systems are currently being developed to provide sub-Kelvin cooling of the bolometer arrays for several ground- and balloon-based experiments which aim to measure the polarization of the Cosmic Microwave Background (QUBIC, LSPE and POLARBEAR-2). The novel tiltable miniaturised system benefits from a lack of external circulation pumps and a mechanically simple design. The condenser of the twin-pumped recirculating diluter is cooled continuously by two 3He/4He sorption coolers. The sorption pumps are operated by convective heat switches. The dilution unit features a thermally separated mixing chamber, still and step heat exchangers. The designs and analyses of both the sorption coolers and the diluter are reported; both systems have been manufactured and are presently under test.
Poster Session: Active Instruments (Active Structures, Active Optics)
Smart warping harnesses for active mirrors and stress polishing
Sabri Lemared, Emmanuel Hugot, Zalpha Challita, et al.
We present two ways to generate or compensate for first order optical aberrations using smart warping harnesses. In these cases, we used the same methodology leading to replace a previous actuation system currently on-sky and to get a freeform mirror intended to a demonstrator. Starting from specifications, a warping harness is designed, followed by a meshing model in the finite elements software. For the two projects, two different ways of astigmatism generation are presented. The first one, on the VLT-SPHERE instrument, with a single actuator, is able to generate a nearly pure astigmatism via a rotating motorization. Two actuators are sufficient to produce the same aberration for the active freeform mirror, main part of the OPTICON-FAME project, in order to use stress-polishing method.
Control system for an alternative actuator for the primary surface of the Large Millimeter Telescope (LMT/GTM)
José Luis Hernández Rebollar, Josefina Lázaro Hernández, Cesar Arteaga Magaña, et al.
The Large Millimeter Telescope/Gran Telescopio Milimétrico (LMT/GTM) is a bi-national project between INAOE in México and UMASS in the USA. It is an open air radio telescope designed for astronomical observations in wavelengths ranging from 0.85 mm to 4 mm. Its 50-meter diameter primary reflector is so massive that its shape deviates from the theoretical parabola due to gravitational effect as it moves in elevation, which ultimately affects gain, one of the most important features of the telescope. To correct this elevation-dependent deformation, the primary surface is divided into 180 segments that are automatically positioned by means of four electro mechanical actuators. Unfortunately, the lifetime expectancy of the interim actuators installed in rings 1 to 3 during 2013, are below specs and the cost of substituting them with the new actuators, now under development for rings 4 and 5, may not be affordable. In this paper an alternative actuator control system that re uses most of the current electronics coupled to a completely redesigned mechanism is presented. The results of the performance tests under load show that the system is capable of achieving positioning with RMS error of 4 micron and that the accuracy is dominated by the LVDT characterization error.
Thermal expansion as a precision actuator
The UK ATC has developed a novel thermal actuator design as part of an OPTICON project focusing on the development of a Freeform Active Mirror Element (FAME). The actuator uses the well understood concept of thermal expansion to generate the required force and displacement. As heat is applied to the actuator material it expands linearly. A resistance temperature device (RTD) is embedded in the centre of the actuator and is used both as a heater and a sensor. The RTD temperature is controlled electronically by injecting a varying amount of current into the device whilst measuring the voltage across it. Temperature control of the RTD has been achieved to within 0.01°C.

A 3D printed version of the actuator is currently being used at the ATC to deform a mirror but it has several advantages that may make it suitable to other applications. The actuator is cheap to produce whilst obtaining a high accuracy and repeatability. The actuator design would be suitable for applications requiring large numbers of actuators with high precision.
Realization and testing of an active mirror mechanism for in-field pointing in eLISA
An active tilt mirror mechanism, meant for correction of the constellation breathing of the evolved Laser Interferometer Space Antenna, has been designed and realized. Its open-loop performance has been characterized in both time and frequency domain. Based on this, a feedback controller has been designed and the resulting closed-loop performance has been assessed. Up to what is measurable in a normal lab environment, these experiments demonstrate compliance with the extreme pointing jitter requirement, even when using the internal encoder as feedback sensor.
