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- Front Matter: Volume 9151
- Telescope Structures
- Active Instruments
- Cryogenic Techniques
- Mirror Materials
- Mirror Finishing
- Test and Metrology
- Test and Metrology of Large Optics
- System Test and Alignment
- Novel Technologies
- Coatings
- Gratings I
- Gratings II
- High Contrast Imaging
- Spectroscopy
- Optical Fibers and Positioners I
- Optical Fibers and Positioners II
- Poster Session
Front Matter: Volume 9151
Front Matter: Volume 9151
Show abstract
This PDF file contains the front matter associated with SPIE Proceedings Volume 9151 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.
Telescope Structures
Mechanical design of SST-GATE, a dual-mirror telescope for the Cherenkov Telescope Array
Show abstract
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 over a wide band of energy, from a few tens of GeV
to more than 100 TeV. Two sites are foreseen to view the whole sky where about 100 telescopes, composed of three
different classes, related to the specific energy region to be investigated, will be installed. Among these, the Small Size
class of Telescopes, SSTs, are devoted to the highest energy region, to beyond 100 TeV. Due to the large number of
SSTs, their unit cost is an important parameter.
At the Observatoire de Paris, we have designed a prototype of a Small Size Telescope named SST-GATE, based on the
dual-mirror Schwarzschild-Couder optical formula, which has never before been implemented in the design of a
telescope. Over the last two years, we developed a mechanical design for SST-GATE from the optical and preliminary
mechanical designs made by the University of Durham. The integration of this telescope is currently in progress.
Since the early stages of mechanical design of SST-GATE, finite element method has been used employing shape and
topology optimization techniques to help design several elements of the telescope. This allowed optimization of the
mechanical stiffness/mass ratio, leading to a lightweight and less expensive mechanical structure. These techniques and
the resulting mechanical design are detailed in this paper. We will also describe the finite element analyses carried out to
calculate the mechanical deformations and the stresses in the structure under observing and survival conditions.
Active optics system of the ASTRI SST-2M prototype for the Cherenkov Telescope Array
Show abstract
ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) SST-2M is an end-to-end prototype of Small Size
class of Telescope for the Cherenkov Telescope Array. It will apply a dual mirror configuration to Imaging Atmospheric
Cherenkov Telescopes. The 18 segments composing the primary mirror (diameter 4.3 m) are equipped with an active
optics system enabling optical re-alignment during telescope slew. The secondary mirror (diameter 1.8 m) can be moved
along three degrees of freedom to perform focus and tilt corrections. We describe the kinematic model used to predict the
system performance as well as the hardware and software design solution that will be implemented for optics control.
Unimorph-type deformable mirror for cryogenic telescopes
Show abstract
Deformable mirrors can be used in cryogenic instruments to compensate for temperature-induced deformations. A
unimorph-type deformable mirror consists of a mirror substrate and a piezoelectric layer bonded on substrates rear
surface. A challenge in the design of the deformable mirror is the lack of knowledge about material properties.
Therefore, we measured the coefficient of thermal expansion (CTE) of the substrate material TiAl6V4 between 295 K
and 86 K. The manufactured mirror is characterized by an adaptive optical measurement setup in front of a test cryostat.
The measured mirror deformations are feedback into a finite element model to calculate the CTE of the piezoelectric
layer. We compare our obtained results to other published CTE-values for the piezoelectric material PIC151.
Design, fabrication and testing of active carbon shell mirrors for space telescope applications
Show abstract
A novel active mirror concept based on carbon fiber reinforced polymer (CFRP) materials is presented. A nanolaminate facesheet, active piezoelectric layer and printed electronics are implemented in order to provide the reflective surface, actuation capabilities and electrical wiring for the mirror. Mirrors of this design are extremely thin (500-850 µm), lightweight (~ 2 kg/m2) and have large actuation capabilities (~ 100 µm peak- to-valley deformation per channel). Replication techniques along with simple bonding/transferring processes are implemented eliminating the need for grinding and polishing steps. An outline of the overall design, component materials and fabrication processes is presented. A method to size the active layer for a given mirror design, along with simulation predictions on the correction capabilities of the mirror are also outlined. A custom metrology system used to capture the highly deformable nature of the mirrors is demonstrated along with preliminary prototype measurements.
Co-phasing of segmented telescopes using dual wavelength digital holography
Show abstract
An approach for the online co-phasing of segmented telescopes using dual wavelength digital holography has been
proposed. In this approach, two digital interferograms, one fringe pattern for one wavelength, which are acquired by a
high speed CCD camera using a Mach-Zehnder point diffraction interferometer, are required. By these two fringes for
two different wavelengths, the phase of a synthetic wavelength for the segmented telescope can be extracted using the
digital holography technique. To overcome errors caused by atmosphere and measurement noises, the piston and tip/tilt
coefficients for each segment of the telescope are acquired by fitting the plane from the phase of the synthetic
wavelength in each segment of the telescope. The performance of the method is tested by co-phasing of a simulated
telescope with 37 hexagonal segments with the existence of turbulence and noise. The numerical experimental results
show that co-phasing of the segmented telescope with high accuracy can be achieved by dual wavelength digital
holography.
Active Instruments
FAME: Freeform Active Mirrors Experiment
Show abstract
This paper discusses the development of a demonstrator freeform active mirror for future
astronomical instruments both on Earth and in space. It consists of a system overview and progress
in various areas of technology in the building blocks of the mirror: an extreme freeform thin face
sheet, an active array, design tools and the metrology and control of the system. The demonstrator
aims to investigate the applicability of the technique in high end astronomical systems, also for space
and cryogenically.
An approach to fabrication of large adaptive optics mirrors
Show abstract
For more than two decades, Northrop Grumman Xinetics has been the principal supplier of small deformable
mirrors that enable adaptive optical (AO) systems for the ground-based astronomical telescope community. With
today’s drive toward extremely large aperture systems, and the desire of telescope designers to include adaptive
optics in the main optical path of the telescope, Xinetics has recognized the need for large active mirrors with the
requisite bandwidth and actuator stoke. Presented in this paper is the proposed use of Northrop Grumman Xinetics’
large, ultra-lightweight Silicon Carbide substrates with surface parallel actuation of sufficient spatial density and
bandwidth to meet the requirements of tomorrow’s AO systems, while reducing complexity and cost.
Design and performances of JPCam actuator system
Joan Manel Casalta,
Manuel Canchado,
Albert Molins,
et al.
Show abstract
JPCam is designed to perform the Javalambre-PAU Astrophysical Survey (J-PAS), a photometric survey of the northern
sky with the new JST telescope being constructed in the Observatorio Astrofísico of Javalambre in Spain by CEFCA
(Centro de Estudios de Física del Cosmos de Aragón).
SENER has been responsible for the design, manufacturing, verification and delivery of the JPCam Actuator System
that will be installed between the Telescope and the cryogenic Camera Subsystem. The main function is to control the
instrument position to guarantee the image quality required during observations in all field of view and compensate
deformations produced by gravity and temperature changes.
The paper summarizes the main aspects of the hexapod design and earliest information related of integration and
performances tests results.
Interface Region Imaging Spectrograph (IRIS) entrance aperture design
Show abstract
The Interface Region Imaging Spectrograph (IRIS) is a complementary follow-on to Solar Dynamics Observatory
Atmospheric Imaging Assembly (SDO-AIA) and funded as a member of the NASA SMEX program. This paper
presents the thermal design of the IRIS telescope front end, with a focus on the IRIS door and entrance aperture
assembly. The challenge of the IRIS entrance aperture, including the door design, was to manage the solar flux, both
before and after the door was opened. This is especially a problem with instruments that are permanently pointed directly
at the sun. Though there is an array of effective flux-rejecting coatings, they are expensive, hard to apply, harder to
measure, delicate, prone to unpredictable performance decay with exposure, and very often a source of contamination.
This paper presents a thermal control and protection method based on robust, inexpensive coatings and materials,
combined to produce high thermal and structural isolation. The end result is a first line of thermal protection whose
performance is easy to predict and well isolated from the instrument it is protecting.
Design and performance of a cryogenic iris aperture mechanism
Show abstract
A cryogenic iris mechanism is under development as part of the ground calibration source for the SAFARI instrument.
The iris mechanism is a variable aperture used as an optical shutter to fine-tune and modulate the absolute power output
of the calibration source. It has 4 stainless steel blades that create a near-circular aperture in every position. The
operating temperature is 4.5 Kelvin to provide a negligible background to the SAFARI detectors, and ‘hot spots’ above
9K should be prevented. Cryogenic testing proved that the iris works at 4K. It can be used in a broad range of cryogenic
optical instruments where optical throughput needs to be controlled.
Challenges in the design include the low cooling power available (5mW) and low friction at cryogenic temperatures. The
actuator is an ‘arc-type’ rotary voice-coil motor. The use of flexural pivots creates a mono-stable mechanism with a
resonance frequency at 26Hz. Accurate and fast position control with disturbance rejection is managed by a PID servo
loop using a hall-sensor as input. At 4 Kelvin, the frequency is limited to 4Hz to avoid excess dissipation and heating.
In this paper, the design and performance of the iris are discussed. The design was optimized using a thermal, magnetic
and mechanical model made with COMSOL Finite Element Analysis software. The dynamical and state-space modeling
of the mechanism and the concept of the electrical control are presented. The performance of the iris show good
agreement to the analytical and COMSOL modeling.
Cryogenic Techniques
The adaptive optics beam steering mirror for the GMT Integral-Field Spectrograph, GMTIFS
Show abstract
To achieve the high adaptive optics sky coverage necessary to allow the GMT Integral-Field Spectrograph to
access key scientific targets, the on-instrument adaptive-optics wavefront-sensing system must patrol the full 180
arcsecond diameter guide field passed to the instrument. Starlight must be held stationary on the wavefront
sensor (accounting for flexure, differential refraction and non-sidereal tracking rates) to ~ 1 milliarcsecond to
provide the stable position reference signal for deep AO observations and avoid introducing image blur. Hence a
tight tolerance of 1/180,000 is placed on the positioning and encoding accuracy for the cryogenic On-Instrument
Wave-Front Sensor feed. GMTIFS will achieve this requirement using a beam-steering mirror system as an
optical relay for starlight from across the accessible guide field. The system avoids hysteresis and backlash by
eliminating friction and avoiding gearing while maintaining high setting speed and accuracy with a precision
feedback loop. Here we present the design of the relay system and the technical solution deployed to meet the
challenging specifications for drive rate, accuracy and positional encoding of the beam-steering system.
Development and characterization of a 2D precision cryogenic chopper for METIS
Show abstract
The Mid-infrared E-ELT Imager and Spectrograph, or METIS, is foreseen as an early instrument for the European
Extremely Large Telescope (E-ELT). A key part of METIS is the Cold Chopper (MCC) which switches the optical beam
between the target and a nearby reference sky during observation for characterization of the fluctuating IR background
signal in post-processing. This paper discusses the development and characterization of the realized MCC demonstrator.
The chopper mirror (Ø64mm) should tip/tilt in 2D with a combined angle of up to 13.6mrad with 1.7μrad stability and
repeatability within 5ms (95% duty cycle at 5Hz) at 80K. As these requirements cannot be met in the presence of friction
or backlash, the mirror is guided by a monolithically integrated flexure mechanism. The angular position is actuated by
three linear actuators and measured by three linear position sensors, resulting in a fast tip, tilt, and focus mirror. Using
the third actuator introduces symmetry, and thus homogeneity in forces and heat flux.
In an earlier paper, Ref. [1], the design of the chopper and the breadboard level testing of the key components were
discussed. Since then, the chopper design has been revised to implement the lessons learned from the breadboard test and
a demonstrator has been realized. This demonstrator has undergone an elaborate test program for characterization and
performance validation in a cryogenic environment, as discussed in this paper.
Development and performance validation of a cryogenic linear stage for SPICA-SAFARI verification
Show abstract
In the context of the SAFARI instrument (SpicA FAR-infrared Instrument) SRON is developing a test environment to
verify the SAFARI performance. The characterization of the detector focal plane will be performed with a backilluminated
pinhole over a reimaged SAFARI focal plane by an XYZ scanning mechanism that consists of three linear
stages stacked together. In order to reduce background radiation that can couple into the high sensitivity cryogenic
detectors (goal NEP of 2•10-19 W/√Hz and saturation power of few femtoWatts) the scanner is mounted inside the
cryostat in the 4K environment. The required readout accuracy is 3 μm and reproducibility of 1 μm along the total travel
of 32 mm. The stage will be operated in “on the fly” mode to prevent vibrations of the scanner mechanism and will
move with a constant speed varying from 60 μm/s to 400 μm/s. In order to meet the requirements of large stroke, low
dissipation (low friction) and high accuracy a DC motor plus spindle stage solution has been chosen.
In this paper we will present the stage design and stage characterization, describing also the measurements setup. The
room temperature performance has been measured with a 3D measuring machine cross calibrated with a laser
interferometer and a 2-axis tilt sensor. The low temperature verification has been performed in a wet 4K cryostat using a
laser interferometer for measuring the linear displacements and a theodolite for measuring the angular displacements.
The angular displacements can be calibrated with a precision of 4 arcsec and the position could be determined with high
accuracy. The presence of friction caused higher values of torque than predicted and consequently higher dissipation.
The thermal model of the stage has also been verified at 4K.
Realization and performance of cryogenic selection mechanisms
Show abstract
Within Infra-Red large wavelength bandwidth instruments the use of mechanisms for selection of observation modes,
filters, dispersing elements, pinholes or slits is inevitable. The cryogenic operating environment poses several challenges
to these cryogenic mechanisms; like differential thermal shrinkage, physical property change of materials, limited use of
lubrication, high feature density, limited space etc.
MATISSE the mid-infrared interferometric spectrograph and imager for ESO's VLT interferometer (VLTI) at Paranal in
Chile coherently combines the light from 4 telescopes. Within the Cold Optics Bench (COB) of MATISSE two concepts
of selection mechanisms can be distinguished based on the same design principles: linear selection mechanisms (sliders)
and rotating selection mechanisms (wheels).Both sliders and wheels are used at a temperature of 38 Kelvin.
The selection mechanisms have to provide high accuracy and repeatability. The sliders/wheels have integrated tracks
that run on small, accurately located, spring loaded precision bearings. Special indents are used for selection of the
slider/wheel position. For maximum accuracy/repeatability the guiding/selection system is separated from the actuation
in this case a cryogenic actuator inside the cryostat.
The paper discusses the detailed design of the mechanisms and the final realization for the MATISSE COB. Limited
lifetime and performance tests determine accuracy, warm and cold and the reliability/wear during life of the instrument.
The test results and further improvements to the mechanisms are discussed.
Mirror Materials
ZERODUR iso-grid design of a 3m class light weighted mirror blank for the E-ELT M5
Show abstract
The tip and tilt M5 mirror of the European Extremly Large Telescope (E-ELT) requires a demanding approach in light
weighting. The approximately 3 m x 2.5 m elliptical plano mirror is specified to a weight of less than 500 kg with high
Eigenfrequencies and low deformation under different inclination angles.
In 2011 SCHOTT has presented a study to develop a design for the M5 mirror blank of the ESO E-ELT. The design
presented was based on a radial square design to achieve the best compromise between performance and
manufacturability. With the fabrication of a prototype section SCHOTT demonstrated its capability to manufacture the
demanding features including pockets with 350 mm depth, thin walls and sloped pocket bottoms.
Now 3 years later SCHOTT presents an iso-grid based design that is in accordance with the manufacturability progress
that has been demonstrated in various ELZM (Extremely Lightweighted ZERODUR Mirrors) publications in the last two
years. The achievements on the specified mechanical parameters are compared to the first approach from 2011. In this
paper the results are presented and the performance parameters are discussed.
Material characteristics of CLEARCERAM-Z HS 1.5m diameter mirror blanks for the TMT M1 segment blanks
Show abstract
Within the Astronomical community there is growing interest in the development and use of very large aperture telescopes which will incorporate the latest advancements in optical materials. One of the most notable of these large size telescope projects, the Thirty Meter Telescope (TMT), will use mirror segments in the actively controlled primary mirrors and Ohara is a supplier of the near zero expansion mirror blanks. In this paper we present the measurement and inspection results on the initial 60 pieces of Ohara CLEARCERAM-Z HS 1.5m diameter blanks, which will be used for the TMT M1 Segment Blanks, including data on the Coefficient of Thermal Expansion (CTE), CTE uniformity, residual stress, bubbles and inclusions.
Production of primary mirror segments for the Giant Magellan Telescope
Show abstract
Segment production for the Giant Magellan Telescope is well underway, with the off-axis Segment 1 completed, off-axis
Segments 2 and 3 already cast, and mold construction in progress for the casting of Segment 4, the center segment. All
equipment and techniques required for segment fabrication and testing have been demonstrated in the manufacture of
Segment 1. The equipment includes a 28 m test tower that incorporates four independent measurements of the segment's
figure and geometry. The interferometric test uses a large asymmetric null corrector with three elements including a 3.75
m spherical mirror and a computer-generated hologram. For independent verification of the large-scale segment shape,
we use a scanning pentaprism test that exploits the natural geometry of the telescope to focus collimated light to a point.
The Software Configurable Optical Test System, loosely based on the Hartmann test, measures slope errors to submicroradian
accuracy at high resolution over the full aperture. An enhanced laser tracker system guides the figuring
through grinding and initial polishing. All measurements agree within the expected uncertainties, including three
independent measurements of radius of curvature that agree within 0.3 mm. Segment 1 was polished using a 1.2 m
stressed lap for smoothing and large-scale figuring, and a set of smaller passive rigid-conformal laps on an orbital
polisher for deterministic small-scale figuring. For the remaining segments, the Mirror Lab is building a smaller, orbital
stressed lap to combine the smoothing capability with deterministic figuring.
Recent development of fabrication of extreme lightweighted ceramic mirrors
Show abstract
This paper will present the recent development achievements of a German SME supply chain to manufacture super lightweighted
HB-Cesic® mirrors for IR to visible applications. We will present recent design developments for achieving
extreme light-weighted mirror substrates with extremely high stiffness and performance and in the second part the newly
established German supply chain for the manufacturing of such extreme light-weighted mirror substrates.
Space invader asteroids: detect and deter with silicon space based lasers
Show abstract
Current strategies for detecting and deflecting/destroying small to medium size asteroids that may possibly enter earth’s atmosphere are in their infancy. Projects such as Sentinel for detection and DE-STAR for deflecting of asteroids are gaining momentum while others are in design stages.
Single crystal silicon (SC-Si) with its unique set of mechanical and thermal properties is eminently suitable for deployment in any and all of these applications. But the superior thermal properties are particularly suited for high energy applications. We show that the exceptionally high thermal conductivity and diffusivity combined with relatively low thermal expansion enable SC-Si mirrors to handle the energy, often without active cooling, while maintaining figure. Both cooled and uncooled SC-Si mirrors have been employed successfully in high energy laser applications.
For IR detection of asteroids, SC-Si lenses, prisms and filters are invaluable in the 1-5 μm range along with SC-Si mirrors at any wavelength. This space qualified material has zero defects and is readily available in a range of sizes to 300 mm diameter from several vendors, but while boules to 460 mm have limited availability, they can be obtained. For applications requiring larger sizes, McCarter’s proprietary glass frit bonding is utilized. Components approaching one meter have been produced.
Mirror Finishing
Ultra-precise manufacturing of aspherical and freeform mirrors for high resolution telescopes
Show abstract
Optical freeforms are increasingly gaining interest for optical systems like telescopes and spectrometers. This is a key topic of discussions for many years; however, the manufacturing process of freeform optics remains a challenging task whose complexity derives from the missing symmetry in freeform surfaces.