Focal plane actuation by hexapod for the development of a high-resolution suborbital telescope
We present a prototype hexapod image stabilization system as the key instrument for a proposed suborbital balloon mission. The unique design thermally isolates an off-the-shelf non-cryogenic hexapod from a liquid nitrogen cooled focal plane, enabling its use in a cryogenic environment. Balloon gondolas currently achieve 1-2 arcsecond pointing error, but cannot correct for unavoidable jitter movements (~20 micron amplitude at 20 Hz at the worst) caused by wind rushing over balloon surfaces, thermal variations, and vibrations from cryocoolers, and reaction wheels. The jitter causes image blur during exposures and limits the resolution of the system. Removal of this final jitter term decreases pointing error by an order of magnitude and allows for true diffraction-limited observation. Tip-tilt pointing systems have been used for these purposes in the past, but require additional optics and introduce multiple reflections. The hexapod system, rather, is compact and can be plugged into the focal point of nearly any configuration. For a 0.8m telescope the improvement in resolution by this system would provide 0.1” angular resolution at 300nm, which is comparable to Hubble for a fraction of the cost. On an actual balloon, the hexapod system would actuate the focal plane to counteract the jitter using position information supplied by guidestar cameras. However, in the lab, we instead simulate guide camera tracking, using a 1024 × 1024 e2v science-grade CCD to take long exposures of a target attached to an XY stage driven with the balloon jitter signal recorded during the STO mission. Further confirmation of the positional accuracy and agility of the hexapod is achieved using a laser and fast-sampling position-sensitive diode. High-resolution time domain multispectral imaging of the gas giants, especially in the UV range, is of particular interest to the planetary community, and a suborbital telescope with the hexapod stabilization in place would provide a wealth of new data. On an Antarctic ~100-day Long-Duration-Balloon (LDB) mission the continued high-resolution imaging of gas giant storm systems would provide cloud formation and evolution data second to only a Flagship orbiter.
Mechanical development of an alternative set of actuators for the LMT/GTM primary surface outer rings: also useful to replace the interim actuators
César Arteaga-Magaña, José Luis Hernández-Rebollar, Josefina Lazaro-Hernandez, et al.
The Large Millimeter Telescope/Gran Telescopio Milimétrico (LMT/GTM) is the world’s largest, single dish radio telescope for observations in millimeter wavelengths. 180 segments arranged in 5 rings form the reflector’s active surface. Each segment supported by four linear actuators. The current interim actuators fill the 3 inner rings only, while allowing the completion of the rest of the surface and the installation of the final actuators. A new modification had, to be made in order to reduce the actuators size for the outer rings and also to improve their performance. The project needs to install at least another 336 actuators for the 2 outer rings of segments. However, the room for those actuators has reduced room underneath the outer rings.

Initially, the present development was intended as alternative for the antenna’s outer rings, but as time went by, we discovered the advantage of installing them as replacements of the current interim actuators, since a system of final actuators for the antenna’s outer rings is already under test and construction. Hence, this new mechanical design of compact geometry is not only capable of fitting in the reduced space, but also of replacing the interim actuators in the inner rings.
The WEAVE focus translation system: from design to construction
Manuel Canchado, Antonio Romero, Óscar Maroto, et al.
WEAVE is a new wide-field spectroscopy facility proposed for the prime focus of the 4.2m William Herschel Telescope (WHT), placed in La Palma, Canary Islands, Spain.

To allow for the compensation of the effects of temperature-induced and gravity-induced image degradation, the WEAVE prime focus assembly will be translated along the telescope optical axis. The assembly comprises the prime focus corrector with integrated ADC, a central mount for the corrector, an instrument rotator and a twin-focal-plane fibre positioner. Translation is accomplished through the use of a set of purpose-built actuators; collectively referred to as the Focus Translation System (FTS), formed by four independently-controlled Focus Translation Units (FTUs), eight vanes connecting the FTUs to a central can, and a central can hosting WEAVE Instrument. Each FTU is capable of providing a maximum stroke of ±4mm with sufficient, combined force to move the five-tonne assembly with a positional accuracy of ±20μm at a resolution of 5μm. The coordinated movement of the four FTUs allows ±3mm WEAVE focus adjustment in the optical axis and ±0.015° tilt correction in one axis. The control of the FTS is accomplished through a PLC-based subsystem that receives positional demands from the higher-level Instrument Control System.

SENER has been responsible for designing, manufacturing and testing the FTS and the equipment required to manipulate and store the FTS together with the instrument.

This manuscript describes the final design of the FTS along with the analyses and simulations that were performed, discusses the manufacturing procedures and the results of early verification prior to integration with the telescope. The plans for mounting the whole system on the telescope are also discussed.
Active optics system for the 4m telescope of the Eastern Anatolia Observatory (DAG)
Gregory P. Lousberg, Emeric Mudry, Christian Bastin, et al.
An active optics system is being developed by AMOS for the new 4m-class telescope for the Turkish Eastern Anatolia Observatory (DAG). It consists in (a) an adjustable support for the primary mirror and (b) two hexapods supporting M2 and M3. The M1 axial support consists of 66 pneumatic actuators (for mirror shape corrections) associated with 9 hydraulic actuators that are arranged in three independent circuits so as to fix the axial position of the mirror. Both M1 support and the hexapods are actively controlled during regular telescope operations, either with look-up tables (openloop control) or using optical feedback from a wavefront sensor (closed-loop control).
Poster Session: Coronography and High Contrast Imaging
A three-layer eight-octant phase mask towards broadband high-contrast observations
Naoshi Murakami, Jun Nishikawa, Natsumi Akaiwa, et al.
We designed and fabricated an achromatic eight-octant phase mask (8OPM) for broadband coronagraphic observations of exoplanets. The fabricated 8OPM is composed of three-layer eight-octant half-wave plates based on photonic crystals. By using Jones calculus, it is shown that the three-layer 8OPM achieves much higher contrast over broad wavelength range than that of the previous single-layer design. We carry out preliminary laboratory experiments of the coronagraph using the fabricated three-layer 8OPM. As a model star, we use several visible laser light sources for characterizing the coronagraphic performance. As a result, we obtain higher contrasts than theoretical ones of the single-layer 8OPM. However, the achieved contrasts are lower than the theoretical values of the three-layer one. At present we suspect that manufacturing errors of the half-wave plates in the 8OPM limit the achieved contrasts.
Advances in starshade technology readiness for an exoplanet characterizing science mission in the 2020's
David Webb, Brian Hirsch, Case Bradford, et al.
The discovery of thousands of exoplanets is generating increasing interest in the direct imaging and characterization of these planets. Starshade, an external occulter, could fly in formation between a telescope and distant star, blocking out the light from the star, and enabling us to focus on the light of any orbiting planets. Recent technology developments in coordination with system level design, has added much needed detail to define the technology requirements for a science mission that could launch in the 2020’s. This paper addresses the mechanical architecture, the successful efforts to date, the current state of design for the mechanical system, and upcoming technology efforts.
Development of speckle nulling technique for the Savart-plate lateral-shearing interferometric nuller for exoplanets (SPLINE)
Kenta Yoneta, Naoshi Murakami, Takayuki Kotani, et al.
The Savart-Plate Lateral-shearing Interferometric Nuller for Exoplanets (SPLINE) is a kind of a visible nulling coronagraph for directly detecting exoplanets. The SPLINE consists of two crossed polarizers and a Savart plate placed between them. Theoretically the SPLINE realizes perfect cancellation of starlight. However, achievable contrast is limited by residual stellar speckles due to wavefront aberration caused by imperfect optical surfaces of the optical elements. For reducing the residual stellar speckles of the SPLINE, we propose a speckle nulling technique using a Liquid-Crystal Spatial Light Modulator (LCSLM) to create a dark hole. For the speckle nulling, we apply the Self-Coherent Camera (SCC) technique to the SPLINE for wavefront sensing in the focal plane. We report our recent progress on computer simulation and preliminary laboratory experiments of the speckle nulling technique applied to the SPLINE.