Ultra-precise manufacturing with diamond tools is an appropriate method to realize optical freeforms. Aspherical off axis mirrors machined similar to freeform or classical freeform mirrors like anamorphic mirrors can be fabricated in a deterministic process by using reference structures and correction loops. Diamond machining offers an excellent technology to meet the requirements regarding small values of surface deviation and low tolerances of position accuracy. Nevertheless, the typical micro-roughness of approximately 5 nm rms and the periodic turning structure set the limitation for diamond machined surfaces. The surfaces fulfill requirements for application in the Near Infrared (NIR) and Infrared (IR) spectral ranges, respectively. For smoothing the periodic structure, the diamond turning is combined with post polishing techniques like MRF (Magnetorheological Finishing) or computer assisted polishing. Therefore, the aluminum mirror has to be coated with amorphous nickel-phosphorous or silicon. Thus, the specification of applications in the visible (VIS) spectral range is reached. This process chain is interesting for a growing number of multi- and hyperspectral imaging devices such as telescopes and spectrometers based on all reflective metal optics.
The paper summarizes the fabrication of an optical bench for a high resolution IR telescope, discusses the results of post polishing mirrors for VIS telescopes, and shows an efficient and easy snap-together alignment strategy. The optical function of the TMA demonstrator built is an afocal imaging for a Limb-Sounder Instrument with a magnification of 4.5:1. Besides the design and manufacturing approach, the snap-together integration of the optical bench is presented, too. The presentation is finished with a forecast of a freeform IR telescope based on anamorphic mirrors.
Ion figuring of large prototype mirror segments for the E-ELT
Show abstract
At INAF-Astronomical Observatory of Brera a study is under way to explore the problems related to the ion beam
figuring of full scale Zerodur hexagonal mirrors of M1 for the European Extremely Large Telescope (E-ELT), having
size of 1.4 m corner to corner. During this study it is initially foreseen the figuring of a scaled down version mirror of the
same material having size of 1 m corner to corner to assess the relevant figuring problems and issues. This specific
mirror has a radius of curvature of 3 m, which allows for easy interferometric measurement. A mechanical support was
designed to minimize its deformations due to the gravity. The Ion Beam Figuring Facility used for this study has been
recently completed in the Brera Observatory and has a figuring area of 140 cm x 170 cm. It employs a Kaufman ion
source having 50 mm grids mounted on three axis. This system has been designed and developed to be autonomous and
self-monitoring during the figuring process. The software and the mathematical tools used to compute the dwell time
solution have been developed at INAF-OAB as well. Aim of this study is the estimation and optimization of the time
requested to correct the surface adopting strategies to mitigate the well-known thermal problems related to the Zerodur
material. In this paper, the results obtained figuring the 1 m corner-to-corner test segment are reported.
Test and Metrology
Lightweighted ZERODUR for telescopes
Show abstract
The glass ceramic ZERODUR® from SCHOTT has an excellent reputation as mirror blank material for
earthbound and space telescope applications. It is known for its extremely low coefficient of thermal expansion
(CTE) at room temperature and its excellent CTE homogeneity. Recent improvements in CNC machining at
SCHOTT allow achieving extremely light weighted substrates up to 90% incorporating very thin ribs and face
sheets. In 2012 new ZERODUR® grades EXPANSION CLASS 0 SPECIAL and EXTREME have been released
that offer the tightest CTE grades ever. With ZERODUR® TAILORED it is even possible to offer ZERODUR®
optimized for customer application temperature profiles. In 2013 SCHOTT started the development of a new
dilatometer setup with the target to drive the industrial standard of high accuracy thermal expansion metrology
to its limit. In recent years SCHOTT published several paper on improved bending strength of ZERODUR® and
lifetime evaluation based on threshold values derived from 3 parameter Weibull distribution fitted to a multitude
of stress data. ZERODUR® has been and is still being successfully used as mirror substrates for a large number
of space missions. ZERODUR® was used for the secondary mirror in HST and for the Wolter mirrors in
CHANDRA without any reported degradation of the optical image quality during the lifetime of the missions.
Some years ago early studies on the compaction effects of electron radiation on ZERODUR® were re analyzed.
Using a more relevant physical model based on a simplified bimetallic equation the expected deformation of
samples exposed in laboratory and space could be predicted in a much more accurate way. The relevant
ingredients for light weighted mirror substrates are discussed in this paper: substrate material with excellent
homogeneity in its properties, sufficient bending strengths, space radiation hardness and CNC machining
capabilities.
Phase measuring deflectometry for thin actuated mirrors
Show abstract
In the development of thin active mirrors for future x-ray telescopes there is a need for full field, non-null methods
of rapidly characterising highly distorted surfaces without contact. Phase measuring deflectometry, due to its
high dynamic range and flexibility, is a promising solution to this problem. In this paper is described a system
developed by the authors at University College London, as well as the results of surface measurements using this
methodology on thin sheets of actuated glass.
Deflectometry for optics evaluation: free form segments of polynomial mirror
Show abstract
Deflectometry is a well-known method for astronomical mirror metrology. This paper describes the method we developed for the characterization of free-form concave mirrors. Our technique is based on the synergy between deflectometry and ray-tracing. The deflectometrical test is performed by illuminating the reflecting surface with a known light pattern in a Ronchi – like configuration and retrieving the slope errors by the observed rays deflection. The ray-tracing code allows us to measure the slopes and to evaluate the mirror optical performance. This technique has two main advantages: it is fast and it is applicable on-site, as an intermediate step in the manufacturing process, preventing that out-of-specification mirrors may proceed towards further production steps. Thus, we obtain a considerable time and cost reduction.
As an example, we describe the results obtained measuring the primary mirror segments of the Cherenkov prototypal telescope manufactured by the Italian National Institute for Astrophysics in the context of the ASTRI Project. This specific case is challenging because the segmentation of the polynomial primary mirror lead to individual mirrors with deviations from the spherical optical design up to a few millimeters.
Test and Metrology of Large Optics
Final acceptance testing of the LSST monolithic primary/tertiary mirror
Show abstract
The Large Synoptic Survey Telescope (LSST) is a three-mirror wide-field survey telescope with the primary and tertiary
mirrors on one monolithic substrate1. This substrate is made of Ohara E6 borosilicate glass in a honeycomb sandwich,
spin cast at the Steward Observatory Mirror Lab at The University of Arizona2. Each surface is aspheric, with the
specification in terms of conic constant error, maximum active bending forces and finally a structure function
specification on the residual errors3. There are high-order deformation terms, but with no tolerance, any error is
considered as a surface error and is included in the structure function. The radii of curvature are very different, requiring
two independent test stations, each with instantaneous phase-shifting interferometers with null correctors. The primary
null corrector is a standard two-element Offner null lens. The tertiary null corrector is a phase-etched computer-generated
hologram (CGH). This paper details the two optical systems and their tolerances, showing that the uncertainty
in measuring the figure is a small fraction of the structure function specification. Additional metrology includes the radii
of curvature, optical axis locations, and relative surface tilts. The methods for measuring these will also be described
along with their tolerances.
Design of an E-ELT M1 segment measurement machine with nanometer accuracy
Show abstract
The baseline design of the European Extremely Large Telescope features a telescope with a 39-meter-class
primary mirror (M1), consisting of 798 hexagonal segments. A measurement machine design is presented based
on a non-contact single-point scanning technique, capable of measuring the form error of each segment with
nanometer uncertainty, fast, and with low operational costs. The implementation of a tactile precision probe
eliminates the need for the CMM in the earlier segment manufacturing process. Preliminary assessment show
nanometer-level uncertainty after calibration.
Metrology of flat mirrors with a computer generated hologram
Show abstract
We designed the interferometric test of a 300 mm flat mirror, based onto a spherical mirror and a dedicated CGH. The spherical beam of the interferometer is quasi collimated to the desired diameter by the spherical mirror, used slightly off-axis, and the CGH performs the residual wavefront correction. We performed tests on a 200 mm and 300 mm flat mirrors, and compared the results to the ones obtained by stitching, showing an accuracy well within the designed value. The possibility to calibrate the cavity by subtracting out the figure errors of the spherical mirror has also been evaluated.
Measuring large mirrors using SCOTS: the Software Configurable Optical Test System
Show abstract
Large telescope mirrors are typically measured using interferometry, which can achieve measurement accuracy of a few nanometers. However, applications of interferometry can be limited by small dynamic range, sensitivity to environment, and high cost. We have developed a range of surface measurement solutions using SCOTS, the Software Configurable Optical Test System, which illuminates the surface under test with light modulated from a digital display or moving source. The reflected light is captured and used to determine the surface slope which is integrated to provide the shape. A range of systems is presented that measures nearly all spatial scales and supports all phases of processing for large telescope mirrors.
Polishers around the globe: an overview on the market of large astronomical mirrors
Thorsten Döhring
Show abstract
Astronomical mirrors are key elements in modern optical telescopes, their dimensions are usually large and their
specifications are demanding. Only a limited number of skilled companies respectively institutions around the world are
able to master the challenge to polish an individual astronomical mirror, especially in dimensions above one meter. This
paper presents an overview on the corresponding market including a listing of polishers around the globe. Therefore
valuable information is provided to the astronomical community: Polishers may use the information as a global
competitor database, astronomers and project managers may get more transparency on potential suppliers, and suppliers
of polishing equipment may learn about unknown potential customers in other parts of the world. An evaluation of the
historical market demand on large monolithic astronomical mirrors is presented. It concluded that this is still a niche
market with a typical mean rate of 1-2 mirrors per year. Polishing of such mirrors is an enabling technology with impact
on the development of technical know-how, public relation, visibility and reputation of the supplier. Within a
corresponding technical discussion different polishing technologies are described. In addition it is demonstrated that
strategic aspects and political considerations are influencing the selection of the optical finisher.
System Test and Alignment
Design, fabrication, integration and commissioning of an upgraded guiding probe for the VLT unit telescope 4
Christoph Frank,
Peter Hammersley,
Bernard Buzzoni,
et al.
Show abstract
As part of the preparation for the arrival of the MUSE instrument to the VLT, it was required to adapt the hosting
telescope (UT4) guide probe, to increase its back focal length. This is to allow enough space for the later deployment of
the MUSE Adaptive Optics module GALACSI, in-between the telescope adapter rotator and the instrument itself. The
UT guide probe is a critical component for the successful operation of the telescope, so its modification to increase the
telescope’s back focal length, while maintaining full compatibility with the existing operation model and other hardware,
was rather demanding.
The design, manufacture, assembly and test for the new supporting arm in the UT guiding probe is presented. It mixes
the use of novel materials (HB-CESIC® for the mirrors substrates) and state of the art manufacturing techniques (3D
printing mould production and rapid casting for the support structure), which allow producing easily a high performance
subsystem. Characterization of the system prior delivery to the telescope, its integration in the UT and results after
commissioning is presented. Its successful implementation has validated new manufacturing techniques that may prove
very useful for future instruments development.
MUSE alignment onto VLT
Show abstract
MUSE (Multi Unit Spectroscopic Explorer) is a second generation Very Large Telescope (VLT) integral field spectrograph developed for the European Southern Observatory (ESO). It combines a 1’ x 1’ field of view sampled at 0.2 arcsec for its Wide Field Mode (WFM) and a 7.5"x7.5" field of view for its Narrow Field Mode (NFM). Both modes will operate with the improved spatial resolution provided by GALACSI (Ground Atmospheric Layer Adaptive Optics for Spectroscopic Imaging), that will use the VLT deformable secondary mirror and 4 Laser Guide Stars (LGS) foreseen in 2015. MUSE operates in the visible wavelength range (0.465-0.93 μm). A consortium of seven institutes is currently commissioning MUSE in the Very Large Telescope for the Preliminary Acceptance in Chile, scheduled for September, 2014.
MUSE is composed of several subsystems which are under the responsibility of each institute. The Fore Optics derotates and anamorphoses the image at the focal plane. A Splitting and Relay Optics feed the 24 identical Integral Field Units (IFU), that are mounted within a large monolithic structure. Each IFU incorporates an image slicer, a fully refractive spectrograph with VPH-grating and a detector system connected to a global vacuum and cryogenic system. During 2012 and 2013, all MUSE subsystems were integrated, aligned and tested to the P.I. institute at Lyon. After successful PAE in September 2013, MUSE instrument was shipped to the Very Large Telescope in Chile where that was aligned and tested in ESO integration hall at Paranal. After, MUSE was directly transported, fully aligned and without any optomechanical dismounting, onto VLT telescope where the first light was overcame the 7th of February, 2014.
This paper describes the alignment procedure of the whole MUSE instrument with respect to the Very Large Telescope (VLT). It describes how 6 tons could be move with accuracy better than 0.025mm and less than 0.25 arcmin in order to reach alignment requirements. The success of the MUSE alignment is demonstrated by the excellent results obtained onto MUSE image quality and throughput directly onto the sky.
The deterministic optical alignment of the HERMES spectrograph
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The High Efficiency and Resolution Multi Element Spectrograph (HERMES) is a four channel, VPH-grating spectrograph fed by two 400 fiber slit assemblies whose construction and commissioning has now been completed at the Anglo Australian Telescope (AAT). The size, weight, complexity, and scheduling constraints of the system necessitated that a fully integrated, deterministic, opto-mechanical alignment system be designed into the spectrograph before it was manufactured. This paper presents the principles about which the system was assembled and aligned, including the equipment and the metrology methods employed to complete the spectrograph integration.
Active optics for space applications: an ESA perspective
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Active optics for Space is relatively new field that takes advantage of lessons learnt on ground, and together with the
tighter constrains of space environment it allows operation of larger mirrors apertures for space telescopes and better
image quality. Technical developments are crucial to guarantee proper technological readiness for applications on new
missions whose performance can be driven also by these novelties. This paper describes the philosophy pursued at ESA,
providing an overview of the activities run within the Agency, as well as perspectives for new developments. The Optics
Section of the Directorate of Technical and Quality Management of ESA/ESTEC is currently running three projects.
Two examples are here addressed.
Novel Technologies
Focal plane actuation to achieve ultra-high resolution on suborbital balloon payloads
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Over the past few years there has been remarkable success flying imaging telescope systems suspended from suborbital balloon payload systems. These imaging systems have covered optical, ultraviolet, sub-‐millimeter and
infrared passbands (i.e. BLAST, STO, SBI, Fireball and others). In recognition of these advances NASA is now considering ambitious programs to promote planetary imaging from high altitude at a fraction of the cost of similar fully orbital systems. The challenge with imaging from a balloon payload is delivering the full diffraction-‐limited resolution of the system from a moving payload. Good progress has been made with damping mechanisms and oscillation control to remove most macroscopic movement in the departures of the imaging focal plane from a static configuration, however a jitter component remains that is difficult to remove using external corrections.
This paper reports on work to demonstrate in the laboratory the utility and performance of actuating a detector focal plane (of whatever type) to remove the final jitter terms using an agile hexapod design. The input to this demonstration is the jitter signal generated by the pointing system of a previously flown balloon mission (the Stratospheric Terahertz Observatory, STO). Our group has a mature jitter compensation system that thermally isolates the control head from the focal plane itself. This allows the hexapod to remain at ambient temperature in a vacuum environment with the focal plane cooled to cryogenic temperatures. Our lab design mounts the focal plane on the hexapod in a custom cryostat and delivers an active optical stimulus together with the corresponding jitter signal, using the actuation of the hexapod to correct for the departures from a static, stable configuration. We believe this demonstration will make the case for inclusion of this technological solution in future balloon-‐borne imaging systems requiring ultra-‐high resolution.
New generation of photonic lanterns for mid-IR astronomy
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We report the fabrication and characterization of prototype femtosecond-laser direct-written integrated photonic lanterns
for operation in the mid-infrared (mid-IR). The devices were inscribed inside the bulk of a commercial gallium
lanthanum sulphide (GLS) chalcogenide glass substrate and the characterization was performed using monochromatic
light with a wavelength of 3.39 μm. We demonstrate that these proof-of-concept devices are capable of coupling specific
multimode states of light into an array of single-modes, and vice-versa, with low-loss. In the future, instruments that
utilize the single-moded output of such components may find applications in areas such as heterodyne spectroscopy,
interferometry and remote sensing.
Image slicing with a twist: spatial and spectral Nyquist sampling without anamorphic optics
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Integral field spectrographs have become mainstream instruments at modern telescopes because of their efficient way of collecting data-cubes. Image slicer based integral field spectrographs achieve the highest fill-factor on the detector, but due to the need to Nyquist-sample the spectra, their spatial sampling on the sky is rectangular. Using anamorphic pre-optics before the image slicer overcomes this effect further maximising the fill-factor, but introduces optical aberrations, throughput losses, and additional alignment and calibration requirements, compromising overall instrument performance. In this paper I present a concept for an image-slicer that achieves both spatial and spectral Nyquist-sampling without anamorphic pre-optics. Rotating each slitlet by 45° with respect to the dispersion direction, and arranging them into a saw-tooth pseudo-slit, leads to a lozenge shaped sampling element on the sky, however, the centres of the lozenges lie on a regular and square grid, satisfying the Nyquist sampling criterion in both spatial directions.
Realizing the diamond annular groove phase masks for the mid infrared region: five years of successful process development of diamond plasma etching
Show abstract
The Annular Groove Phase Mask (AGPM) is a circularly symmetric half wave plate consisting of a circular high aspect
ratio sub-wavelength grating. Here we present a method for realizing such structures in diamond. To improve the AGPM
performance, antireflective sub-wavelength gratings are etched on the backside of the components, and such gratings are
also discussed. Components for the N-band (around 10 μm) and the L-band (around 3.8 μm) have been successfully
fabricated. We are currently developing the process further to improve the precision of the gratings and produce an
AGPM for the K-band (around 2.2 μm).
Starbug fibre positioning robots: performance and reliability enhancements
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Starbugs are miniature piezoelectric ‘walking’ robots that can be operated in parallel to position many payloads (e.g.
optical fibres) across a telescope’s focal plane. They consist of two concentric piezo-ceramic tubes that walk with micron
step size. In addition to individual optical fibres, Starbugs have moved a payload of 0.75kg at several millimetres per
second. The Australian Astronomical Observatory previously developed prototype devices and tested them in the
laboratory. Now we are optimising the Starbug design for production and deployment in the TAIPAN instrument, which
will be capable of configuring 300 optical fibres over a six degree field-of-view on the UK Schmidt Telescope within a
few minutes. The TAIPAN instrument will demonstrate the technology and capability for MANIFEST (Many Instrument
Fibre-System) proposed for the Giant Magellan Telescope. Design is addressing: connector density and voltage
limitations, mechanical reliability and construction repeatability, field plate residues and scratching, metrology stability,
and facilitation of improved motion in all aspects of the design for later evaluation. Here we present the new design
features of the AAO TAIPAN Starbug.
Coatings
Progress and new techniques for protected-silver coatings
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We describe progress in the on-going effort at the University of California Observatories Advanced Coatings Lab to
develop efficient, durable silver-based coatings for telescope mirrors. We have continued to improve previously
identified recipes produced with e-beam ion-assisted deposition (IAD). We have started exploring nitride adhesion and
barrier layers added to or replacing layers in promising recipes. Our coating chamber now has one magnetron installed,
and two more will be added shortly so we can perform direct comparisons of e-beam IAD and sputtering processes for
the same recipes. We report on recent tests and findings relevant to protected-Ag coatings, including e-beam vs sputter
deposited silver; our current work with nitrides; and a comparison of certain fluorides. While focused on telescope
mirror coatings, we have also developed and tested two Ag-based coatings suitable for AO and for CCD-range
instruments. We also report on field-testing of earlier samples that have been exposed in the dome of the 3-m telescope
at Lick Observatory for a period of 2 years. Finally, we describe results of a pilot study using atomic-layer deposition
(ALD), a chemical vapor deposition technique, to produce barrier layers over silver. Optical quality ALD films are
smooth, conformal and have excellent uniformity and thickness control, and their barrier properties look extremely
promising for protecting silver from corrosion.
Gratings I
Bulk silica transmission grating made by reactive ion etching for NIR space instruments
Show abstract
A GRISM, made of a grating on a prism, allow combining image and spectroscopy of the same field of view with the
same optical system and detector, thus simplify instrument concept. New GRISM designs impose technical
specifications difficult to reach with classical grating manufacturing processes: large useful aperture (>100mm), low
groove frequency (<30g/mm), small blaze angle (<3°) and, last but not least, line curvature allowing wavefront
corrections. In addition, gratings are commonly made of resin which may not be suitable to withstand the extreme space
environment. Therefore, in the frame of a R&D project financed by the CNES, SILIOS Technologies developed a new
resin-free grating manufacturing process and realized a first 80mm diameter prototype optically tested at LAM. We
present detailed specifications of this resin-free grating, the manufacturing process, optical setups and models for optical
performance verification and very encouraging results obtained on the first 80mm diameter grating prototype: >80%
transmitted efficiency, <30nm RMS wavefront error, groove shape and roughness very close to theory and uniform over
the useful aperture.