Development and characterization of Four-Quadrant Phase Mask coronagraph (FQPM)
M. Bonafous, R. Galicher, P. Baudoz, et al.
The goal of a coronagraph is to reduce the flux of a bright object (e.g. a star) in order to distinguish its faint neighborhood (e.g. exoplanets and disks). In this context, we proposed one coronagraph that uses a four quadrant phase mask (FQPM). Since 2000, we fabricated several monochromatic FQPM working in visible and near-infrared light at the Paris Observatory. We have developed systematic procedures for fabrication and characterization of the phase masks. Visual inspections with an optical microscope are performed for every component and a coronagraphic performance measurement based on inclination of the component is done on a dedicated bench that is set up in a clean room. This procedure gives a quick feedback on the quality and performance of the component. Depending on the results, images of the central transition can be recorded with an electron microscope to understand the limitations of the fabrication process. This procedure allowed us to understand the influence of various parameters such as the width of the transitions between the quadrants, the alignment of the transitions or the step depth. Based on these results, we modified the mask design and the fabrication process to improve our success rate to nearly 100% when building a FQPM for any given optimal wavelength in visible or near-infrared. Moreover, we improved the performance of the components, reaching attenuations of more than 20,000 on the central peak in raw images for most coronagraphs. The best of these components are now used on the THD bench, an optical/NIR bench developed for the study of high contrast imaging techniques, reaching 10-8 contrast level routinely.
Analysis of nulling phase functions suitable to image plane coronagraphy
François Hénault, Alexis Carlotti, Christophe Vérinaud
Coronagraphy is a very efficient technique for identifying and characterizing extra-solar planets orbiting in the habitable zone of their parent star, especially in a space environment. An important family of coronagraphs is actually based on phase plates located at an intermediate image plane of the optical system, and spreading the starlight outside the "Lyot" exit pupil plane of the instrument. In this commutation we present a set of candidate phase functions generating a central null at the Lyot plane, and study how it propagates to the image plane of the coronagraph. These functions include linear azimuthal phase ramps (the well-known optical vortex), azimuthally cosine-modulated phase profiles, and circular phase gratings. Nnumerical simulations of the expected null depth, inner working angle, sensitivity to pointing errors, effect of central obscuration located at the pupil or image planes, and effective throughput including image mask and Lyot stop transmissions are presented and discussed. The preliminary conclusion is that azimuthal cosine functions appear as an interesting alternative to the classical optical vortex of integer topological charge.
Development of PIAA Complex Mask Coronagraphs for large aperture ground-based telescopes
Kevin Newman, Dan Sirbu, Ruslan Belikov, et al.
The Phase Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC) is an architecture for directly observing extra-solar planets, and can achieve performance near the theoretical limits for any direct-detection instrument. The PIAACMC architecture includes aspheric PIAA optics, and a complex phase-shifting focal plane mask that provides a pi phase shift to a portion of the on-axis starlight. The phase-shifted starlight is forced to interfere destructively with the un-shifted starlight, causing the starlight to be eliminated, and allowing a region for high-contrast imaging near the star.

The PIAACMC architecture can be designed for segmented and obscured apertures, so it is particularly well suited for ground-based observing with the next generation of large telescopes. There will be unique scientific opportunities for directly observing Earth-like planets around nearby low-mass stars. We will discuss design strategies for adapting PIAACMC for the next generation of large ground-based telescopes, and present progress on the development of the focal plane mask technology. We also present simulations of wave-front control with PIAACMC, and suggest directions to apply the coronagraph architecture to future telescopes.