New metrology techniques improve the production of silicon diffractive optics
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Silicon immersion gratings and grisms offer significant advantages in compactness and performance over frontsurface gratings and over grisms made from lower-index materials. At the same time, the high refractive index of Si (3.4) leads to very stringent constraints on the allowable groove position errors, typically rms < 20 nm over 100 mm and repetitive error of <5 nm amplitude. For both types of devices, we produce grooves in silicon using photolithography, plasma etching, and wet etching. To date, producers have used contact photolithography to pattern UV sensitive photoresist as the initial processing step, then transferred this pattern to a layer of silicon nitride that, in turn, serves as a hard mask during the wet etching of grooves into silicon.
For each step of the groove production, we have used new and sensitive techniques to determine the contribution of that step to the phase non-uniformity. Armed with an understanding of the errors and their origins, we could then implement process controls for each step. The plasma uniformity was improved for the silicon nitride mask etch process and the phase contribution of the plasma etch step was measured. We then used grayscale lithography, a technique in which the photoresist is deliberately underexposed, to measure large-scale nonuniformities in the UV exposure system to an accuracy of 3-5%, allowing us to make corrections to the optical alignment. Additionally, we used a new multiple-exposure technique combined with laser interferometry to measure the relationship between UV exposure dose and line edge shift. From these data we predict the contribution of the etching and photolithographic steps to phase error of the grating surface. These measurements indicate that the errors introduced during the exposure step dominate the contributions of all the other processing steps. This paper presents the techniques used to quantify individual process contributions to phase errors and steps that were taken to improve overall phase uniformity.
Gratings II
Developing ultrafast laser inscribed volume gratings
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Due to their high efficiency and broad operational bandwidths, volume phase holographic gratings (VPHGs) are often
the grating technology of choice for astronomical instruments, but current VPHGs exhibit a number of drawbacks
including limits on their size, function and durability due to the manufacturing process. VPHGs are also generally made
using a dichromated gelatine substrate, which exhibits reduced transmission at wavelengths longer than ~2.2 μm,
limiting their ability to operate further into the mid-infrared.
An emerging alternative method of manufacturing volume gratings is ultrafast laser inscription (ULI). This technique
uses focused ultrashort laser pulses to induce a localised refractive index modification inside the bulk of a substrate
material. We have recently demonstrated that ULI can be used to create volume gratings for operation in the visible,
near-infrared and mid-infrared regions by inscribing volume gratings in a chalcogenide glass. The direct-write nature of
ULI may then facilitate the fabrication of gratings which are not restricted in terms of their size and grating profile, as is
currently the case with gelatine based VPHGs.
In this paper, we present our work on the manufacture of volume gratings in gallium lanthanum sulphide (GLS)
chalcogenide glass. The gratings are aimed at efficient operation at wavelengths around 1 μm, and the effect of applying
an anti-reflection coating to the substrate to reduce Fresnel reflections is studied.
Echelle volume phase holographic gratings: experimental results
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The develop of Volume Phase Holographic Gratings (VPHGs) working as echelle grating is reported based on binary structures. A mask lithography process was developed to produce the patterns on SU8 photoresist. The binary pattern with 50 l/mm were regular and defect free. The samples were characterized by different duty cycles, which is a key parameter in defining the diffraction efficiency in such binary gratings. The efficiency has been measured at different wavelengths and for different orders. The results have been compared with those obtained by simulations.
Mass production of volume phase holographic gratings for the VIRUS spectrograph array
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The Visible Integral-field Replicable Unit Spectrograph (VIRUS) is a baseline array of 150 copies of a simple, fiber-fed integral field spectrograph that will be deployed on the Hobby-Eberly Telescope (HET). VIRUS is the first optical astronomical instrument to be replicated on an industrial scale, and represents a relatively inexpensive solution for carrying out large-area spectroscopic surveys, such as the HET Dark Energy Experiment (HETDEX). Each spectrograph contains a volume phase holographic (VPH) grating with a 138 mm diameter clear aperture as its dispersing element. The instrument utilizes the grating in first-order for 350 < λ (nm) < 550. Including witness samples, a suite of 170 VPH gratings has been mass produced for VIRUS. Here, we present the design of the VIRUS VPH gratings and a discussion of their mass production. We additionally present the design and functionality of a custom apparatus that has been used to rapidly test the first-order diffraction efficiency of the gratings for various discrete wavelengths within the VIRUS spectral range. This device has been used to perform both in-situ tests to monitor the effects of adjustments to the production prescription as well as to carry out the final acceptance tests of the gratings' diffraction efficiency. Finally, we present the as-built performance results
for the entire suite of VPH gratings.
Curved VPH gratings for novel spectrographs
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The introduction of volume phase holographic (VPH) gratings into astronomy over a decade ago opened new
possibilities for instrument designers. In this paper we describe an extension of VPH grating technology that will have
applications in astronomy and beyond: curved VPH gratings. These devices can disperse light while simultaneously
correcting aberrations. We have designed and manufactured two different kinds of convex VPH grating prototypes for
use in off-axis reflecting spectrographs. One type functions in transmission and the other in reflection, enabling Offnerstyle
spectrographs with the high-efficiency and low-cost advantages of VPH gratings. We will discuss the design
process and the tools required for modelling these gratings along with the recording layout and process steps required to
fabricate them. We will present performance data for the first convex VPH grating produced for an astronomical
spectrograph.
A set of innovative immersed grating based spectrometer designs for METIS
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We present innovative, immersed grating based optical designs for the SMO (Spectrograph Main Optics) module of the
Mid-infrared E-ELT Imager and Spectrograph, METIS. The immersed grating allows a significant reduction of SMO
volume compared to conventional echelle grating designs, because the diffraction takes place in high refractive index
silicon. Additionally, using novel optimization techniques and technical solutions in silicon micromachining offered by
the semiconductor industry, further improvements can be achieved. We show optical architectures based on compact,
double-pass Three Mirror Anastigmat (TMA) designs, which appear advantageous in terms of one or several of the
following: optical performance, reduction of volume, ease of manufacturing and testing. We explore optical designs,
where the emphasis is put on manufacturability and we investigate optical solutions, where the ultimate goal is the
highest possible optical performance. These novel, silicon immersed grating based design concepts are applicable for
future earth and space based spectrometers.
High Contrast Imaging
Prime focus wide-field corrector designs with lossless atmospheric dispersion correction
Show abstract
Wide-Field Corrector designs are presented for the Blanco and Mayall telescopes, the CFHT and the AAT. The designs
are Terezibh-style, with 5 or 6 lenses, and modest negative optical power. They have 2.2°-3° ields of view, with curved
and telecentric focal surfaces suitable for fiber spectroscopy. Some variants also allow wide-field imaging, by changing
the last WFC element. Apart from the adaptation of the Terebizh design for spectroscopy, the key feature is a new
concept for a ‘Compensating Lateral Atmospheric Dispersion Corrector’, with two of the lenses being movable laterally
by small amounts. This provides excellent atmospheric dispersion correction, without any additional surfaces or
absorption. A novel and simple mechanism for providing the required lens motions is proposed, which requires just 3
linear actuators for each of the two moving lenses.
Image quality analysis of a fluid atmospheric dispersion corrector
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The image quality of a fluid atmospheric dispersion corrector (FADC) is analysed and presented.
The FADC is located at Cassegrain focus to correct the atmospheric dispersion passively at
Australian astronomical telescope. It is shown that an FADC with diameter 10mm can correct
the atmospheric dispersion over a field diameter of 20” in the spectral range between 0.4μm to
1μm, and to zenith distance of 52º. The FADC image quality is well controlled within 0.27” for
the telescope operation range. The residual atmospheric dispersion is 0.1” for on axis field and
0.20” at the extreme field (10”) at zenith distance of 52º. The FADC-induced shift in the
centroids of the images is less than 10μm (0.067”). The FADC can work up to the field of 5’ at
Cassegrain focus and its image quality is similar to its on axis performance. Our on-sky
demonstration results agree well with our simulations.
Stop-less Lyot coronagraph for exoplanet characterization: design, manufacturing, and tests of the apodizer
Show abstract
Upcoming high-contrast imagers will all provide spectroscopic capabilities for the characterization of directly detected giant planets in wide orbits. While integral field spectroscopy (IFS) can provide both spatial and spectral information, it is usually limited in terms of field of view and resolution. The alternative is to use long slit spectroscopy coupled with coronagraphy (LSC), which can easily provide higher resolution and larger field of view. The SPHERE instrument for the VLT provides a LSC mode in its near-infrared imager and spectrograph, IRDIS. However, the fact that the occulting coronagraphic mask is merged in the focal plane with the slit reduces significantly its capacity to attenuate the diffraction, limiting the high-contrast capabilities of the instrument at close angular separations (0.3"-0.4"). To improve the diffraction suppression of the LSC in IRDIS, we recently proposed to use the stop-less Lyot coronagraph (SLLC) to build an apodized long slit coronagraph (ALSC), and we demonstrated that it improves notably the performance at small angular separation, allowing the spectral analysis of colder planets. The design of the SLLC apodizer has been optimized for an implementation in SPHERE/IRDIS, and it has recently been manufactured before being inserted into the instrument during reintegration of SPHERE in Paranal. In the current work, we present the final design of the SLLC apodizer, its specifications for the manufacturing step, and the first results obtained on SPHERE. We compare the results between the simple LSC and the new ALSC, and we draw the conclusions on the advantages and drawbacks of our design.
Successful Starshade petal deployment tolerance verification in support of NASA’s technology development for exoplanet missions
Show abstract
A Starshade is a sunflower-shaped satellite with a large inner disk structure surrounded by petals. A Starshade flies in formation with a space-borne telescope, creating a deep shadow around the telescope over a broad spectral band to permit nearby exoplanets to be viewed. Removing extraneous starlight before it enters the observatory optics greatly loosens the tolerances on the telescope and instrument that comprise the optical system, but the nature of the Starshade dictates a large deployable structure capable of deploying to a very precise shape. These shape requirements break down into key mechanical requirements which include the rigid-body position and orientation of each of the petals that ring the periphery of the Starshade. To verify our capability to meet these requirements, we modified an existing flight-like Astromesh reflector, provided by Northrup Grumman, as the base ring to which the petals attach. The integrated system, including 4 of the 30 flight-like subscale petals, truss, connecting spokes and central hub, was deployed tens of times in a flight-like manner using a gravity compensation system. After each deployment, discrete points in prescribed locations covering the petals and truss were measured using a highly-accurate laser tracker system. These measurements were then compared against the mechanical requirements, and the as-measured data shows deployment accuracy well within our milestone requirements and resulting in a contrast ratio consistent with exoplanet detection and characterization.
A novel high-contrast imaging technique based on optical tunneling to search for faint companions around bright stars at the limit of diffraction
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We present a novel application of optical tunneling in the context of high-angular resolution, high-contrast techniques with the aim of improving direct imaging capabilities of faint companions in the vicinity of bright stars. In contrast to existing techniques like coronagraphy, we apply well-established techniques from integrated optics to exclusively extinct a very narrow angular direction coming from the sky. This extinction is achieved in the pupil plane and does not suffer from diffraction pattern residuals. We give a comprehensive presentation of the underlying theory as well as first laboratory results.
The vector apodizing phase plate coronagraph: prototyping, characterization and outlook
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The Apodizing Phase Plate (APP) is a phase-only pupil-plane coronagraph that suppresses starlight in a D-shaped region from 2 to 7 λ D around a target star. Its performance is insensitive to residual tip-tilt variations from the AO system and telescope structure. Using liquid crystal technology we develop a novel and improved version of the APP: the broadband vector Apodizing Phase Plate (vAPP). The vAPP prototype consists of an achromatic half-wave retarder pattern with a varying fast axis encoding phase structure down to 25 microns. The fast axis encodes the required phase pattern through the vector phase, while multiple twisting liquid crystal layers produce a nearly constant half-wave retardance over a broad bandwidth. Since pupil phase patterns are commonly designed to be antisymmetric, two complementary PSFs are produced with dark holes on opposite sides.
We summarize results of the characterization of our latest vAPP prototype in terms of pupil phase reconstruction and PSF contrast performance. The liquid crystal patterning technique allows us to manufacture more extreme phase patterns than was possible before. We consider phase-only patterns that yield higher contrasts and better inner working angles than previous APPs, and patterns that produce dark regions 360 degrees around the PSF core. The possibility of including a phase ramp into the coronagraph is demonstrated, which simplifies the vAPP into a single optic. This additional phase ramp removes the need for a quarter-wave plate and a Wollaston prism, and enables the simplified implementation of a vAPP in a filter wheel at a pupil-plane location. Since the phase ramp is analogous to a polarization grating, it generates a (polarized) spectrum of a planet inside the dark hole, and thus allows for instantaneous characterization of the planet.
Spectroscopy
Development of micro image slicer of integral field unit for spaceborne solar spectrograph
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We present an innovative optical design for image slicer integral field unit (IFU) and manufacturing method which overcome optical limitation of metallic mirrors. Our IFU consists of micro image slicer of 45 arrayed highly-narrow flat metallic mirrors and a pseudo pupil mirror array of off-axis conic aspheres forming three pseudo slits of re-arranged slicer images. A prototype IFU demonstrates their optical quality high enough for a visible light spectrograph. The each slicer mirror is 1.58 mm in length and 30μm in width with surface roughness < 1 nm rms, edge sharpness < 0.1μm, etc. This IFU is small-sized and can be implemented in a multi-slit spectrograph without any moving mechanism and fore optics in which one slit is real and the others are of pseudo slits from the IFU. Those properties are well suitable for space-borne spectrograph to be aboard such as a next Japanese solar mission SOLAR-C.
Robust reflective pupil slicing technology
Show abstract
Tornado Spectral Systems (TSS) has developed the High Throughput Virtual Slit (HTVSTM), robust all-reflective
pupil slicing technology capable of replacing the slit in research-, commercial- and MIL-SPEC-grade spectrometer
systems. In the simplest configuration, the HTVS allows optical designers to remove the lossy slit from pointsource
spectrometers and widen the input slit of long-slit spectrometers, greatly increasing throughput without
loss of spectral resolution or cross-dispersion information.
The HTVS works by transferring etendue between image plane axes but operating in the pupil domain rather
than at a focal plane. While useful for other technologies, this is especially relevant for spectroscopic applications
by performing the same spectral narrowing as a slit without throwing away light on the slit aperture. HTVS can
be implemented in all-reflective designs and only requires a small number of reflections for significant spectral
resolution enhancement–HTVS systems can be efficiently implemented in most wavelength regions. The etendueshifting
operation also provides smooth scaling with input spot/image size without requiring reconfiguration
for different targets (such as different seeing disk diameters or different fiber core sizes). Like most slicing
technologies, HTVS provides throughput increases of several times without resolution loss over equivalent slitbased
designs.
HTVS technology enables robust slit replacement in point-source spectrometer systems. By virtue of pupilspace
operation this technology has several advantages over comparable image-space slicer technology, including
the ability to adapt gracefully and linearly to changing source size and better vertical packing of the flux
distribution. Additionally, this technology can be implemented with large slicing factors in both fast and slow
beams and can easily scale from large, room-sized spectrometers through to small, telescope-mounted devices.
Finally, this same technology is directly applicable to multi-fiber spectrometers to achieve similar enhancement.
HTVS also provides the ability to anamorphically “stretch” the slit image in long-slit spectrometers, allowing
the instrument designer to optimize the plate scale in the dispersion axis and cross-dispersion axes independently
without sacrificing spatial information. This allows users to widen the input slit, with the associated gain of
throughput and loss of spatial selectivity, while maintaining the spectral resolution of the spectrometer system.
This “stretching” places increased requirements on detector focal plane height, as with image slicing techniques,
but provides additional degrees of freedom to instrument designers to build the best possible spectrometer
systems.
We discuss the details of this technology for an astronomical context, covering the applicability from small
telescope mounted spectrometers through long-slit imagers and radial-velocity engines. This powerful tool provides
additional degrees of freedom when designing a spectrometer, enabling instrument designers to further
optimize systems for the required scientific goals.
MUSE field splitter unit: fan-shaped separator for 24 integral field units
Show abstract
MUSE (Multi Unit Spectroscopic Explorer) is a second generation Very Large Telescope (VLT) integral field
spectrograph developed for the European Southern Observatory (ESO). It combines a 1’ x 1’ field of view sampled at 0.2
arcsec for its Wide Field Mode (WFM) and a 7.5"x7.5" field of view for its Narrow Field Mode (NFM). Both modes will
operate with the improved spatial resolution provided by GALACSI (Ground Atmospheric Layer Adaptive Optics for
Spectroscopic Imaging), that will use the VLT deformable secondary mirror and 4 Laser Guide Stars (LGS) foreseen in
2015. MUSE operates in the visible wavelength range (0.465-0.93 μm). A consortium of seven institutes is currently
commissioning MUSE in the Very Large Telescope for the Preliminary Acceptance in Chile, scheduled for September,
2014.
MUSE is composed of several subsystems which are under the responsibility of each institute. The Fore Optics derotates
and anamorphoses the image at the focal plane. A Splitting and Relay Optics feed the 24 identical Integral Field Units
(IFU), that are mounted within a large monolithic instrument mechanical structure. Each IFU incorporates an image
slicer, a fully refractive spectrograph with VPH-grating and a detector system connected to a global vacuum and
cryogenic system. During 2012 and 2013, all MUSE subsystems were integrated, aligned and tested to the P.I. institute at
Lyon. After successful PAE in September 2013, MUSE instrument was shipped to the Very Large Telescope in Chile
where it was aligned and tested in ESO integration hall at Paranal. After, MUSE was directly transferred in monolithic
way onto VLT telescope where the first light was achieved.
This paper describes the MUSE main optical component: the Field Splitter Unit. It splits the VLT image into 24
subfields and provides the first separation of the beam for the 24 Integral Field Units. This talk depicts its manufacturing
at Winlight Optics and its alignment into MUSE instrument. The success of the MUSE alignment is demonstrated by the
excellent results obtained onto MUSE positioning, image quality and throughput onto the sky. MUSE commissioning at
the VLT is planned for September, 2014.
The problem of scattering in fibre-fed VPH spectrographs and possible solutions
Show abstract
All spectrographs unavoidably scatter light. Scattering in the spectral direction is problematic for sky subtraction, since atmospheric spectral lines are blurred. Scattering in the spatial direction is problematic for fibre fed spectrographs, since it limits how closely fibres can be packed together. We investigate the nature of this scattering and show that the scattering wings have both a Lorentzian component, and a shallower (1/r) component. We investigate the causes of this from a theoretical perspective, and argue that for the spectral PSF the Lorentzian wings are in part due to the profile of the illumination of the pupil of the spectrograph onto the diffraction grating, whereas the shallower component is from bulk scattering. We then investigate ways to mitigate the diffractive scattering by apodising the pupil. In the ideal case of a Gaussian apodised pupil, the scattering can be significantly improved. Finally we look at realistic models of the spectrograph pupils of fibre fed spectrographs with a centrally obstructed telescope, and show that it is possible to apodise the pupil through non-telecentric injection into the fibre.
Development of integrated photonic-dicers for reformatting the point-spread-function of a telescope
Show abstract
Spectroscopy is a technique of paramount importance to astronomy, as it enables the chemical composition, distances
and velocities of celestial objects to be determined. As the diameter of a ground-based telescope increases, the pointspread-
function (PSF) becomes increasingly degraded due to atmospheric seeing. A degraded PSF requires a larger
spectrograph slit-width for efficient coupling and current spectrographs for large telescopes are already on the metre
scale. This presents numerous issues in terms of manufacturability, cost and stability.
As proposed in 2010 by Bland-Hawthorn et al, one approach which may help to improve spectrograph stability
is a guided wave transition, known as a “photonic-lantern”. These devices enable the low-loss reformatting of a
multimode PSF into a diffraction-limited source (in one direction). This pseudo-slit can then be used as the input to a
traditional spectrograph operating at the diffraction limit. In essence, this approach may enable the use of diffractionlimited
spectrographs on large telescopes without an unacceptable reduction in throughput.
We have recently demonstrated that ultrafast laser inscription can be used to realize “integrated” photoniclanterns,
by directly writing three-dimensional optical waveguide structures inside a glass substrate. This paper presents
our work on developing ultrafast laser inscribed devices capable of reformatting a multimode telescope PSF into a
diffraction-limited slit.
Optical Fibers and Positioners I
Advances in the Echidna fiber-positioning technology
Show abstract
We present advances in the patented Echidna 'tilting spine' fiber positioner technology that has been in operation since 2007 on the SUBARU telescope in the FMOS system. The new Echidna technology is proposed to be implemented on two large fiber surveys: the Dark Energy Spectroscopic Instrument (DESI) (5000 fibers) as well the Australian ESO Positioner (AESOP) for 4MOST, a spectroscopic survey instrument for the VISTA telescope (~2500 fibers). The new 'superspine' actuators are stiffer, longer and more accurate than their predecessors. They have been prototyped at AAO, demonstrating reconfiguration times of ~15s for errors of <5 microns RMS. Laboratory testing of the prortotype shows accurate operation at temperatures of -10 to +30C, with an average heat output of 200 microwatts per actuator during reconfiguration. Throughput comparisons to other positioner types are presented, and we find that losses due to tilt will in general be outweighed by increased allocation yield and reduced fiber stress FRD. The losses from spine tilt are compensated by the gain in allocation yield coming from the greater patrol area, and quantified elsewhere in these proceedings. For typical tilts, f-ratios and collimator overspeeds, Echidna offers a clear efficiency gain versus current r-that or theta-phi positioners.
Developing engineering model Cobra fiber positioners for the Subaru Telescope’s prime focus spectrometer
Show abstract
The Cobra fiber positioner is being developed by the California Institute of Technology (CIT) and the Jet Propulsion
Laboratory (JPL) for the Prime Focus Spectrograph (PFS) instrument that will be installed at the Subaru Telescope on
Mauna Kea, Hawaii. PFS is a fiber fed multi-object spectrometer that uses an array of Cobra fiber positioners to rapidly
reconfigure 2394 optical fibers at the prime focus of the Subaru Telescope that are capable of positioning a fiber to
within 5μm of a specified target location. A single Cobra fiber positioner measures 7.7mm in diameter and is 115mm
tall. The Cobra fiber positioner uses two piezo-electric rotary motors to move a fiber optic anywhere in a 9.5mm
diameter patrol area. In preparation for full-scale production of 2550 Cobra positioners an Engineering Model (EM)
version was developed, built and tested to validate the design, reduce manufacturing costs, and improve system
reliability. The EM leveraged the previously developed prototype versions of the Cobra fiber positioner. The
requirements, design, assembly techniques, development testing, design qualification and performance evaluation of EM
Cobra fiber positioners are described here. Also discussed is the use of the EM build and test campaign to validate the
plans for full-scale production of 2550 Cobra fiber positioners scheduled to begin in late-2014.
IFU simulator: a powerful alignment and performance tool for MUSE instrument
Show abstract
MUSE (Multi Unit Spectroscopic Explorer) is a second generation Very Large Telescope (VLT) integral field
spectrograph (1x1arcmin² Field of View) developed for the European Southern Observatory (ESO), operating in the
visible wavelength range (0.465-0.93 μm). A consortium of seven institutes is currently commissioning MUSE in the
Very Large Telescope for the Preliminary Acceptance in Chile, scheduled for September, 2014.
MUSE is composed of several subsystems which are under the responsibility of each institute. The Fore Optics derotates
and anamorphoses the image at the focal plane. A Splitting and Relay Optics feed the 24 identical Integral Field Units
(IFU), that are mounted within a large monolithic instrument mechanical structure. Each IFU incorporates an image
slicer, a fully refractive spectrograph with VPH-grating and a detector system connected to a global vacuum and
cryogenic system. During 2012 and 2013, all MUSE subsystems were integrated, aligned and tested to the P.I. institute at
Lyon. After successful PAE in September 2013, MUSE instrument was shipped to the Very Large Telescope in Chile
where that was aligned and tested in ESO integration hall at Paranal. After, MUSE was directly transferred in monolithic
way without dismounting onto VLT telescope where the first light was overcame.
This talk describes the IFU Simulator which is the main alignment and performance tool for MUSE instrument. The IFU
Simulator mimics the optomechanical interface between the MUSE pre-optic and the 24 IFUs. The optomechanical
design is presented. After, the alignment method of this innovative tool for identifying the pupil and image planes is
depicted. At the end, the internal test report is described. The success of the MUSE alignment using the IFU Simulator is
demonstrated by the excellent results obtained onto MUSE positioning, image quality and throughput.
MUSE commissioning at the VLT is planned for September, 2014.
Astronomical optical frequency comb generation and test in a fiber-fed MUSE spectrograph
Show abstract
We here report on recent progress on astronomical optical frequency comb generation at innoFSPEC-Potsdam and
present preliminary test results using the fiber-fed Multi Unit Spectroscopic Explorer (MUSE) spectrograph. The
frequency comb is generated by propagating two free-running lasers at 1554.3 and 1558.9 nm through two dispersionoptimized
nonlinear fibers. The generated comb is centered at 1590 nm and comprises more than one hundred lines with
an optical-signal-to-noise ratio larger than 30 dB. A nonlinear crystal is used to frequency double the whole comb
spectrum, which is efficiently converted into the 800 nm spectral band. We evaluate first the wavelength stability using
an optical spectrum analyzer with 0.02 nm resolution and wavelength grid of 0.01 nm. After confirming the stability
within 0.01 nm, we compare the spectra of the astro-comb and the Ne and Hg calibration lamps: the astro-comb exhibits
a much larger number of lines than lamp calibration sources. A series of preliminary tests using a fiber-fed MUSE
spectrograph are subsequently carried out with the main goal of assessing the equidistancy of the comb lines. Using a
P3d data reduction software we determine the centroid and the width of each comb line (for each of the 400 fibers
feeding the spectrograph): equidistancy is confirmed with an absolute accuracy of 0.4 pm.
Optical Fibers and Positioners II
Commissioning the SALT fiber instrument feed
Show abstract
Lessons learnt during the design, integration and commissioning of the Southern African Large Telescope’s Fiber
Instrument Feed and its guidance probe are presented along with initial on-sky performance results.
Advances over the original design include enhanced target acquisition efficiency by including an imaging camera
directly on the fiber-positioning stage, use of linear optical encoders on all motion stages, thermally compensated
designs, a guidance probe optical design with much improved throughput as well as a new non-contact metrology
process to accurately model the guider.
The system has since been successfully used in commissioning the SALT High-Resolution Spectrograph.
Photonic lantern behaviour and implications for instrument design
Show abstract
Photonic lanterns are an important enabling technology for astrophotonics with a wide range of potential applications including fibre Bragg grating OH suppression, integrated photonic spectrographs and fibre scramblers for high resolution spectroscopy. The behaviour of photonic lanterns differs in several important respects from the conventional fibre systems more frequently used in astronomical instruments and a detailed understanding of this behaviour is required in order to make the most effective use of this promising technology. To this end we have undertaken a laboratory study of photonic lanterns with the aim of developing an empirical model for the mapping from input to output illumination distributions. We have measured overall transmission and near field output light distributions as a function of input angle of incidence for photonic lanterns with between 19 and 61 cores. We present the results of this work, highlight the key differences between photonic lanterns and conventional fibres, and illustrate the implications for instrument design via a case study, the design of the PRAXIS spectrograph. The empirical photonic lantern model was incorporated into an end-to-end PRAXIS performance model which was used to optimise the design parameters of the instrument. We describe the methods used and the resulting conclusions. The details of photonic lantern behaviour proved particularly important in selecting the optimum on sky field of view per fibre and in modelling of the instrument thermal background.
The FRD and transmission of the 270-m GRACES optical fiber link and a high numerical aperture fiber for astronomy
Show abstract
We report results of the extensive development work done on the 270-m optical fiber link for the GRACES project and a preliminary investigations into a high numerical aperture fiber for astronomy. The Gemini Remote Access CFHT ESPaDOnS Spectrograph (GRACES) is an instrumentation experiment to link ESPaDOnS, a bench-mounted highresolution optical spectrograph at CFHT, to the Gemini-North telescope with an optical fiber link. A 270-m fiber link with less than 14% Focal Ratio Degradation (FRD) has been developed jointly by HIA and FiberTech Optica for the experiment. A preliminary study has been conducted by HIA into a high numerical aperture fiber (0.26 numerical aperture) with the intended application of wide field optical spectrographs fiber fed from the telescope prime focus. The Laboratory test results of FRD, transmission, and stability for the GRACES fiber link and preliminary FRD measurements of the high numerical aperture fiber tests are reported.
Poster Session
Manufacture of the combined primary and tertiary mirrors of the Large Synoptic Survey Telescope
Show abstract
The Steward Observatory Mirror Lab is nearing completion of the combined primary and tertiary mirrors of the Large
Synoptic Survey Telescope. Fabrication of the combined mirror requires simulation of an active-optics correction that
affects both mirror surfaces in a coordinated way. As is common for large mirrors, the specification allows correction of
large-scale figure errors by a simulated bending of the substrate with the 156 mirror support actuators. Any bending
affects both mirrors, so this active-optics correction is constrained by the requirement of bending the substrate so both
mirrors meet their figure specifications simultaneously. The starting point of the simulated correction must be
measurements of both mirrors with the substrate in the same shape, i. e. the same state of mechanical and thermal stress.
Polishing was carried out using a 1.2 m stressed lap for smoothing and large-scale figuring, and a set of smaller passive
rigid-conformal laps on an orbital polisher for deterministic small-scale figuring. The primary mirror is accurate to about
25 nm rms surface after the active-optics correction, while work continues toward completion of the tertiary.
Wavefront detection of extended-beacon based on phase-space optics
Show abstract
The extended beacon belongs to partially coherent light field, and Phase space optics is an effective
tool for analyzing such light field, especially in calculating the phase of multi-angle and multi-layer.
Due to the lack of directly detection means, the research of this theory is crippled a long time, in this
paper an optical structure to obtain the spectrogram which is a kind of phase space distribution has
been presented, while the resolution problem and an improved method for possible has been
discussed too, and two new methods to get high quality astronomical Image are appearing with such
algorithm.
The design of force actuator used in extreme low temperature environment
Show abstract
The existing force actuators cannot work properly in the Antarctic under the condition of low temperature. In this paper,
a new design scheme of force actuator is put forward. Combined with the actual situation and the requirement of thin
mirror active optical experiment system, we design the force actuator structure which combined the active support and
passive support, and use a s-shaped load-cell to realize the force feedback controlling. Passive support part is responsible
for the adjustment of large travel with low precision, and active support part driven by PZT is responsible for the
adjustment with high accuracy. Finally, test was carried out through the open loop and closed loop experiment in low
temperature environment. The experimental results show that: The force actuator’s output force is 120 ~ 280N, the
accuracy is better than 0.05 N, meeting the requirement for the high precision under low temperature. The new kind of
force actuator can be applied to the active optical support system in the Antarctic, and at the same time can also be
applied to other structures of precision adjustment.
Adapting large lightweight primary mirror to space active optics capabilities
Show abstract
The next generation of space telescope will use large primary mirrors to achieve high angular resolution. Due
to weight constrain, these large mirrors will have a very low mass per unit area. This ultra-light-weighting
leads to deformations of the primary mirror optical surface due to gravity load difference between ground and
space. Active Optics systems then become essential to maintain the quality of the output wavefront. The
supporting structures and surface quality specifications of the mirror must be optimized regarding the active
optics capabilities. The case of a two meters lightweight primary mirror will be presented.
Baseline design of the LSST hexapods and rotator
Show abstract
The Large Synoptic Survey Telescope (LSST) is a large (8.4 meter) wide-field (3.5 degree) survey telescope, which will
be located on the Cerro Pachón summit in Chile. Both the Secondary Mirror (M2) Cell Assembly and Camera utilize
hexapods to facilitate optical positioning relative to the Primary/Tertiary (M1M3) Mirror. Geometric considerations
preclude the use of a conventional hexapod arrangement for the M2 Hexapod. A rotator resides between the Camera and
its hexapod to facilitate tracking. The requirements of the M2 Hexapod and Camera Hexapod are very similar;
consequently to facilitate maintainability both hexapods will utilize identical actuators.
Performance testing of the LMT/GTM primary surface actuators
Show abstract
The design of the Large Millimeter Telescope (LMT/GTM) relies on an active surface system for the primary
reflector in order to meet its surface accuracy specifications. Each of the 180 segments of the 50m diameter
primary surface is supported by four linear actuators, one for each corner. Currently, while awaiting completion
of the reflector panels needed to populate the outer portion of the surface, the precision reflecting surface is
limited to the inner three rings of segments, which provide a 32.5m diameter primary with 84 segments. The
interim active surface system must provide control for these segments long enough to allow completion of the
rest of the reflecting surface and procurement and installation of the final actuators.
During the LMT/GTM first light program in 2011, the interim active surface system of 336 actuators suffered
from a wide range of problems that prevented useful operation. Following extensive diagnostic testing in 2012, the
project implemented a substantial actuator improvement program in the first half of 2013, including mechanical
modifications and replacement of the control system electronics. Later, each actuator was characterized on
an external test bench, providing demonstration of actuator operation and readiness, as well as calibration
information to further improve absolute accuracy.
In this paper, we present results of the accuracy and repeatability of the refurbished actuators, as well as
results from the accelerated lifetime testing on site. The uniformity of the calibration coefficients is also presented
and discussed. The results demonstrate that the refurbished actuator units are sufficient for the needs of the
interim active surface system.
Mechanical improvement of the interim LMT/GTM primary surface actuators
Show abstract
The design of the Large Millimeter Telescope/Gran Telescopio Milimétrico (LMT/GTM) is such that it
relies on an active surface system for the primary reflector in order to meet its surface accuracy specifications.
Specifically, each of the primary surface segments is supported by four linear actuators, one for each corner. In
the current state of the telescope, the interim active surface system must provide control for 84 segments long
enough to allow completion of the rest of the reflecting surface and installation of the final actuators.
During the LMT/GTM first light program in 2011, the interim active surface system of 336 actuators
suffered from a wide range of problems that prevented useful operation. As a result, the LMT engineering team
began an extensive testing program in 2012 to determine the sources of the problem and to evaluate possible
mitigation strategies. As a result of these tests, the project implemented a substantial improvement program
to the actuators in the first half of 2013, including a set of mechanical modifications and a replacement of the
control system electronics.
In this paper, we present the original mechanical design of the actuator, the design issues, and the modifications
that were implemented. Details are provided about how the actuators have been improved from the
perspective of repeatability, accuracy, and robustness. Finally, additional comments and recommendations are
made for applying the lessons learned to the final actuator system.
Performance testing of the LMT/GTM M2 positioner
Show abstract
Prior to the early science campaign of Spring 2013, the engineering team at the Large Millimeter Telescope/
Gran Telescopio Milimétrico (LMT/GTM) conducted a series of performance tests on the hexapod used
for positioning the secondary reflector (M2 mirror). The tests were of particular interest to the project due to
the high mass of the existing aluminum M2 mirror.
The testing was conducted in a lower foundation room at the LMT site on a fixture that allowed the positioner
and mirror to be oriented at both zenith and horizon orientations. In each of these positions, the repeatability of
the system zero position was tested, along with both single degree-of-freedom (DOF) and combined DOF motions.
Additionally, the tests investigated the stability of the system at constant command position to changes in the
orientation of the unit with respect to gravity. Throughout these tests, a laser tracker was used for measurement
of the position of targets on both the fixed base of the hexapod and on the outer rim of the M2 mirror. In this
way, motions of the tracker head or of the support fixture could be eliminated from the analysis.
In this paper, we present results of the accuracy and repeatability of the system, as well as comments on
the effects of the laser tracker measurement geometry with respect to the system at the zenith and horizon
orientations.
Design of a radio telescope surface segment actuator based on a form-closed eccentric cam
Show abstract
As radio telescopes have reached larger diameters and higher frequencies, it is typically not possible to
meet their surface accuracy specifications using passive homology-based designs. The most common solution to
this problem in the current generation of large, high-frequency radio telescopes is to employ a system of linear
actuators to correct the surface shape of the primary reflector.
The exact specifications of active surface actuators vary with the telescope. However, they have many common
features, some of which drive their design. In general, these actuators must provide precise and repeatable
positioning under significant loads during operation and they must withstand even higher loads for survival conditions.
For general safety, they typically must hold position in the event of a power failure and must incorporate
position limits, whether electrical, mechanical, or both. Because the number of actuators is generally high for
large active surfaces (hundreds or even thousands of actuators), they must also be reliable and of reasonable
individual cost. Finally, for maximum flexibility in their installation, they must be compact.
This paper presents a concept for an active surface actuator based on a form-closed eccentric cam (kinematically,
a Scotch Yoke mechanism). Such a design is limited in stroke, but offers potential advantages in
terms of manufacture, compactness, measurement, and survival loading. The paper demonstrates that some of
the expected advantages cannot be practically realized, due to dimensions that are driven by survival loading
conditions. As a result, this concept is likely to offer an advantage over conventional screw-type actuators only
for cases where actuator runaway and stall are the driving considerations.
Research on high dynamic range information capture of GEO camera
Show abstract
A high dynamic range imaging method of GEO staring imaging is proposed based on radiance simulation
of GEO remote sensing targets and analysis of foreign and domestic remote sensing payload characteristics. Due
to the high temporal resolution of GEO staring imaging, multiple exposure method is used and image sequences
are captured with different integration times; Then a high dynamic range image is obtained after fusion with the
contrast of neighborhood pixel values being the weighting factor. Finally experiments are done in lab with
visible plane array 2048*2048 imaging system for verifying multiple exposure test. It can be proved that using
multiple exposure capture fusion method can obtain an 11 bit high dynamic range image. The essence of the
method is that it sacrifices time resolution in exchange for high dynamic range, which overcomes the defect of
small dynamic range of single exposure and is of practical significance in terms of GEO high dynamic range
information capture.
Jitter studies for the secondary and tertiary mirror systems on the Thirty Meter Telescope
Show abstract
The Secondary Mirror System (M2S) and Tertiary Mirror System (M3S) of the Thirty Meter Telescope (TMT) consist of
passively mounted mirrors supported in kinematic cell assemblies that are moved during telescope tracking to counteract
effects of changing zenith angle and thermal gradients within the telescope structure. TMT is concerned that the
requirements for pointing jitter during Adaptive Optics tracking for the M2 and M3 Systems are very challenging with a
risk of requiring complex stabilization systems for compliance. Both systems were researched to determine whether
similar un-stabilized hardware exists that can meet the TMT jitter requirements. Tests using representative TMT
tracking motions were then performed to measure jitter on similar existing hardware. The results of these hardware tests
have been analyzed. Test results, remaining risk assessment and further testing plans are presented.
Fast active optics control of wide-field telescopes based on science image analysis
Show abstract
We study a novel active optics control scheme at the VST on Cerro Paranal, an f/5:5 survey telescope with a 1x1 degree field of view and a 2.6m primary mirror. This scheme analyzes the elongation pattern of the star PSFs across the full science image (267 Mpixels) and compares their second moments with an analytical model based on 5th-order geometrical optics, comprising 9 degrees of freedom in mirror misalignments and deformations. Using a numerical optimization method, we can complete the star extraction and fitting process in under one minute, fast enough for effective closed-loop active optics control in survey observing cadences.
The architecture of the active surface control system of the Large Millimeter Telescope
Show abstract
One of the fundamental design principles of the LMT is that its segmented primary surface must be active: the position and orientation of each of the segments must be moved in order to maintain the precise parabolic surface that is required by the specifications. Consequently, a system of actuators, one at the corner of each segment, is used to move the segments to counteract surface deformations attributed to gravity or thermal effects.