WFIRST/AFTA coronagraph contrast performance sensitivity studies: simulation versus experiment
The WFIRST/AFTA 2.4 m space telescope currently under study includes a stellar coronagraph for the imaging and the spectral characterization of extrasolar planets. The coronagraph employs sequential deformable mirrors to compensate for phase and amplitude errors. Using the optical model of an Occulting Mask Coronagraph (OMC) testbed at the Jet Propulsion Laboratory, we have investigated through modeling and simulations the sensitivity of dark hole contrast in a Hybrid Lyot Coronagraph (HLC) for several error cases, including lateral and longitudinal translation errors of two deformable mirrors, DM1 and DM2, lateral and/or longitudinal translation errors of an occulting mask and a Lyot-Stop, clocking errors of DM1 and DM2, and the mismatch errors between the testbed and the model sensitivity matrices. We also investigated the effects of a control parameter, namely the actuator regularization factor, on the control efficiency and on the final contrast floor. We found several error cases which yield contrast results comparable to that observed on the HLC testbed. We present our findings in this paper.
Design and construction of a 76m long-travel laser enclosure for a space occulter testbed
Michael Galvin, Yunjong Kim, N. Jeremy Kasdin, et al.
Princeton University is upgrading our space occulter testbed. In particular, we are lengthening it to ~76m to achieve flightlike Fresnel numbers. This much longer testbed required an all-new enclosure design. In this design, we prioritized modularity and the use of commercial off-the-shelf (COTS) and semi-COTS components. Several of the technical challenges encountered included an unexpected slow beam drift and black paint selection. Herein we describe the design and construction of this long-travel laser enclosure.
Poster Session: Atmospheric Compensation and Polarimetry
CHOUGH: petite ADC for a high-order adaptive optics system
Daniel Hölck, Nazim Ali Bharmal, Martin Black, et al.
We discuss the design of a 50mm diameter Atmospheric Dispersion Corrector (ADC) for The CANARY-Hosted Upgrade for High-Order Adaptive Optics (CHOUGH). Usually to avoid pupil actuator-lenslet array mismatch, the ADC is Customarily placed very close to the pupil plane. This design aims to achieve a non-pupil conjugated ADC suitable to be located in any place inside the collimated beam path, this is due to the restrictions given by CHOUGH optical relay. The ADC also needs to satisfy the very small pupil shift requirement, for pupil stability. The ADC is of the Amici prism type, made up of two plates of cemented double prisms. The two plates counter rotate correcting for the different Zenith angles, from the Zenith up to 60°.
Daytime sky polarization calibration limitations
The daytime sky has been recently demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K Inouye Solar Telescope (DKIST) and other large telescopes under construction can benefit from precise polarimetric calibration of large off-axis mirrors. Several atmospheric phenomena and instrumental errors potentially limit the techniques accuracy. At the 3.67m AEOS telescope on Haleakala, we have performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting consistent calibrations. Effective sampling of the telescope optical configurations and filtering of data for several derived parameters provide robustness to the derivedMueller matrix calibrations. Second-order scattering models of the sky show that this method is relatively insensitive to assumptions about telescope induced polarization provided the mirror coatings are highly reflective. Zemax-derived polarization models show agreement between predictions and on-sky calibrations.
Progress in modeling polarization optical components for the Daniel K. Inouye Solar Telescope
The DKIST will have a suite of first-light polarimetric instrumentation requiring precise calibration of a complex articulated optical path. The optics are subject to large thermal loads caused by the ~300Watts of collected solar irradiance across the 5 arc minute field of view. The calibration process requires stable optics to generate known polarization states. We present modeling of several optical, thermal and mechanical effects of the calibration optics, the first transmissive optical elements in the light path, because they absorb substantial heat. Previous studies showed significant angle of incidence effects from the f/13 converging beam and the 5 arc minute field of view, but were only modeled at a single nominal temperature. New thermal and polarization modeling of these calibration retarders shows heating causes significant stability limitations both in time and with field caused by the bulk temperature rise along with depth and radial thermal gradients. Modeling efforts include varying coating and material absorption, Mueller matrix stability estimates and mitigation efforts.