A new control system was designed and built within the project to implement an active surface at the LMT. The technical concept for the active surface control system is to provide a set of bus boxes with built-in control and I/O capabilities to run four actuators each. Bus boxes read the LVDT sensor position and limit switch status for each actuator and use this information to drive the actuator’s DC motor, closing the position loop. Each bus box contains a DC power supply for the electronics, a second DC power supply for the motors, an embedded controller with I/O to close the position loop, and a custom printed circuit board to condition the LVDT signals and drive the motors. An interface printed circuit board resides in each actuator providing a single connector access to the LVDT, the motor, and the limit switches. During the fall of 2013, 84 bus boxes were commissioned to control the 336 actuators of the inner three rings of the telescope. The surface correction model was determined using holography measurements and the active surface system has been in regular use during the scientific observation at the LMT.
LSST Inner Loop Controller (ILC) design and characterization
Show abstract
The Large Synoptic Survey Telescope (LSST) utilizes active optics on its three mirrors to maintain image quality. This
paper describes the philosophy behind the design and characterization of the inner loop controllers for the LSST project.
A custom approach was selected in order to satisfy the stringent requirements of the active optics control system
resulting in a very low power, robust and compact solution. The tough metrology requirements were translated into an
analog front end capable of performing with high accuracy under the varying ambient conditions, mainly temperature.
Networking capabilities are embedded in the design to adapt to different distributed control configurations. All basic
applications and some additional uses are discussed, and test results are presented.
Freeform mirror based optical systems for FAME
Show abstract
In this paper we present the design of freeform mirror based optical systems that have the potential to be used in future
astronomical instrumentation in the era of extremely large ground based telescopes. Firstly we describe the optical
requirements followed by a summary of the optimization methodology used to design the freeform surface. The intention
is to create optical architectures, which not only have the numerous advantages of freeform based systems (increased
optical performance and/or reduction of mass and volume), but also can be manufactured and tested with today’s
manufacturing techniques and technologies.
The team plans to build a demonstrator based on one of the optical design examples presented in this paper. The
demonstrator will be built and tested as part of the OPTICON FP7 Freeform Active Mirror Experiment (FAME) project.
A hydroforming technique developed as part of the previous OPTICON FP7 project will be used to produce an accurate,
compact and stable freeform mirror. The manufacturing issues normally experienced in the production of freeform
mirrors are solved through the hydroforming of thin polished substrates, which then will be supported with an active
array structure. The active array will be used to compensate for residual manufacturing errors, thermo-elastic
deformation and gravity-induced errors.
Active array design for FAME: Freeform Active Mirror Experiment
Show abstract
In this paper a status report is given on the development of the FAME (Freeform Active Mirror Experiment) active array.
Further information regarding this project can be found in the paper by Venema et al. (this conference). Freeform optics
provide the opportunity to drastically reduce the complexity of the future optical instruments. In order to produce these
non-axisymmetric freeform optics with up to 1 mm deviation from the best fit sphere, it is necessary to come up with
new design and manufacturing methods. The way we would like to create novel freeform optics is by fine tuning a preformed
high surface-quality thin mirror using an array which is actively controlled by actuators. In the following we
introduce the tools deployed to create and assess the individual designs. The result is an active array having optimal
number and lay-out of actuators.
Active scale changer for anamorphic, distortion free, telecentric zoom system
Show abstract
We present the conception of an anamorphic and telecentric scale changer with no distortion, able to provide magnifications in the range of 2 to 30 without any interchangeable optics, dedicated to ground or space applications. Several optical designs are investigated and the final configuration is based on off-axis five mirrors system with no moving elements. Four active mirrors are adapted to four different zoom configurations. A specific mechanical profile with variable thickness distribution is simulated and optimized on each mirror to allow using a minimal number of actuators. An opto-mechanical design will be presented, showing the implementation of actuators on the system. This work is done in the frame of the ANR project OASIX and will produce a lab prototype in 2015.
ZERODUR TAILORED for cryogenic application
Show abstract
ZERODUR® glass ceramic from SCHOTT is known for its very low thermal expansion coefficient (CTE) at room
temperature and its excellent CTE homogeneity. It is widely used for ground-based astronomical mirrors but also for
satellite applications. Many reference application demonstrate the excellent and long lasting performance of
ZERODUR® components in orbit. For space application a low CTE of the mirror material is required at cryogenic
temperatures together with a good match of the thermal expansion to the supporting structure material.
It is possible to optimize the coefficient of thermal expansion of ZERODUR® for cryogenic applications. This paper
reports on measurements of thermal expansion of ZERODUR® down to cryogenic temperatures of 10 K performed by
the PTB (Physikalisch Technische Bundesanstallt, Braunschweig, Germany, the national metrology laboratory). The
ZERODUR® TAILORED CRYO presented in this paper has a very low coefficient of thermal expansion down to 70 K.
The maximum absolute integrated thermal expansion down to 10 K is only about 20 ppm.
Mirror blanks made from ZERODUR® TAILORED CRYO can be light weighted to almost 90% with our modern
processing technologies. With ZERODUR® TAILORED CRYO, SCHOTT offers the mirror blank material for the next
generation of space telescope applications.
ZERODUR: bending strength data for etched surfaces
Show abstract
In a continuous effort since 2007 a considerable amount of new data and information has been gathered on the bending
strength of the extremely low thermal expansion glass ceramic ZERODUR®.
By fitting a three parameter Weibull distribution to the data it could be shown that for homogenously ground surfaces
minimum breakage stresses exist lying much higher than the previously applied design limits.
In order to achieve even higher allowable stress values diamond grain ground surfaces have been acid etched, a
procedure widely accepted as strength increasing measure.
If surfaces are etched taking off layers with thickness which are comparable to the maximum micro crack depth of the
preceding grinding process they also show statistical distributions compatible with a three parameter Weibull
distribution. SCHOTT has performed additional measurement series with etch solutions with variable composition
testing the applicability of this distribution and the possibility to achieve further increase of the minimum breakage
stress.
For long term loading applications strength change with time and environmental media are important. The parameter
needed for prediction calculations which is combining these influences is the stress corrosion constant. Results from the
past differ significantly from each other. On the basis of new investigations better information will be provided for
choosing the best value for the given application conditions.
Process optimization for polishing large aspheric mirrors
Show abstract
Large telescope mirrors have stringent requirements for surface irregularity on all spatial scales. Large scale errors, typically represented with Zernike polynomials, are relatively easy to control. Errors with smaller spatial scale can be more difficult because the specifications are tighter. Small scale errors are controlled with a combination of natural smoothing from large tools and directed figuring with precisely controlled small tools. The optimization of the complete process builds on the quantitative understanding of natural smoothing, convergence of small tool polishing, and confidence in the surface measurements. This paper provides parametric models for smoothing and directed figuring that can be used to optimize the manufacturing process.
SST dual-mirror telescope for Cherenkov Telescope Array: an innovative mirror manufacturing process
Show abstract
The Observatoire de Paris is constructing a prototype Small-Sized Telescope (SST) for the Cherenkov Telescope Array
(CTA), named SST-GATE, based on the dual-mirror Schwarzschild-Couder optical design. Considering the mirrors size
and its specific curvature and the optical requirements for the Cherenkov imaging telescope, a non-conventional process
has been used for designing and manufacturing the mirrors of the SST-GATE prototype. Based on machining, polishing
and coating of aluminium bulk samples, this process has been validated by simulation and tests that will be detailed in
this paper after a discussion on the Schwarzschild-Couder optical design which so far has never been used to design
ground based telescopes.
Even if the SST-GATE is a prototype for small size telescopes of the CTA array, the primary mirror of the telescope is 4
meters diameter, and it has to be segmented. Due to the dual-mirror configuration, the alignment is a complex task that
needs a well defined and precise process that will be discussed in this paper.
The glass cold-shaping technology for the mirrors of the Cherenkov Telescope Array
Show abstract
The next generation of imaging atmospheric Cherenkov telescopes will require the production of thousands of mirror
segments; an unprecedented amount of optical surface. To accomplish this, the Italian Istituto Nazionale di AstroFisica
(INAF) has recently developed a successful technique.
This method, called glass cold-shaping, is mainly intended for the manufacturing of mirrors for optical systems with an
angular resolution of a few arcminutes, intended to operate in extreme environments. Its principal mechanical features
are very low weight and high rigidity of the resulting segments, and its cost and production time turn out to be very
competitive as well. The process is based on the shaping of thin glass foils by means of forced bending at room
temperature; a sandwich structure is then assembled for retaining the imposed shape. These mirrors are composted of
commercial, off-the-shelf materials.
In this contribution we give an overview of the latest results achieved in the manufacturing of the pre-production series
of mirrors for the Medium Size and Small Size Telescopes of the Cherenkov Telescope Array observatory.
Production of thin glass mirrors by hot slumping for x-ray telescopes: present process and ongoing development
Show abstract
Thin glass foils are considered good candidates to build a segmented X-ray telescope with effective area as large as 2 m2
and angular resolution better than 5 arcsec. In order to produce thin glass mirror segments, we developed a direct hot
slumping technique assisted by pressure, in which the shape of a mould is replicated onto the optical surface of the glass.
In this paper we present the result obtained with AF32 (by Schott) and EAGLE XG (by Corning) glass types. The
selected mould material is Zerodur K20, as it does not require any anti-sticking layer and has a good matching, in terms
of Coefficient of Thermal Expansion, with both glass types. Our group already produced a few prototypes, reaching
angular resolution near 20 arcsec. In this work, relevant steps forward aimed at attaining a 5 arcsec angular resolution are
described, along with the tuning of few key parameters in the slumping process. The results obtained on a newly
procured cylindrical Zerodur K20 mould are presented.
An inverse-polished mirror for wavefront correction of space-based telescopes
K. Enya,
K. Haze,
Y. Chibu,
et al.
Show abstract
In this report we describe our development of a prototype inverse-polished mirror for the passive correction of the static
and predictable wavefront errors (WFE) of space-based telescopes, in particular, especially for infrared coronagraphs.
An artificial WFE pattern with a root mean square (rms) value of 350 nm was numerically generated to facilitate the
design of the prototype mirror. The surface of the mirror is approximately flat, is 50.0 mm in diameter and 15.0 mm
thick at the edge. The designed WFE pattern was constructed on the mirror surface by micro-polishing. Both the figure
and roughness of the mirror surface were evaluated. The rms value of the measured surface figure was reduced to 135
nm after subtraction of the designed surface figure. The benefit of subtraction to mid-infrared coronagraph performance
was simulated, which showed the contrast was improved by a factor of ~100 close to the core (closer than 10 λ/D where
λ and D are the wavelength and telescope aperture diameter, respectively) of the coronagraphic image of a point source.
An analysis of the power spectrum density shows that the lower frequencies in the WFE are well reproduced on the
mirror, while the higher frequencies remain due to the limitations imposed on the controllable spatial resolution by the
fabrication process. In this study, inverse-polished mirrors combined with deformable mirrors and their application to
ground-based telescopes are also discussed. To fully explore the potential of the inverse-polished mirror, a systematic
allocation of the error budget is essential taking into account not only the fabrication accuracy of the mirror but also an
evaluation of the telescope and other factors with non-predictable uncertainties.
Composite telescope technology
Show abstract
We report the development of optical mirrors based on polymer matrix composite materials. Advantages of this
technology are low cost and versatility. By using appropriate combinations of polymers and various metallic and
nonmetallic particles and fibers, the properties of the materials can be tailored to suit a wide variety of applications. We
report the fabrication and testing of flat and curved mirrors made with metal powders, multiple mirrors replicated with
high degree of uniformity from the same mandrels, cryogenic testing, mirrors made of ferromagnetic materials that can
be actively or adaptively controlled by non-contact actuation, optics with very smooth surfaces made by replication, and
by spincasting. We discuss development of a new generation of ultra-compact, low power active optics and 3D printing
of athermal telescopes.
Production of the 4.26 m ZERODUR mirror blank for the Advanced Technology Solar telescope (ATST)
Show abstract
The Daniel K. Inouye Solar Telescope (DKIST, formerly the Advanced Technology Solar Telescope, ATST)
will be the most powerful solar telescope in the world. It is currently being built by the Association of
Universities for Research in Astronomy (AURA) in a height of 3000 m above sea level on the mountain
Haleakala of Maui, Hawaii. The primary mirror blank of diameter 4.26 m is made of the extremely low thermal
expansion glass ceramic ZERODUR® of SCHOTT AG Advanced Optics.
The DKIST primary mirror design is extremely challenging. With a mirror thickness of only 78 to 85 mm it is
the smallest thickness ever machined on a mirror of 4.26 m in diameter. Additionally the glassy ZERODUR®
casting is one of the largest in size ever produced for a 4 m class ZERODUR® mirror blank. The off axis
aspherical mirror surface required sophisticated grinding procedures to achieve the specified geometrical
tolerance. The small thickness of about 80 mm required special measures during processing, lifting and
transport. Additionally acid etch treatment was applied to the convex back-surface and the conical shaped outer
diameter surface to improve the strength of the blank. This paper reports on the challenging tasks and the
achievements on the material property and dimensional specification parameter during the production of the
4.26 m ZERODUR® primary mirror blank for AURA.
A FEM coupling model for properties prediction during the curing of an epoxy adhesive for a novel assembly of radio telescope panel
Show abstract
The curing of epoxy adhesives is a complex phenomenon where the thermal, the chemistry and the mechanics are
coupled. Corresponding material properties such as mechanical and physics properties are evolving with the curing. This
paper focuses on their predictions by a multiphisics FEM approach of the thermal, chemical and mechanical couplings
involved by the curing for a novel assembly of radio telescope panel. The first part presents the constitutive model of an
epoxy adhesive that is considered for the curing. The numerical solving, performed with a specific user subroutine of
Ansys, is detailed and allows the study of real three-dimensional structure parts. Residual stresses and strains of different
metallic membranes and internal adhesives in the interconnect during the assembly of radio telescope panel are
investigated. The mechanical response of the interconnect is analyzed with respect to the poisson’s ratio, relaxation time
and adhesive thickness. It is shown that, although the overall residual stresses at the interconnect increase with the
adhesive curing, the local strains have different evolving trends, indicating the possibility of damage and decohesion that
might compromise mechanical integrity and interrupt the component processing precision.
FAME: Freeform Active Mirrors Experiment: manufacturing process development
Show abstract
Extreme freeform mirrors couple a non-axisymmetrical shape and an extreme asphericity, i.e. more than one millimeter
of deviation from the best fit sphere. In astronomical instrumentation, such a large asphericity allows compact
instruments, using less optical components. However, the lack of freeform mirrors manufacturing facilities is a real issue.
We present the concept and development of an innovative manufacturing process based on plasticity forming which
allow imprinting permanent deformations on mirrors, following a pre-defined mold. The aim of this activity, pursued in
the frame of the OPTICON-FAME (Freeform Active Mirrors Experiment) project, is to demonstrate the suitability of
this method for VIS/NIR/MIR applications. The process developed can operate on thin and flat polished initial
substrates. Three study cases have been highlighted by FEA (Finite Element Analysis) and the real tests associated were
performed on thin substrates in AISI420b stainless steel with 100 mm optical diameter. A comparison between FEA and
tests is performed to study the evolution of the mechanical behaviour and the optical quality. The opto-mechanical
results will allow a fine tuning of FEA parameters to optimize the residual form errors obtained through this process to
converge toward an innovative and recurrent process.
Mechanical design and analysis of focal plate for gravity deformation
Show abstract
The surface accuracy of astronomical telescope focal plate is a key indicator to precision stellar observation. To conduct
accurate deformation measurement for focal plate in different status, a 6-DOF hexapod platform was used for attitude
adjustment. For the small adjustment range of a classic 6-DOF hexapod platform, an improved structural arrangement
method was proposed in the paper to achieve ultimate adjustment of the focal plate in horizontal and vertical direction.
To validate the feasibility of this method, an angle change model which used ball hinge was set up for the movement and
base plate. Simulation results in MATLAB suggested that the ball hinge angle change of movement and base plate is
within the range of the limiting angle in the process of the platform plate adjusting to ultimate attitude. The proposed
method has some guiding significance for accurate surface measurement of focal plate.
Minimizing the bimetallic bending for cryogenic metal optics based on electroless nickel
Show abstract
Ultra-precise metal optics are key components of sophisticated scientific instruments in astronomy and space
applications. Especially for cryogenic applications, a detailed knowledge and the control of the coefficient of thermal
expansion (CTE) of the used materials are essential. Reflective optical components in IR- and NIR-instruments primarily
consist of the aluminum alloy Al6061. The achievable micro-roughness of diamond machined and directly polished
Al6061 does not fulfill the requirements for applications in the visible spectral range. Electroless nickel enables the
reduction of the mirror surface roughness to the sub-nm range by polishing. To minimize the associated disadvantageous
bimetallic effect, a novel material combination for cryogenic mirrors based on electroless nickel and hypereutectic
aluminum-silicon is investigated. An increasing silicon content of the aluminum material decreases the CTE in the
temperature range to be considered. This paper shows the CTE for aluminum materials containing about 42 wt% silicon
(AlSi42) and for electroless nickel with a phosphorous content ranging from 10.5 to 13 %. The CTE differ to about
0.5 × 10-6 K-1 in a temperature range from -185 °C (LN2) to 100 °C. Besides, the correlations between the chemical
compositions of aluminum-silicon materials and electroless nickel are shown. A metrology setup for cryo-interferometry
was developed to analyze the remaining and reversible shape deviation at cryogenic temperatures. Changes could be
caused by different CTE, mounting forces and residual stress conditions. In the electroless nickel layer, the resulting
shape deviation can be preshaped by deterministic correction processes such as magnetorheological finishing (MRF) at
room temperature.
New developments in rotating and linear motion mechanisms used in contamination sensitive space telescopes
Show abstract
We have been developing a rotating mechanism and a linear motion mechanism for their usage in contamination
sensitive space telescopes. They both are needed for ~1.4 meter optical telescope and its focal plane instrument onboard
SOLAR-C, the next-generation spaceborne solar observatory following Hinode. Highly reliable long life performance,
low outgassing properties, and low level of micro-vibration are required along with their scientific performance. With
the proto-type mechanisms, the long life performance and outgassing properties of the mechanisms have been evaluated
in vacuum chambers. The level of micro-vibration excited during the operations of the rotating mechanism was
measured by operating it on the Kestler table. This paper provides the overall descriptions of our mechanism
developments.
A good attitude towards improved space telescope observations
Show abstract
The quality of space telescope observations greatly depends on the pointing performance of the spacecraft. In
this paper, recent advances in star tracker algorithms are discussed. This paper discusses efficient star tracker
algorithms that improve the pointing performance of the satellite, resulting in observations of higher quality.
Furthermore, the greatly reduced computational cost of these algorithms facilitates the inclusion of astronomical
payload measurements in the attitude determination and control loop. When the payload is used as an additional
star tracker, the pointing performance of the spacecraft increases drastically, which in its turn improves the quality
of the scientific measurements. Simulations show that with these improvements, the absolute pointing error of
the spacecraft can be reduced considerably.
Analysis, testing, and control of telescope's high-precision drive system in low-temperature environment
Show abstract
Antarctic is perfect site for astronomic observatory. But Antarctic also challenge the telescope design because of low temperature. The low temperature can impact characterization of telescope control system, especially for drive system. The following phenomenon can be produced due to low temperature. 1. The viscosity of grease will increase. 2. The clearance of bearing and gear will decrease. These two factors can lead to the increase in load torque of drive system with temperature drop. This would cause the bad tracking accuracy and low speed creeping. In order to overcome the impact of low temperature and improve the telescope’s track accuracy. In this paper, we describe some methods to overcome the effect of low temperature. First, the motor’s electromagnetism and lubrication in low temperature are analyzed. It shows that motor’s electromagnetism is little affected by temperature if the suitable material is selected. But the characterization of grease change dramatically with temperature. Second, the other lubricant material, solid lubricant, instead of lubricating grease is proposed. Contrasting experiment on two lubricant material proved that the solid lubricant is better than lubricating grease in low temperature environment. Third, besides the mechanical solution, a method from control point view is proposed to reduce the temperature influence. In this paper, the friction feedforward algorithm is used to compensate the torque change. Laboratory testing results will be presented verifying that friction feedforward can increase the tracking accuracy in low temperature environment.
Low vibration cooling using a pulse tube cooler and cryostat for the GRAVITY beam combiner instrument at the VLTI
Show abstract
GRAVITY is a second generation VLTI instrument for high-precision narrow-angle astrometry and phase-referenced
interferometric imaging in the astronomical K-band. The cryostat of the beam combiner instrument provides the required
temperatures for the various subunits ranging from 40K to 290K with a milli-Kelvin temperature stability for some
selected units. The bath cryostat is cooled with liquid nitrogen and makes use of the exhaust gas to cool the main optical
bench to an intermediate temperature of 240K. The fringe tracking detector will be cooled separately by a single-stage
pulse tube cooler to a temperature of 40K. The pulse tube cooler is optimized for minimum vibrations. In particular its
warm side is connected to the 80K reservoir of the LN2 cryostat to minimize the required input power. All temperature
levels are actively stabilized by electric heaters. The cold bench is supported separately from the vacuum vessel and the
liquid nitrogen reservoir to minimize the transfer of acoustic noise onto the instrument.
The LSST camera 500-watt -130 degC mixed refrigerant cooling system
Gordon B. Bowden,
Brian J. Langton,
William A. Little,
et al.
Show abstract
The LSST Camera has a higher cryogenic heat load than previous CCD telescope cameras due to its large size (634 mm
diameter focal plane, 3.2 Giga pixels) and its close coupled front-end electronics operating at low temperature inside the
cryostat. Various refrigeration technologies are considered for this telescope/camera environment. MMR-Technology’s
Mixed Refrigerant technology was chosen. A collaboration with that company was started in 2009. The system, based on
a cluster of Joule-Thomson refrigerators running a special blend of mixed refrigerants is described. Both the advantages
and problems of applying this technology to telescope camera refrigeration are discussed. Test results from a prototype
refrigerator running in a realistic telescope configuration are reported. Current and future stages of the development
program are described.
Grism cryogenic mount for the Euclid-NISP mission
Show abstract
The spectroscopic channel of the Euclid Near Infrared SpectroPhotometer (NISP) relies on four grisms mounted on a
wheel via Invar mounts. The mount design was studied to maintain the optical performances and alignment at cryogenic
operating temperature (120K), and to survive launch vibrations. We designed two stages of radially compliant blades:
one set of 9 blades is bonded to the Silica grism and the second set of 3 blades is located at interface points with the
wheel. Severe packaging and mass constraints yielded us to design a ring mount with strong weight relief. In fall 2013
we proceeded to thermal cycling (323K-105K), vibration tests (10.7 g rms) to successfully qualify the grism mount in the
Euclid environment. Thanks to detailed finite element analyses, we correlated simulations and tests.
Conceptual design of a cryogenic pupil mechanism with continuous complex movements for HARMONI
Show abstract
In order to improve the signal-to-noise ratio of HARMONI (E-ELT first light visible and near-infrared integral field VIR
spectrometer), a pupil mask has been identified to be included at the fore-optics to limit the background radiation coming
into the spectrographs. This mask should rotate synchronously with the telescope pupil during observations, taking into
account the combined effects of the telescope tracking and the de-rotation of the FOV. The implementation of the pupil
mask functionality will require complex movements with high precision at cryogenic temperatures which implies an
important technological challenge.
This paper details a set of experiments completed to gain knowledge and experience in order to accomplish the design
and control of cryogenic mechanisms reaching this type of pupil motion. The conceptual design of the whole mechanism
started from the feedback acquired from those experiments is also described in the following sections.
The mechanical design of CHARIS: an exoplanet IFS for the Subaru Telescope
Show abstract
Princeton University is designing and building an integral field spectrograph (IFS), the Coronagraphic High Angular
Resolution Imaging Spectrograph (CHARIS), for integration with the Subaru Corona Extreme Adaptive Optics
(SCExAO) system and the AO188 adaptive optics system on the Subaru Telescope. CHARIS and SCExAO will
measure spectra of hot, young Jovian planets in a coronagraphic image across J, H, and K bands down to an 80
milliarcsecond inner working angle. Here we present the current status of the mechanical design of the CHARIS
instrument.
Optimized analysis of geometry parameters for honeycomb sandwich mirror
Show abstract
The relationship of geometry parameters, specific stiffness, surface figure and natural frequency was investigated based
on modified Gibson theory, sandwich theory, Hoff theory and vibration theory. By theoretical analysis and finite element
method, we demonstrated the geometric parameters had non-linear influence on dimensionless specific stiffness in
different directions with the honeycomb core was equivalent as modified solid material. Approximate expressions of
deformation, natural frequency and geometric parameters were obtained. The results showed the optimal solidity ratio
and face plate thickness ratio were in the range of 0.03 ~ 0.1 and 0.02 ~0.05, respectively.
Using the DP-190 glue for adhesive attachment of a large space mirror and its rim
Show abstract
The glue DP-190 is widely used for adhesive attachment of astrositall (zerodur) lightweight large-size space
astronomical mirrors (diameter of 1.7 m and more) with elements of their frames of invar. Peculiarities of physicalmechanical
behavior of the glue DP-190 when exposed to the environment during the ground operation and in orbit
cause instability of the reflective surface quality of mirrors. In this report we show that even a small (around 1%-5%)
volumetric deformation of a cylindrical adhesive layer with a thickness of 0.8 mm between the mirror and the rim
element causes significant mirrors deformation. We propose to use adhesive layer of special form that allows to reduce
volumetric deformations of the glue DP-190 up to three times. Here we present results based on primary mirror tests of
the WSO-UV project.
The servo control system of KDUST telescope
Show abstract
The KDUST telescope would be installed in Antarctic Dome A, where is extremely cold, high, dry, but have a very stable,
calm atmosphere for astronomical observation. According to project requirement, the position following error should be
less than 1''. To achieve project target, a direct drive method is used in the project. Normal PID control algorithm is used
in controller. It can meet the target in the room temperature. But the following error increased too significantly in the
cryogenic environment. In this paper, the expert PID algorithm is applied to control system. The control parameter can
be adjusted by amplitude and variation of following error. Experiment proved that expert PID has an obvious advantage
in both start-up and tracking process under different temperature. Moreover expert PID also can improve the stability of
whole system.
Tests characterization and alignment for the optics of the ASTRI SST-2M telescope prototype for the Cherenkov Telescope Array
Show abstract
ASTRI SST-2M is a prototype dual mirror Small Size class of Telescope for the Cherenkov Telescope Array (CTA). Its
innovative design based on a Schwarzschild-Couder configuration will permit use Silicon Photo-multipliers as focalplane
detectors. The dual mirror configuration is a challenge for the realization of both the primary and secondary.
Accurate tests and characterization are mandatory to understand the behavior of the optical configuration, its limits and
the possibility of improvements of the full CTA array. Moreover, optical alignment requires solutions and procedures
that have not been used so far on Cherenkov telescopes. The aim of paper is to provide an analysis of these topics in the
context of the ASTRI SST-2M telescope.
The optical AIV of the infrared multi-slit spectrophotometer for GTC, EMIR
Show abstract
EMIR is a second generation GTC instrument. EMIR's external structure is a huge cylindrical vacuum chamber of 2.2m
length and 1.8m in diameter with a weight of around 5 tons, with the instrument inside on a floating optical bench at the
LN2 temperature in a cryogenicaly poumped system. The instrument is mounted on the optical bench over a double cold
base.
The verification of the optical system and its alignment on the optical bench, as well as the alignment of the optical axis
with respect to the vacuum chamber and the whole set to the rotator, are the critical tasks that will validate the feasibility
and functionality of this instrument.
Experimental validation of Advanced Dispersed Fringe Sensing (ADFS) algorithm using Advanced Wavefront Sensing and Correction Testbed (AWCT)
Show abstract
Large aperture telescope commonly features segment mirrors and a coarse phasing step is needed
to bring these individual segments into the fine phasing capture range. Dispersed Fringe Sensing
(DFS) is a powerful coarse phasing technique and its alteration is currently being used for JWST.
An Advanced Dispersed Fringe Sensing (ADFS) algorithm is recently developed to improve the
performance and robustness of previous DFS algorithms with better accuracy and unique
solution. The first part of the paper introduces the basic ideas and the essential features of the
ADFS algorithm and presents the some algorithm sensitivity study results. The second part of the
paper describes the full details of algorithm validation process through the advanced wavefront
sensing and correction testbed (AWCT): first, the optimization of the DFS hardware of AWCT
to ensure the data accuracy and reliability is illustrated. Then, a few carefully designed algorithm
validation experiments are implemented, and the corresponding data analysis results are shown.
Finally the fiducial calibration using Range-Gate-Metrology technique is carried out and a
<10nm or <1% algorithm accuracy is demonstrated.
Field application of moment-based wavefront sensing to in-situ alignment and image quality assessment of astronomical spectrographs: results and analysis of aligning VIRUS unit spectrographs
Show abstract
Teague introduced a phase retrieval method that uses the image shape moments. More recently, an independent
study arrived at a similar technique, which was then applied to in-situ full-field image-quality evaluation of
spectroscopic systems. This moment-based wavefront sensing (MWFS) method relies on the geometric relation
between the image shape moments and the geometric wavefront modal coefficients. The MWFS method allows
a non-iterative determination of the modal coefficients from focus-modulated images at arbitrary spatial resolutions.
The determination of image moments is a direct extension of routine centroid and image size calculation,
making its implementation easy. Previous studies showed that the MWFS works well in capturing large low-order
modes, and is quite suitable for in-situ alignment diagnostics. At the Astronomical Instrumentation conference
in 2012, we presented initial results of the application of the moment-based wavefront sensing to a fiber-fed astronomical
spectrograph, called VIRUS (a set of replicated 150 identical integral-field unit spectrographs contained
in 75 unit pairs). This initial result shows that the MWFS can provide accurate full-field image-quality assessment
for efficiently aligning these 150 spectrographs. Since then, we have assembled more than 24 unit pairs
using this technique. In this paper, we detail the technical update/progress made so far for the moment-based
wavefront sensing method and the statistical estimates of the before/after alignment aberrations, image-quality,
and various efficiency indicators of the unit spectrograph alignment process.
Hybrid phase retrieval using moment-based wavefront sensing and Gerchberg-Saxton iterative transform method
Show abstract
The iterative phase retrieval method has the intrinsic weakness in computational speed. However, its ability
to capture fine spatial phase structure is clearly the benefit much needed in characterizing image quality of an
instrument in an end-to-end fashion. The empirical wisdom of any iterative process is that the convergence
becomes a lot faster when the initial guess is close to the phase solution. As an adaptation of this wisdom, we
present the hybrid phase retrieval method where phase retrieval is conducted in combination of the moment-
base wavefront sensing (MWFS) method and the Gerchberg-Saxton (GS) type iterative transform method. The
moment-based method captures the large low-order phase based on the linear relation between the modal phase
coefficients and the focal plane image moments. The MWFS estimate is then fed to the GS iterative method
as the initial phase guess and diversity. At each GS iteration, the estimated phase is updated to the phase
diversity. The iteration continues until the phase update is smaller than a pre-defined limit. For coarse spatial
resolution systems, the MWFS estimate can be sufficient to determine the phase, while the hybridization with
the GS process permits capturing much finer scale phase structures for systems requiring diffraction-scale spatial
resolution. A case study is presented.
Experiences with global laser tracker alignment of the 32.5-m LMT primary surface
Show abstract
The Large Millimeter Telescope Alfonso Serrano (LMT) currently has a primary reflector of 32.5m diameter composed
of 84 panels, each having a surface area of approximately 10 square meters. Each panel is supported on four electromechanical
actuators, allowing for the correction of tip-tilt, piston and twist. The actuators are designed to perform active
surface compensation of gravity deformations as a function of elevation.
Following the setting and installation of individual panels, an approximation for global alignment of the primary surface
is carried out using a total station. An RMS error of 200 - 500μm is expected for this process. Final global alignment is
conducted using holography at 12GHz for elevations corresponding to the location of geostationary sources. As an
intermediate alignment option for the antenna at zenith, the use of a laser tracker has been explored. Global alignment of
a large primary surface with a laser tracker presents the common problems related to the contact measurement of a large
object in a non-metrology environment. Key issues are the stable location of fiducial points and the relatively slow data
collection rate. Additionally the high altitude site (4600m, 15000ft) with mean temperatures around zero degrees Celsius,
presents a challenge for our interferometer-equipped trackers.
In this paper we present first results using a tracker located near the antenna vertex, and mechanical adjusters in place of
actuators. An RMS error of around 100μm was achieved. Limiting factors included inadequate fiducials and slow
mapping speed. Proposals for reduced data collection times and improved metrology robustness are presented.
Metrology and surface figuring of the LMT secondary mirror
Show abstract
The Large Millimeter Telescope Alfonso Serrano (LMT) is a 50-meter (currently 32m) diameter single-dish telescope optimized for astronomical observations at millimeter wavelengths in the range 0.85 mm < λ < 4 mm. During initial operation, the LMT makes use of the central 1.7 meters of a 2.5m hyperbolic secondary reflector constructed of cast and machined aluminum. Following the first light campaign in 2011, a program of iterative surface sanding was carried out to reduce the surface error of the central area to a level compatible with that presently achieved for the primary reflector. Metrology during the sanding process was conducted using a Leica laser tracker. A total of 22 sanding iterations were interspersed with tracker measurements at differing spatial resolutions, allowing the RMS surface error to be reduced from 63 to 35 microns. Maps for the final iterations were repeated for distinct scan patterns to check for systematic variance. Since the work was carried out in early 2013, repeat measurements of the dismounted secondary have confirmed the stability of this reflector.
In this paper we present details of the surface improvement program with emphasis on the metrology techniques used throughout the process. We discuss issues such as data sampling, measurement geometry, and mirror orientation. We also consider the steps taken to ensure tight control of the sanding task itself, since this process was carried out entirely by hand. Finally we present some comparative metrology results obtained using our laser tracker and photogrammetry equipment.
Metrology and surface adjustment of primary reflector panels on the LMT
Show abstract
The primary reflector of the Large Millimeter Telescope (LMT) Alfonso Serrano is presently composed of 84 surface panels arranged in three concentric rings, providing a 32.5 meter collecting area. Each panel comprises 8 precision composite subpanels having electro-formed nickel skins bonded to an aluminum honeycomb core. Differential thread adjusters beneath each subpanel allow for the manual removal of tip/tilt and piston errors, in addition to facilitating some fine tuning of the surface shape. An assembled panel provides a surface area of approximately 8-12 square meters.
Preparation of surface panels in 2012 and 2013 for Early Science observations made use of a Leica laser tracker. Measurement and adjustment of panels was carried out off the antenna, achieving a mean panel RMS surface error of 29.5μm for the 67 panels processed to date, with a spread of 23-37μm. A panel stability check consisting of surface walk-on tests and repeat metrology resulted in an increase in the mean surface error to 31.0μm. Following installation, in situ tracker measurements of 19 panels showed a final mean error of 45.3μm. Panels are adjusted by hand using an iterative process. In-house data processing uses fiducial marks scribed onto the subpanel molds and replicated during manufacture, to achieve accurate registration of the surface point cloud during data fitting. The number of iterations varies, depending mainly on the behavior of the differential adjusters. A well-behaved panel may be set within around 7 hours. In this paper we describe the iterative panel surface adjustment process used to date. We focus on metrology technique and data processing using the laser tracker, and present comparisons with trial photogrammetry measurements.
Data processing for fabrication of GMT primary segments: raw data to final surface maps
Show abstract
The Giant Magellan Telescope (GMT) primary mirror is a 25 meter f/0.7 surface composed of seven 8.4 meter circular
segments, six of which are identical off-axis segments. The fabrication and testing challenges with these severely
aspheric segments (about 14 mm of aspheric departure, mostly astigmatism) are well documented. Converting the raw
phase data to useful surface maps involves many steps and compensations. They include large corrections for: image
distortion from the off-axis null test; misalignment of the null test; departure from the ideal support forces; and
temperature gradients in the mirror. The final correction simulates the active-optics correction that will be made at the
telescope. Data are collected and phase maps are computed in 4D Technology's 4SightTM software. The data are saved
to a .h5 (HDF5) file and imported into MATLAB® for further analysis. A semi-automated data pipeline has been
developed to reduce the analysis time as well as reducing the potential for error. As each operation is performed, results
and analysis parameters are appended to a data file, so in the end, the history of data processing is embedded in the file.
A report and a spreadsheet are automatically generated to display the final statistics as well as how each compensation
term varied during the data acquisition. This gives us valuable statistics and provides a quick starting point for
investigating atypical results.
Metrology system for the calibration of multi-dof mechanisms
Show abstract
This paper presents a novel absolute position metrology system developed in our institute based on a concept using industrial vision by which USB cameras observe targets provided with special dots patterns. The system was originally devised for precision 2D measurements, then extended to 6-degree-of-freedom setups. This particular metrology system has been developed for testing and calibrating the precision hexapods aligning the secondary mirrors of the ESO VLTI auxiliary telescopes but its principle can be used for measuring the accuracy of any multi-degree-of-freedom mechanisms. The accuracy/resolution of the metrology system is typically 2-5 μm along linear degrees of freedom, respectively 5 arcsec for tip-tilt. This method is particularly affordable in cost, robust, yet accurate enough for most precision measurements in astronomical optomechanics.
Efficient diagnosis of radiotelescopes from far-field data
Amedeo Capozzoli,
Claudio Curcio,
Giuseppe D'Elia,
et al.
Show abstract
The paper presents an innovative method for the diagnosis of reflector antennas in radio astronomical applications,
which optimizes the number and the location of the far field sampling points exploited to retrieve the antenna status in
terms of feed misalignments. In this way the measurement time length process is drastically reduced to minimize the
effects of the time variations of the measurement setup, as well as the idle time forced by the maintenance activity. The
effects of the feed misalignment are modeled in terms of an aberration function, properly expanded on a set of basis
functions in order to preserve the linear relationship between the unknown parameters defining the antenna status and the
far field pattern, assumed measured in amplitude and phase. Thanks to the optimization of the Singular Values behavior
of the relevant linear operator, in its discrete form, the number and the position of the samples is found. The numerical
analysis shows the effectiveness of the method in the simple case of a phase aperture affected by tilt only, even if the
approach can be extended also to higher order aberrations. The performances are estimated with a comparison with a
standard approach, based on the acquisition of the far field pattern by means of a uniform Cartesian grid defined
according the Nyquist criterion and requiring a number of field samples significantly larger.
Precise topographic surface measurements of warm and cold large image detectors for astronomical instrumentations
Show abstract
This paper describes ESO’s surface measurement device for large image detectors in astronomy. The machine was equipped with a sub-micrometer laser displacement sensor and is fully automated with LabView. On the example of newly developed curved CCDs, which are envisaged for future astronomical instruments, it was demonstrated that this machine can exactly determine the topographic surfaces of detectors. This works even at cryogenic temperatures through a dewar window. Included is the calculation of curvature radii from these cold curved CCDs after spherical fitting with MATLAB. In addition (and interesting for calibration of instruments) the micro-movements of the detector inside the cryostat are mapped.
CARMENES ultra-stable cooling system: very promising results
Show abstract
CARMENES is a high resolution spectrograph to detect planets through the variation of radial velocity, destined for the
Calar Alto Observatory in Almeria, Spain. The optical bench has a working temperature of 140K with a 24 hours
stability of ±0,1K; goal ±0,01K. It is enclosed with a radiation shield actively cooled with thermalized nitrogen gas that
flows through strategically positioned heat exchangers to remove its radiative load. The cooling system has an external
preparation unit (N2GPU), which provides the nitrogen gas through actively vaporizing liquid nitrogen with heating
resistances and a three stage circuit flow, each one controlled by an independent PID.
Since CARMENES is still in the construction phase, a dedicated test facility has been built in order to simulate the
instrument and correctly establish the N2GPU parameters. Furthermore, the test facility allows a wide range of
configurations set-ups, which enables a full characterization of the N2GPU and the cooling system.