Polarization modeling and predictions for DKIST part 1: telescope and example instrument configurations
We outline polarization performance calculations and predictions for the Daniel K. Inouye Solar Telescope (DKIST) optics and show Mueller matrices for two of the first light instruments. Telescope polarization is due to polarization dependent mirror reflectivity and rotations between groups of mirrors as the telescope moves in altitude and azimuth. The Zemax optical modeling software has polarization ray-trace capabilities and predicts system performance given a coating prescription. We develop a model coating formula that approximates measured witness sample polarization properties. Estimates show the DKIST telescope Mueller matrix as functions of wavelength, azimuth, elevation, and field angle for the Cryogenic Near Infra-Red Spectro-Polarimeter and for the Visible SpectroPolarimeter (ViSP). Footprint variation is substantial and shows vignetted field points will have strong polarization effects. We estimate 2% variation of some Mueller matrix elements over the 5 arc minute CryoNIRSP field. We validate the Zemax model by show limiting cases for flat mirrors in collimated and powered designs that compare well with theoretical approximations and are testable with lab ellipsometers.
An efficient stable optical polariser module for calibration of the S4UVN earth observation satellite
Stephen Rolt, Ariadna Calcines, Bartosz Lomanowski, et al.
We describe here an optical polariser module intended to deliver well characterised polarised light to an imaging spectrometer instrument. The instrument in question is the Sentinel-4/UVN Earth observation imaging spectrometer due to be deployed in 2019 in a geostationary orbit. The polariser module described here will be used in the ground based calibration campaign for this instrument. One critical task of the calibration campaign will be the highly accurate characterisation of the polarisation sensitivity of instrument. The polariser module provides a constant, uniform source of linearly polarised light whose direction can be adjusted without changing the output level or uniformity of the illumination.

A critical requirement of the polariser module is that the illumination is uniform across the exit pupil. Unfortunately, a conventional Glan-Taylor arrangement cannot provide this uniformity due to the strong variation in transmission at a refractive surface for angles close to the critical angle. Therefore a modified prism arrangement is proposed and this is described in detail. Detailed tolerance modelling and straylight modelling is also reported here.
Poster Session: Revolutionary Technologies
Noise modeling and analysis of an IMU-based attitude sensor: improvement of performance by filtering and sensor fusion
Nirmal K., Sreejith A. G., Joice Mathew, et al.
We describe the characterization and removal of noises present in the Inertial Measurement Unit (IMU) MPU- 6050, which was initially used in an attitude sensor, and later used in the development of a pointing system for small balloon-borne astronomical payloads. We found that the performance of the IMU degraded with time because of the accumulation of different errors. Using Allan variance analysis method, we identified the different components of noise present in the IMU, and verified the results by the power spectral density analysis (PSD). We tried to remove the high-frequency noise using smooth filters such as moving average filter and then Savitzky Golay (SG) filter. Even though we managed to filter some high-frequency noise, these filters performance wasn't satisfactory for our application. We found the distribution of the random noise present in IMU using probability density analysis and identified that the noise in our IMU was white Gaussian in nature. Hence, we used a Kalman filter to remove the noise and which gave us good performance real time.
Photocontrolled deformable mirrors as potential technology for astronomical instrumentation
Martino Quintavalla, Stefano Bonora, Dario Natali, et al.
Deformable mirrors are key optical elements in modern astronomical telescopes and instrumentation both for active and adaptive optical systems. Different technological approaches have been exploited for the realization of the deformable mirrors, especially for adaptive optics devices. A new approach is here presented, namely the photo-controlled deformable mirrors, where the size and density of actuators is set by an illumination pattern projected on the back side of a photoconductor. The working principle and an electric model are presented highlighting the features of the material that affect the performances of the mirror in terms of dynamic range and response time. Based on these results, a prototype exploiting ZnSe as photoconductor is reported together with its characterization.
Low cost photonic comb for sub-m/s wavelength calibration
Christopher H. Betters, Maxime Hermouet, Thomas Blanc, et al.
A fundamental limitation of precision radial velocity measurements is the accuracy and stability of the calibration source. Here we present a low-cost alternative to more complex laser metrology based systems that utilises a single-mode fibre Fabry-Perot etalon. There are three key elements on this photonic comb: i) an optical fibre etalon with thermo-electric coolers; ii) a Rubidium Saturation Absorption Spectroscopy (SAS) setup; and iii) an optical fibre switch system for simultaneous laser locking of the etalon. We simultaneously measure the Rubidium D2 transitions around 780.2 nm and the closest etalon line. A PID loop controls the etalon temperate to maintain the position of its peak with an RMS error of <10cm/s for 10 minute integration intervals in continous operation. The optical fibre switch system allows for a time multiplexed coupling of the etalon to a spectrograph and SAS system.