The N2GPU has been designed to offer a wide temperature range of thermally stabilized nitrogen gas flow, which apart
from CARMENES could also be used to provide ultra-high thermal stability in other cryogenic instruments. The present
paper shows the testing of the cooling performance, the hardware used and the very promising results obtained.
Cryogenic optical mounting for short-wave infrared spectrometers
Show abstract
In order to measure atmospheric concentrations of carbon monoxide, methane, water and carbon dioxide from spaceborne
platforms, Short-Wave Infrared (SWIR) immersed grating spectrometers are employed. Due to the need to
minimise detector dark current and internal black body radiation from the spectrometer’s own structure, these
instruments are operated at cryogenic temperatures. ESA’s Sentinel 5-Precursor is a small satellite science mission; the
platform comprises the Tropospheric Monitoring Instrument (TROPOMI), which includes a SWIR module.
Optical mounts have been developed for the SWIR module which meet the requirements to cope with the differences in
thermal expansion between the optical elements and their structural mounts over cryogenic temperature ranges, be robust
against the mechanical environment during launch, and maintain optical alignment stability with a tight volume
constraint.
Throughout the design of the SWIR spectrometer, flexures were deployed to control deformations due to thermal
expansion, the design of interfaces between materials of differing coefficient of thermal expansion was carefully
managed, and the geometry of adhesive pads was tightly controlled. Optical mounting concepts were evaluated using
finite element analysis (FEA). A breadboard programme was undertaken to verify these concepts. FEA and breadboard
results were correlated to provide confidence in the design.
The breadboard programme consisted of thermal cycling and pull-testing of adhesive joints, as well as environmental
and optical testing of representative subsystems.
Analysis and breadboarding demonstrated that the optical mounting design will survive the mechanical and thermal
environments, and verified the stability of the optical alignment requirements.
Novel optical mounting structures have been designed, analysed, assembled, tested and integrated into the optical
assemblies of the TROPOMI SWIR spectrometer, creating a compact and robust state of the art instrument. These
concepts are applicable to instruments for astronomical missions aiming to characterise exoplanets, as well as Earth
observation missions.
Extremely low-temperature properties of silicone compound used for thermal coupling in cryostat of SWIR/ASTER on TERRA
Show abstract
The SWIR(Short Wave-length Infrared Radiometer) is one of the optical sensors in ASTER(Advanced Space-borne
Thermal Emission and Reflection Radiometer). ASTER is installed in the EOS(Earth Observing System) TERRA
spacecraft of NASA. TERRA was launched on December18, 1999, and is employed still on the orbit for 14 years in
January, 2014, The detector of SWIR is cooled at temperature 77K by cryocooler with the optimum sensitivity. SWIR
had continued to take the numerous image data for more than five years of the mission period on orbit, and the
cryocooler is still operating normally. However, a gradual rise in temperature of the detector has been seen after launch.
Silicone compound have been used in order to achieve heat transfer between the detector and the cryocooler. On
investigation, we have found that thermal conductivity of the silicone compound has been gradually reduced. We
evaluated the low temperature properties (such as thermal conductivity, strength etc.) of the silicone compound. In
addition, we analyzed the temperature conditions and the thermal stress values of cryostat in the orbit. As a result, the
silicone compound solidified at low temperature shows a behavior similar to adhesive. Depending on the thermal stress
generated at a low temperature, there is a possibility that destruction such as peeling occurs.
Design of the KOSMOS oil-coupled spectrograph camera lenses
Show abstract
We present the design details of oil-coupled lens groups used in the KOSMOS spectrograph camera. The oil-coupled
groups use silicone rubber O-rings in a unique way to accurately center lens elements with high radial and axial stiffness
while also allowing easy assembly. The O-rings robustly seal the oil within the lens gaps to prevent oil migration. The
design of an expansion diaphragm to compensate for differential expansion due to temperature changes is described.
The issues of lens assembly, lens gap shimming, oil filling and draining, bubble mitigation, material compatibility,
mechanical inspection, and optical testing are discussed.
Development of the camera for the large size telescopes of the Cherenkov Telescope Array
Show abstract
The Large Size Telescopes, LSTs, located at the center of the Cherenkov Telescope Array, CTA, will be sensitive
for low energy gamma-rays. The camera on the LST focal plane is optimized to detect low energy events based
on a high photon detection efficiency and high speed electronics. Also the trigger system is designed to detect
low energy showers as much as possible. In addition, the camera is required to work stably without maintenance
in a few tens of years. In this contribution we present the design of the camera for the first LST and the status
of its development and production.
Restraint deformation and corrosion protection of gold deposited aluminum mirrors for cold optics of mid-infrared instruments
Show abstract
We report the restraint deformation and the corrosion protection of gold deposited aluminum mirrors for mid-infrared
instruments. To evaluate the deformation of the aluminum mirrors by thermal shrinkage, monitoring measurement of the
surface of a mirror has been carried out in the cooling cycles from the room temperature to 100 K. The result showed
that the effect of the deformation was reduced to one fourth if the mirror was screwed with spring washers.
We have explored an effective way to prevent the mirror from being galvanically corroded. A number of samples have
been prepared by changing the coating conditions, such as inserting an insulation layer, making a multi-layer and overcoating
water blocking layer, or carrying out precision cleaning before coating. Precision cleaning before the deposition
and protecting coat with SiO over the gold layer seemed to be effective in blocking corrosion of the aluminum. The SiO
over-coated mirror has survived the cooling test for the mid-infrared use and approximately 1 percent decrease in the
reflectance has been detected at 6-25 microns compared to gold deposited mirror without coating.
ZnSe immersion grating in the short NIR region
Show abstract
ZnSe has a high refractive index (n~ 2.45) and low optical loss (< 0.1/cm) from 0.8 to 12 um. Therefore ZnSe immersion
gratings can enable high-resolution spectroscopy over a wide wavelength range. We are developing ZnSe immersion
gratings for a ground-based NIR high-resolution spectrograph WINERED. We previously produced a large prism-shaped
ZnSe immersion grating with a grooved area 50 mm x 58 mm (Ikeda et al. 2010). However, we find two problems as
NIR immersion grating: (i) serious chipping of the grooves, and (ii) inter-order ghosts in the diffraction pattern. We
believed the chipping to be due to micro cracks just beneath surface present prior to diamond machining. Therefore we
removed this damaged region, a few tens of microns thick, by etching the ZnSe grating blank with a mixture of HCl and
HNO3. Ghosts appearing halfway between main diffraction orders originate from small differences in spacing between
odd and even grooves. Apparently the blank shifts repeatably by about 120 nm in the direction orthogonal to the grooves
depending on whether the translation stage holding the blank is moving right to left or left to right. Therefore we remachined
the grating only cutting grooves with the stage moving from right to left. After re-cutting, we also deposit the
Cu coating with an enhanced interface layer of SiO2 on the groove, which is developed in our previous study. We
evaluated the optical performances of this immersion grating. It shows light scattering of 3.8 % at 1μm, no prominent
ghosts, and a spectral resolution of 91,200 at 1 μm. However we measured an absolute diffraction efficiency of only
27.3% for TE and 25.9 % for TM waves at 1.55 μm. A non-immersed measurement of the diffraction efficiency of the
facet blazed near 20º exceeded 60%, much closer to theoretical predictions. We plan to carry out more tests to resolve
this discrepancy.
Polish device for FOCCoS/PFS slit system
Show abstract
The Fiber Optical Cable and Connector System, ”FOCCoS”, for the Prime Focus Spectrograph, “PFS”, is responsible
for transporting light from the Subaru Telescope focal plane to a set of four spectrographs. Each spectrograph will be fed
by a convex curved slit with 616 optical fibers organized in a linear arrangement. The slit frontal surface is covered with
a special dark composite, made with refractory oxide, which is able to sustain its properties with minimum quantities of
abrasives during the polishing process; this stability is obtained This stability is obtained by the detachment of the
refractory oxide nanoparticles, which then gently reinforce gently the polishing process and increase its the efficiency.
The surface roughness measured in several samples after high performance polishing was about 0.01 microns.
Furthermore, the time for obtaining a polished surface with this quality is about 10 times less than the time required for
polishing a brass, glass or ceramic surface of the same size. In this paper, we describe the procedure developed for high
quality polishing of this type of slit. The cylindrical polishing described here, uses cylindrical concave metal bases on
which glass paper is based. The polishing process consists to use grid sequences of 30μm, 12μm, 9μm, 5μm, 3μm, 1μm
and, finally, a colloidal silica on a chemical cloth. To obtain the maximum throughput, the surface of the fibers should
be polished in such a way that they are optically flat and free from scratches. The optical fibers are inspected with a
microscope at all stages of the polishing process to ensure high quality. The efficiency of the process may be improved
by using a cylindrical concave composite base as a substrate suitable for diamond liquid solutions. Despite this process
being completely by hand, the final result shows a very high quality.
Towards freeform microlens arrays for near infrared astronomical instruments
Show abstract
A key requirement for astronomical instruments in next generation Extremely Large Telescopes (ELTs) is the
development of large-aperture Integral Field Units (IFUs) that enable the efficient and spatially contiguous sampling of
the telescope image plane for coupling stellar light onto a spectrometer. Current IFUs are complex to fabricate and suffer
from stray light issues, which limits their application in high-contrast studies such as exoplanet imaging. In this paper,
we present our work on the development of freeform microlens arrays using the rapidly maturing technique of ultrafast
laser inscription and selective wet chemical etching. Using the focus spot from a femtosecond laser source as a tool with
an essentially unrestricted “tool-path”, we demonstrate that it is possible to directly write the surface of a lenslet in three
dimensions within the volume of a transparent material. We further show that high surface quality of the lenses can be
achieved by using an oxy-natural gas flame to polish the lens surface roughness that is characteristic of the post-etched
structures. Using our technique, the shape and position of each lenslet can be tailored to match the spatial positioning of
a two-dimensional multimode fiber array, which can be monolithically integrated with the microlens array.
GRACES: Gemini remote access to CFHT ESPaDOnS spectrograph through the longest astronomical fiber ever made: experimental phase completed
Show abstract
The Gemini Remote Access to CFHT ESPaDONS Spectrograph has achieved first light of its experimental phase in May
2014. It successfully collected light from the Gemini North telescope and sent it through two 270 m optical fibers to the
the ESPaDOnS spectrograph at CFHT to deliver high-resolution spectroscopy across the optical region. The fibers gave
an average focal ratio degradation of 14% on sky, and a maximum transmittance of 85% at 800nm. GRACES achieved
delivering spectra with a resolution power of R = 40,000 and R = 66,000 between 400 and 1,000 nm. It has a ~8%
throughput and is sensitive to target fainter than 21st mag in 1 hour. The average acquisition time of a target is around 10 min. This project is a great example of a productive collaboration between two observatories on Maunakea that was
successful due to the reciprocal involvement of the Gemini, CFHT, and NRC Herzberg teams, and all the staff involved
closely or indirectly.
Interferometers and spectrographs on silicon-platform for astrophysics: trends of astrophotonics
Show abstract
We present results of comprehensive re-design of an arrayed waveguide grating (AWG)-based integrated photonic spectrograph (IPS), using Silica-on-Silicon (SOS) technology, to tailor specific performance parameters of interest to high-resolution (resolving power, R = λ/Δλ= 60,000) exoplanet astronomy and stellar seismology. The compactness, modularity, stability, replicability and small-lightweight-payload of the IPS are a few promising and innovative features in the design of high-resolution spectrographs for astronomy or other areas of sciences. The IPS is designed to resolve up to 646 spectral lines per spectral order, with a wavelength spacing of 25 pm, at a central wavelength of 1630 nm (Hband). The fabricated test waveguides have been stress engineered in order to compensate the inherent birefringence of SOS waveguides. The birefringence values of fabricated test structures were quantified, to be on the order 10-6 (theoretical value required to avoid the formation of ghost-images), through inscription of Bragg-gratings on straight waveguides and subsequent measurement of Bragg-reflection spectra. An interferometer system has been integrated with the SOS-IPS (in the same chip) for the characterization of phase errors of the waveguide array. Moreover, promising results of first fabricated key photonics components to form other complex integrated photonic circuits (IPCs), such as astro-interferometers, using silicon nitride-on-insulator (SNOI) technology are also presented. The fabricated IPCs include multimode interference based devices (power splitter/combiners, optical cross/bar-switches), directional-couplers with varying power ratios, Mach-Zehnder interferometers and an AWG. The first results of annealed, low-hydrogen SNOI based devices are promising and comparable to SOI and commercial devices, with device excess-loss less than 2 dB and under 1 dB/cm waveguide-loss in the IR-wavelength.
Development of a slicer integral field unit for the existing optical spectrograph FOCAS: progress
Show abstract
We are developing an integral field unit (IFU) with an image slicer for the existing optical spectrograph, Faint Object Camera And Spectrograph (FOCAS), on the Subaru Telescope. The slice width is 0.43 arcsec, the slice number is 23, and the field of view is 13.5 × 9.89 arcsec2. Sky spectrum separated by about 5.7 arcmin from an object field can be simultaneously obtained, which allows us precise background subtraction. Slice mirrors, pupil mirrors and slit mirrors are all glass, and their mirror surfaces are fabricated by polishing. Our IFU is about 200 mm × 300 mm × 80 mm in size and 1 kg in weight. It is installed into a mask storage in FOCAS along with one or two mask plates, and inserted into the optical path by using the existing mask exchange mechanism. This concept allow us flexible operation such as Targets of Opportunity observations. High reflectivity of multilayer dielectric coatings offers high throughput (>80%) of the IFU. In this paper, we will report a final optical layout, its performances, and results of prototyping works.
Astronomical near-infrared echelle gratings
Show abstract
High-resolution near-infrared echelle spectrographs require coarse rulings in order to match the free spectral range to the
detector size. Standard near-IR detector arrays typically are 2 K x 2 K or 4 K x 4 K. Detectors of this size combined
with resolutions in the range 30000 to 100000 require grating groove spacings in the range 5 to 20 lines/mm.
Moderately high blaze angles are desirable to reduce instrument size. Echelle gratings with these characteristics have
potential wide application in both ambient temperature and cryogenic astronomical echelle spectrographs. We discuss
optical designs for spectrographs employing immersed and reflective echelle gratings. The optical designs set constraints
on grating characteristics. We report on market choices for obtaining these gratings and review our experiments with
custom diamond turned rulings.
The instrument focal plane mask program at the Large Binocular Telescope
Show abstract
Facility Instruments at the Large Binocular Telescope (LBT) include two spectrograph pairs, the LBT
Near-IR Spectroscopic Utility with Camera and Integral Field Unit for Extragalactic Research (LUCI), a
near-infrared imager and spectrograph pair, and the Multi-Object Double Spectrograph (MODS), a pair of
dual-beam long-slit spectrographs. Both spectrograph designs utilize focal plane masks for long-slit and
multi-slit observations. This paper describes the mask configuration and specification process for each
instrument, as well as the steps in mask fabrication, handling, and installation.
Photonic lantern with cladding-removable fibers
Show abstract
Recently, spectral measurement becomes an important tool in astronomy to find exoplanets etc. The fibers are used
to transfer light from the focal plate to spectrometers. To get high-resolution spectrum, the input slits of the
spectrometers should be as narrow as possible. In opposite, the light spots from the fibers are circle, which diameters are
clearly wider than the width of the spectrometer slits. To reduce the energy loss of the fiber-guide star light, many kinds
of image slicers were designed and fabricated to transform light spot from circle to linear. Some different setup of fiber
slicers are introduced by different research groups around the world. The photonic lanterns are candidates of fiber slicers.
Photonic lantern includes three parts: inserted fibers, preform or tubing, taped part of the preform or tubing. Usually the
optical fields concentrate in the former-core area, so the light spots are not uniform from the tapered end of the lantern.
We designed, fabricated and tested a special kind of photonic lantern. The special fibers consist polymer cladding and
doped high-index core. The polymer cladding could be easily removed using acetone bath, while the fiber core remains
in good condition. We inserted the pure high-index cores into a pure silica tubing and tapered it. During the tapering
process, the gaps between the inserted fibers disappeared. Finally we can get a uniform tapered multimode fiber end. The
simulation results show that the longer the taper is, the lower the loss is. The shape of the taper should be controlled
carefully. A large-zone moving-flame taper machine was fabricated to make the special photonic lantern. Three samples
of photonic lanterns were fabricated and tested. The lanterns with cladding-removable fibers guide light uniform in the
tapered ends that means these lanterns could collect more light from those ends.
Slit device for FOCCoS-PFS-Subaru
Show abstract
The Fiber Optical Cable and Connector System, ”FOCCoS”, subsystem of the Prime Focus Spectrograph,
“PFS”, for Subaru telescope, is responsible to feed four spectrographs with a set of optical fibers cables. The light
injection for each spectrograph is assured by a convex curved slit with a linear array of 616 optical fibers. In this paper
we present a design of a slit that ensures the right direction of the fibers by using masks of micro holes. This kind of
mask is made by a technique called electroforming, which is able to produce a nickel plate with holes in a linear
sequence. The precision error is around 1-μm in the diameter and 1-μm in the positions of the holes. This nickel plate
may be produced with a thickness between 50 and 200 microns, so it may be very flexible. This flexibility allows the
mask to be bent into the shape necessary for a curved slit. The concept requires two masks, which we call Front Mask,
and Rear Mask, separated by a gap that defines the thickness of the slit. The pitch and the diameter of the holes define
the linear geometry of the slit; the curvature of each mask defines the angular geometry of the slit. Obviously, this
assembly must be mounted inside a structure rigid and strong enough to be supported inside the spectrograph. This
structure must have a CTE optimized to avoid displacement of the fibers or increased FRD of the fibers when the device
is submitted to temperatures around 3 degrees Celsius, the temperature of operation of the spectrograph. We have
produced two models. Both are mounted inside a very compact Invar case, and both have their front surfaces covered by
a dark composite, to reduce stray light. Furthermore, we have conducted experiments with two different internal
structures to minimize effects caused by temperature gradients.
This concept has several advantages relative to a design based on Vgrooves, which is the classical option. It is
much easier and quicker to assemble, much cheaper, more accurate, easier to adjust; and it also offers the possibility of
making a device much more strong, robust and completely miniaturized.
Temperature control system for optical elements in astronomical instrumentation
Show abstract
Extremely low temperatures may damage the optical components assembled inside of an astronomical instrument due to the crack in the resin or glue used to attach lenses and mirrors. The environment, very cold and dry, in most of the astronomical observatories contributes to this problem.
This paper describes the solution implemented at SOAR for remotely monitoring and controlling temperatures inside of a spectrograph, in order to prevent a possible damage of the optical parts. The system automatically switches on and off some heat dissipation elements, located near the optics, as the measured temperature reaches a trigger value. This value is set to a temperature at which the instrument is not operational to prevent malfunction and only to protect the optics. The software was developed with LabVIEWTM and based on an object-oriented design that offers flexibility and ease of maintenance.
As result, the system is able to keep the internal temperature of the instrument above a chosen limit, except perhaps during the response time, due to inertia of the temperature. This inertia can be controlled and even avoided by choosing the correct amount of heat dissipation and location of the thermal elements. A log file records the measured temperature values by the system for operation analysis.
Developing a new technology in the construction of fiber lenslet IFUs
Show abstract
In this paper we describe the recent advances in the development of new technologies applied in the
construction of Integral Field Units (IFUs) at Laboratório Nacional de Astrofísica (LNA). Our prototype is the
Eucalyptus lenslet IFU constructed for the 1.6m telescope at Pico dos Dias Observatory (OPD), Brazil. This first
concept was the basis to build two other IFUs with significantly improved concepts: the SOAR Integral Field Unit
Spectrograph (SIFS) and FRODOSPEC. All the new technologies used in the construction of these IFUs are described
in detail in this paper and can be replicated in similar instruments with optical fibers, with considerable advantages over
the traditional technologies.