An astro-comb calibrated solar telescope to search for the radial velocity signature of Venus
David F. Phillips, Alex G. Glenday, Xavier Dumusque, et al.
We recently demonstrated sub-m/s sensitivity in measuring the radial velocity (RV) between the Earth and Sun using a simple solar telescope feeding the HARPS-N spectrograph at the Italian National Telescope, which is calibrated with a green astro-comb. We are using the solar telescope to characterize the effects of stellar (solar) RV jitter due to activity on the solar surface with the goal of detecting the solar RV signal from Venus, thereby demonstrating the sensitivity of these instruments to detect true Earth-twin exoplanets.
NASA's Physics of the Cosmos and Cosmic Origins programs manage Strategic Astrophysics Technology (SAT) development
Thai Pham, Harley Thronson, Bernard Seery, et al.
The strategic astrophysics missions of the coming decades will help answer the questions "How did our universe begin and evolve?" "How did galaxies, stars, and planets come to be?" and "Are we alone?" Enabling these missions requires advances in key technologies far beyond the current state of the art. NASA’s Physics of the Cosmos2 (PCOS), Cosmic Origins3 (COR), and Exoplanet Exploration Program4 (ExEP) Program Offices manage technology maturation projects funded through the Strategic Astrophysics Technology (SAT) program to accomplish such advances. The PCOS and COR Program Offices, residing at the NASA Goddard Space Flight Center (GSFC), were established in 2011, and serve as the implementation arm for the Astrophysics Division at NASA Headquarters. We present an overview of the Programs' technology development activities and the current technology investment portfolio of 23 technology advancements. We discuss the process for addressing community-provided technology gaps and Technology Management Board (TMB)-vetted prioritization and investment recommendations that inform the SAT program. The process improves the transparency and relevance of our technology investments, provides the community a voice in the process, and promotes targeted external technology investments by defining needs and identifying customers. The Programs’ priorities are driven by strategic direction from the Astrophysics Division, which is informed by the National Research Council’s (NRC) "New Worlds, New Horizons in Astronomy and Astrophysics" (NWNH) 2010 Decadal Survey report [1], the Astrophysics Implementation Plan (AIP) [2] as updated, and the Astrophysics Roadmap “Enduring Quests, Daring Visions” [3]. These priorities include technology development for missions to study dark energy, gravitational waves, X-ray and inflation probe science, and large far-infrared (IR) and ultraviolet (UV)/optical/IR telescopes to conduct imaging and spectroscopy studies. The SAT program is the Astrophysics Division’s main investment method to mature technologies that will be identified by study teams set up to inform the 2020 Decadal Survey process on several large astrophysics mission concepts.
Development of high-resolution arrayed waveguide grating spectrometers for astronomical applications: first results
Pradip Gatkine, Sylvain Veilleux, Yiwen Hu, et al.
Astrophotonics is the next-generation approach that provides the means to miniaturize near-infrared (NIR) spectrometers for upcoming large telescopes and make them more robust and inexpensive. The target requirements for our spectrograph are: a resolving power of 3000, wide spectral range (J and H bands), free spectral range of about 30 nm, high on-chip throughput of about 80% (-1dB) and low crosstalk (high contrast ratio) between adjacent on-chip wavelength channels of less than 1% (-20 dB). A promising photonic technology to achieve these requirements is Arrayed Waveguide Gratings (AWGs). We have developed our first generation of AWG devices using a silica-on-silicon substrate with a very thin layer of Si3N4 in the core of our waveguides. The waveguide bending losses are minimized by optimizing the geometry of the waveguides. Our first generation of AWG devices are designed for H band have a resolving power of ~1500 and free spectral range of ~10 nm around a central wavelength of 1600 nm. The devices have a footprint of only 12 mm × 6 mm. They are broadband (1450-1650 nm), have a peak on-chip throughput of about 80% (~-1 dB) and contrast ratio of about 1.5% (-18 dB). These results confirm the robustness of our design, fabrication and simulation methods. Currently, the devices are designed for Transverse Electric (TE) polarization and all the results are for TE mode. We are developing separate J- and H-band AWGs with higher resolving power, higher throughput and lower crosstalk over a wider free spectral range to make them better suited for astronomical applications.
Auxiliary free space optical communication project to ensure continuous transfer of data for DAG the 4m telescope
The continuity of the amount of data that the 4m DAG (Eastern Anatolia Observatory in Turkish) telescope will produce and transfer to Ataturk University is critical not to jeopardize the science programs. Though¬ the fiber optics and radio link infrastructures are in place, these systems are still volatile against earthquakes, and possible excavation damages. Thus the 4m DAG telescope will be equipped with a free space optical communication system to ensure the continuity of the data transfer as a backup system. In order to cope with the disturbances introduced by the atmospheric turbulence, the transceiver FSO system will be equipped with a wavefront corrector. In this paper, the Cassegrain optical design, and working principle of this system as well as expected performance analyses will be presented.
Adaptive optics fed single-mode spectrograph for high-precision Doppler measurements in the near-infrared
We present the design for a high resolution near-infrared spectrograph. It is fed by a single-mode fiber coupled to a high performance adaptive optics system, leading to an extremely stable instrument with high total efficiency. The optical design is a cross-dispersed Echelle spectrograph based on a white pupil layout. The instrument uses a R6 Echelle grating with 13.3 grooves per mm, enabling very high resolution with a small beam diameter. The optical design is diffraction limited to enable optimal performance; this leads to subtle differences compared to spectrographs with large input slits.
Back-propagating the light of field stars to probe telescope mirrors aberrations
F. Soulez, F. Courbin, M. Unser
We propose a wavefront-based method to estimate the PSF over the whole field of view. This method estimate the aberrations of all the mirrors of the telescope using only field stars. In this proof of concept paper, we described the method and present some qualitative results.
Optimization of high sensitivity parametric transducers for the Gravitational Wave Detector "Mario Schenberg"
V. Liccardo, E. C. Ferreira, O. D. Aguiar, et al.
Parametric transducers may be used to monitor vibrational modes of resonant-mass Gravitational Wave (GW) detectors. This work focuses on the development and optimization of electromechanical transducers for the Brazilian GW detector "Mario Schenberg". We present the design and preliminary results for the optimization process of the last generation of the Schenberg parametric transduction system. Thanks to the results obtained from the last run of tests improvements for the transduction system will be planned and performed, in order to make the detector suitable for the searching of GW signals. The aim would be to obtain information about the direction and polarization of waves produced by astrophysical sources in the frequency bands of the spherical detector resonant modes.
The reconnaissance and early-warning optical system design for dual field of space-based "solar blind ultraviolet"
Wen-cong Wang, Dong-dong Jin, Fei Shao, et al.
With the development of modern technology, especially the development of information technology at high speed, the ultraviolet early warning system plays an increasingly important role. In the modern warfare, how to detect the threats earlier, prevent and reduce the attack of precision-guided missile has become a new challenge. Because the ultraviolet warning technology has high environmental adaptability, the low false alarm rate, small volume and other advantages, in the military field applications it has been developed rapidly. According to current application demands for solar blind ultraviolet detection and warning, this paper proposes a reconnaissance and early-warning optical system, which covers solar blind ultraviolet (250nm-280nm) and dual field. This structure takes advantage of a narrow field of view and long focal length optical system to achieve the target object detection, uses wide-field and short focal length optical system to achieve early warning of the target object. It makes use of an ultraviolet beam-splitter to achieve the separation of two optical systems. According to the detector and the corresponding application needs of two visual field of the optical system, the calculation and optical system design were completed. After the design, the MTF of the two optical system is more than 0.8@39lp/mm. A single pixel energy concentration is greater than 80%.