Fiber optical cable and connector system (FOCCoS) for PFS/ Subaru
Show abstract
FOCCoS, "Fiber Optical Cable and Connector System" has the main function of capturing the direct light from the focal plane of Subaru Telescope using optical fibers, each one with a microlens in its tip, and conducting this light through a route containing connectors to a set of four spectrographs. The optical fiber cable is divided in 3 different segments called Cable A, Cable B and Cable C. Multi-fibers connectors assure precise connection among all optical fibers of the segments, providing flexibility for instrument changes. To assure strong and accurate connection, these sets are arranged inside two types of assemblies: the Tower Connector, for connection between Cable C and Cable B; and the Gang Connector, for connection between Cable B and Cable A. Throughput tests were made to evaluate the efficiency of the connections. A lifetime test connection is in progress. Cable C is installed inside the PFI, Prime Focus Instrument, where each fiber tip with a microlens is bonded to the end of the shaft of a 2-stage piezo-electric rotatory motor positioner; this assembly allows each fiber to be placed anywhere within its patrol region, which is 9.5mm diameter.. Each positioner uses a fiber arm to support the ferrule, the microlens, and the optical fiber. 2400 of these assemblies are arranged on a motor bench plate in a hexagonal-closed-packed disposition. All optical fibers from Cable C, protected by tubes, pass through the motors’ bench plate, three modular plates and a strain relief box, terminating at the Tower Connector. Cable B is permanently installed at Subaru Telescope structure, as a link between Cable C and Cable A. This cable B starts at the Tower Connector device, placed on a lateral structure of the telescope, and terminates at the Gang Connector device. Cable B will be routed to minimize the compression, torsion and bending caused by the cable weight and telescope motion. In the spectrograph room, Cable A starts at the Gang Connector, crosses a distribution box and terminates in a slit device. Each slit device receives approximately 600 optical fibers, linearly arrayed in a curve for better orientation of the light to the spectrograph collimator mirror. Four sets of Gang Connectors, distribution boxes and Slit devices complete one Cable A. This paper will review the general design of the FOCCoS subsystem, methods used to manufacture the involved devices, and the needed tests results to evaluate the total efficiency of the set.
Mechanical and thermal design challenges in building a semi-cold near infrared spectrograph: the Robert Stobie -Near Infrared Spectrograph for SALT
Show abstract
The near infrared upgrade to the Robert Stobie Spectrograph (RSS/NIR) for the Southern African Large Telescope
(SALT) extends the capabilities of the visible arm RSS into the Near Infrared (NIR). In order to extend into the NIR
range, the upgrade components of the instrument are required to be cooled. Thus the NIR arm is predominantly housed
in the instrument pre-dewar which is cooled to -40°C, at ambient pressure. The multiple modes, prime focus location and
partially cooled instrument introduce interesting engineering considerations. The NIR spectrograph has an ambient
temperature collimator, a cooled (-40°C) dispersers and camera and a cryogenic detector. The cryogenic dewar and
many of the mechanisms are required to operate within the cooled, atmospheric environment. Cooling the pre-dewar to -
40°C at prime focus of the telescope is also an engineering challenge. Mechanical and thermal aspects of the design are
addressed in this paper with a particular emphasis on the unique considerations of building a semi-warm infrared
spectrograph.
Polarization gratings for visible and near-infrared astronomy
Show abstract
We report on the development of polarization gratings that can be used for polarimetry and/or high throughput broadband spectroscopy in astronomy. Polarization gratings are able to overcome fundamental limitations on the diffraction efficiency of conventional gratings to provide near 100% diffraction efficiency over a broad bandwidth. The broad spectral coverage of these devices will be useful for observations of gamma-ray bursts and supernovae of unknown the redshift, where spectral features may fall over a range of wavelengths. As a spectropolarimeter a polarization grating would be ideal, for example, for the study of dusts and hazes, whose polarimetric properties vary with wavelength. We present the results of a series of laboratory measurements of the diffraction efficiency and modulation efficiency of a prototype grating designed for operation from 500 to 900 nm. We find that the grating is able to achieve greater than 90% diffraction efficiency from 500 to 850 nm and modulate incident circular polarized light with an efficiency of ~ 99%. Our future plans include on-sky testing at a small local telescope, with an eventual goal of incorporating a polarization grating into the design of a microshutter array- based multi-object visible/NIR spectrograph for a 10m class facility.
A comparison of methods for the reduction of fiber modal noise in high-resolution spectrographs
Show abstract
The use of optical fibers to couple spectrographs to telescopes has been important in the search for extrasolar planets using radial velocity measurements. The ability of an optical fiber to partially scramble the input illumination enables a fiber feed to provide more uniform illumination to the spectrograph optics, but a limiting factor in fiber coupling is modal noise. Agitation of the fiber has been shown to reduce modal noise, but altering fiber transmission parameters by varying the length of the fiber may offer advantages. We report on tests comparing some of the alternative devices for reducing modal noise.
FRIDA integral field unit manufacturing
Show abstract
FRIDA (inFRared Imager and Dissector for the Adaptive optics system of the Gran Telescopio Canarias) has been
designed as a cryogenic and diffraction limited instrument that will offer broad and narrow band imaging and integral
field spectroscopy (IFS). Both, the imaging mode and IFS observing modes will use the same Teledyne 2Kx2K detector.
This instrument will be installed at Nasmyth B station, behind the GTC Adaptive Optics system (GTCAO). FRIDA will
provide the IFS mode using a 30 slices Integral Field Unit (IFU). This IFU design is based on the University of Florida
FISICA where the mirror block arrays are diamond turned on monolithic metal blocks.
The FRIDA IFU is of the slicer type; conformed mainly by 3 mirror blocks with 30 spherical mirrors each. It also has a
Schwarzschild relay based on two off axis spherical mirrors and an afocal system of two parabolic off axis mirrors.
Including two insertion mirrors the IFU holds 96 metal mirrors.
All the mirrors have been manufactured by diamond turning techniques on monolithic blocks of aluminum 6061-T6
coated by a Nickel alloy. Except for the Schwarzschild relay and the insertion mirrors, 92 mirrors were manufactured by
Corning in Keene NH, USA. The different blocks and mirrors are mounted on an opto-mechanical support that ensures
the image quality and integrity of the complete IFU.
In this work advances on the manufacturing of the FRIDA IFU components are described. Furthermore, the mirror
blocks individual verification tests and are also described.
FRIDA integral field spectroscopy PSF quality simulations
Show abstract
FRIDA (inFRared Imager and Dissector for the Adaptive optics system of the Gran Telescopio Canarias) has been designed as a cryogenic and diffraction limited instrument that will offer broad and narrow band imaging and integral field spectroscopy (IFS). Both, the imaging mode and IFS observing modes will use the same Teledyne 2Kx2K detector. This instrument will be installed at Nasmyth B station, behind the GTC Adaptive Optics system (GTCAO). FRIDA will provide the IFS mode using a 30 slices Integral Field Unit (IFU). This IFU design is based on University of Florida FISICA where the mirror block arrays are diamond turned on monolithic metal blocks.
FRIDA IFU is conformed mainly by 2 mirror blocks with 30 spherical mirrors each. The image slicing is performed by a block of 30 cylindrical mirrors each of 400 μm width. It also has a Schwarzschild relay based on two off axis spherical mirrors that adapts the GTCAO corrected PSF to the slicer mirrors dimensions. To readapt the sliced PSF to the spectrograph input numerical aperture the IFU has an afocal system of two parabolic off axis mirrors. The AO PSF is bigger than the slice mirror dimensions and this produces diffraction effects. These diffraction effects combined with the intrinsic IFU and spectrograph aberrations produce the final instrumental PSF of the IFS mode.
In order to evaluate the instrumental PSF quality of the FRIDA IFS, modeling simulations were performed by the ZEMAX Physical Optics Propagation (POP) module. In this work the simulations are described and the PSF quality and uniformity on a reconstructed IFS image is evaluated. It is shown the PSF quality of the IFS mode including the instrument manufacturing tolerances fulfills the specifications.
Concept and optical design of the near-infrared integral field unit for SWIMS
Show abstract
SWIMS-IFU is an integral field unit for a near-infrared imaging spectrograph SWIMS (Simultaneous-color
Wide-field Infrared Multi-object Spectrograph), which is being developed as one of the first-generation instruments for the University of Tokyo Atacama Observatory (TAO) 6.5-m infrared telescope and will be also mounted
on the Cassegrain focus of the Subaru telescope in its initial phase (2015-). As SWIMS has a wide wavelength
coverage which is implemented by a dichroic mirror placed into the collimated beam which splitting it into blue
(0.9-1.4 μm) and red (1.4-2.5 μm) arms, the IFU module enables us to simultaneously obtain spatially resolved
entire NIR spectrum from 0.9 to 2.5 μm in a wide-field of view of 14 ′′ x 10.′′4. The concept of the IFU module is
"easy realization" of an integral filed spectroscopy (IFS) mode without modification of an existing spectrograph
optics. Our IFU can be installed in a mask storage of SWIMS like other slit mask holders, so we can easily
carry out IFS observation by just inserting the IFU module into a focal plane stage. The IFU optics consists of
a pre-optics, an image slicer, a pupil mirror array, and a pseudo-slit mirror array. All the components will be
aligned on an aluminum frame which has a floor size of < 170mm x 220mm) and a height of <60mm. Compared
to existing near-infrared IFU instruments, our IFU has wider field coverage and is more sensitive for extended
sources due to its coarser spatial sampling optimized for seeing-limited observations. In this paper, we report
the concept and detailed optical design of the SWIMS-IFU.
Research on key problems for LAMOST optical fiber detection system
Show abstract
The large sky area multi-object fiber spectroscopic telescope (LAMOST) is an innovative reflecting schmidt telescope,
promising a very high spectrum acquiring rate of several ten-thousands of spectra per night. By using the parallel
controllable fiber positioning technique, LAMOST makes reconfiguration of fibers accurately according to the positions
of objects in minutes and fine adjusting the fibers. During telescope observation period, each optical fiber unit positional
accuracy directly determines the quality of subsequent spectrum acqusition, yet for real-time optical fiber positional
accuracy, there only exists an internal information feedback which focus on the corresponding stepper motor driving
conditions, however, this available information is not comprehensive, it can not offer the actual positional information
for each fiber unit. Considering the LAMOST on-site environment, a novel real-time optical fiber positional accuracy
detection system which can be integrated in the existing observation and control system need to be developed to solve
this problem. During the observation interval, this system can offer a comprehensive and effective information feedback
about the focal optical fiber positional accuracy. Based on this feedback, the observation assistants can properly adjust
the observation strategies to ensure the effectiveness and accuracy of acquired spectrum. Furthermore, this fiber
positional accuracy feedback can provide prior spectral quality information to the spectral processing personnel and
optimal the spectrum processing efficiency.
A high-density integrated optical focal plane positioning system
Show abstract
Modern multi-spectral sky survey requires the use of greater quantity and smaller size of the fiber positioner. This paper
presents a high-density integrated optical focal plane positioning system, which includes 150 groups fiber positioning
module and a 1 meter diameter honeycomb-shaped focal plane framework in that have about 150 hexagonal hole. Each
module has a pedestal includes 37 holes and 37 fiber positioner of 11.8 mm diameter. 37 fiber positioner integrated can
greatly reduce the difficulty of the design and installation. The modular structure also facilitates maintenance and
replacement in the field of telescope, and greatly reduce the difficulty of the drive system design. Numerical simulation
results show that: the honeycomb-shaped focal plane framework whose thickness is 100mm and who is in a variety of
working positions and load conditions, its maximum deformation is about 0.02mm. This meet the needs of the
general astronomical telescopes. The positioning accuracy of test 12mm diameter fiber positioner is about 0.04 mm,
and it is expected to reach 0.01mm if have the closed-loop control.
Development of the fibres of MOONS
Show abstract
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 0.8 - 1.8 μm,
with a multiplex capability of approximately 1000 fibres. Each triple arm spectrograph will produce spectra for half of
the targets simultaneously. 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 1.05 arcseconds 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).
This paper presents the overall design of the fibre system and describes the specific developments required to optimise
its performance. The design of the fibre input optics, the choice of the fibre connector, and the layout of the slit end are
described. The results of preliminary tests to measure the effect of twisting on the FRD performance of prototype fibres
are also discussed.
Miniaturized Shack-Hartmann wavefront sensors for starbugs
Show abstract
The ability to position multiple miniaturized wavefront sensors precisely over large focal surfaces are advantageous to
multi-object adaptive optics. The Australian Astronomical Observatory (AAO) has prototyped a compact and lightweight
Shack-Hartmann wavefront-sensor that fits into a standard Starbug parallel fibre positioning robot. Each device makes
use of a polymer coherent fibre imaging bundle to relay an image produced by a microlens array placed at the telescope
focal plane to a re-imaging camera mounted elsewhere. The advantages of the polymer fibre bundle are its high-fill
factor, high-throughput, low weight, and relatively low cost. Multiple devices can also be multiplexed to a single lownoise
camera for cost efficiencies per wavefront sensor. The use of fibre bundles also opens the possibility of
applications such as telescope field acquisition, guiding, and seeing monitors to be positioned by Starbugs. We present
the design aspects, simulations and laboratory test results.
Laser speckle suppression by the phase modulation of input beam in a multimode fiber
Show abstract
When a multimode fiber transmits a laser beam, the speckle will form in its output field. A dynamic fiber scrambler
could be used to suppress the speckle. No matter what actual way of suppressing speckle is, such as fiber scrambling or
using a rotating phase plate, the suppression is the result of exerting disturbance in the process of the speckle forming.
We could disturb the phase of the input beam with specific method to weaken the speckle effect. To get a speckle image
formed by a multimode fiber, we simulate different diffraction patterns under different phase conditions using diffraction
model, in which the phase of the input beam is modulated by a rough surface, and then sum them to form the
instantaneous speckle. To study the speckle suppression, we superpose instantaneous speckles, and as a consequence the
final speckle is suppressed. The simulation would help us understand the speckle suppression experiment with the input
beam phase modulation conducted in our lab.
Generation of optical frequency combs in fibres: an optical pulse analysis
Marina Zajnulina,
Michael Böhm,
Keith Blow,
et al.
Show abstract
The innovation of optical frequency combs (OFCs) generated in passive mode-locked lasers has provided astronomy
with unprecedented accuracy for wavelength calibration in high-resolution spectroscopy in research areas such as the
discovery of exoplanets or the measurement of fundamental constants. The unique properties of OCFs, namely a highly
dense spectrum of uniformly spaced emission lines of nearly equal intensity over the nominal wavelength range, is not
only beneficial for high-resolution spectroscopy. Also in the low- to medium-resolution domain, the OFCs hold the
promise to revolutionise the calibration techniques. Here, we present a novel method for generation of OFCs. As
opposed to the mode-locked laser-based approach that can be complex, costly, and difficult to stabilise, we propose an
all optical fibre-based system that is simple, compact, stable, and low-cost. Our system consists of three optical fibres
where the first one is a conventional single-mode fibre, the second one is an erbium-doped fibre and the third one is a
highly nonlinear low-dispersion fibre. The system is pumped by two equally intense continuous-wave (CW) lasers. To be
able to control the quality and the bandwidth of the OFCs, it is crucial to understand how optical solitons arise out of the
initial modulated CW field in the first fibre. Here, we numerically investigate the pulse evolution in the first fibre using
the technique of the solitons radiation beat analysis. Having applied this technique, we realised that formation of higherorder
solitons is supported in the low-energy region, whereas, in the high-energy region, Kuznetsov-Ma solitons appear.
PRAXIS: low thermal emission high efficiency OH suppressed fibre spectrograph
Show abstract
PRAXIS is a second generation instrument that follows on from GNOSIS, which was the first instrument using fibre
Bragg gratings for OH suppression to be deployed on a telescope. The Bragg gratings reflect the NIR OH lines while
being transparent to the light between the lines. This gives in principle a much higher signal-noise ratio at low resolution
spectroscopy but also at higher resolutions by removing the scattered wings of the OH lines. The specifications call for
high throughput and very low thermal and detector noise so that PRAXIS will remain sky noise limited even with the
low sky background levels remaining after OH suppression. The optical and mechanical designs are presented. The
optical train starts with fore-optics that image the telescope focal plane on an IFU which has 19 hexagonal microlenses
each feeding a multi-mode fibre. Seven of these fibres are attached to a fibre Bragg grating OH suppression system while
the others are reference/acquisition fibres. The light from each of the seven OH suppression fibres is then split by a
photonic lantern into many single mode fibres where the Bragg gratings are imprinted. Another lantern recombines the
light from the single mode fibres into a multi-mode fibre. A trade-off was made in the design of the IFU between field of
view and transmission to maximize the signal-noise ratio for observations of faint, compact objects under typical seeing.
GNOSIS used the pre-existing IRIS2 spectrograph while PRAXIS will use a new spectrograph specifically designed for
the fibre Bragg grating OH suppression and optimised for 1.47 μm to 1.7 μm (it can also be used in the 1.09 μm to 1.26
μm band by changing the grating and refocussing). This results in a significantly higher transmission due to high
efficiency coatings, a VPH grating at low incident angle and optimized for our small bandwidth, and low absorption
glasses. The detector noise will also be lower thanks to the use of a current generation HAWAII-2RG detector.
Throughout the PRAXIS design, from the fore-optics to the detector enclosure, special care was taken at every step along
the optical path to reduce thermal emission or stop it leaking into the system. The spectrograph design itself was
particularly challenging in this aspect because practical constraints required that the detector and the spectrograph
enclosures be physically separate with air at ambient temperature between them. At present, the instrument uses the
GNOSIS fibre Bragg grating OH suppression unit. We intend to soon use a new OH suppression unit based on multicore
fibre Bragg gratings which will allow an increased field of view per fibre. Theoretical calculations show that the gain in
interline sky background signal-noise ratio over GNOSIS may very well be as high as 9 with the GNOSIS OH
suppression unit and 17 with the multicore fibre OH suppression unit.
Research of fiber position measurement by multi CCD cameras
Show abstract
Parallel controlled fiber positioner as an efficiency observation system, has been used in LAMOST for four years, and
will be proposed in ngCFHT and rebuilt telescope Mayall. The fiber positioner research group in USTC have designed a
new generation prototype by a close-packed module robotic positioner mechanisms. The prototype includes about 150
groups fiber positioning module plugged in 1 meter diameter honeycombed focal plane. Each module has 37 12mm
diameter fiber positioners. Furthermore the new system promotes the accuracy from 40 um in LAMOST to 10um in MSDESI.
That’s a new challenge for measurement. Close-loop control system are to be used in new system. The CCD camera
captures the photo of fiber tip position covered the focal plane, calculates the precise position information and feeds back
to control system. After the positioner rotated several loops, the accuracy of all positioners will be confined to less than
10um. We report our component development and performance measurement program of new measuring system by using
multi CCD cameras. With the stereo vision and image processing method, we precisely measure the 3-demension position
of fiber tip carried by fiber positioner. Finally we present baseline parameters for the fiber positioner measurement as a
reference of next generation survey telescope design.
The fiber positioner for 4MOST: exploration of an alternative R-θ design
Allar Saviauk,
Frank Dionies,
Roelof de Jong,
et al.
Show abstract
We present a new type of R-θ fiber actuator named PotsPos for multi-object astronomical spectroscopy. It was initially intended to use as an alternative positioner design for the 4-meter Multi-Object Spectroscopic Telescope (4MOST). Although another tilting spine technology developed by the Australian Astronomical Observatory (AAO) is currently chosen for the 4MOST, PotsPos design has some technical advantages over other positioning technologies and might therefore be an attractive choice for future spectrographs. The design would be similar in positioning capabilities to the tilting spine design but would not suffer from telecentricity and defocus errors. The mechanical design, suitable motors, computational results from dynamic simulations and finite element analyses are presented.
Studying focal ratio degradation of optical fibers for Subaru's Prime Focus Spectrograph
Show abstract
Focal Ration Degradation (FRD) is a change in light’s angular distribution caused by fiber optics. FRD is important to
fiber-fed, spectroscopic astronomical systems because it can cause loss of signal, degradation in spectral resolution, or
increased complexity in spectrograph design. Laboratório Nacional de Astrofísica (LNA) has developed a system that
can accurately and precisely measures FRD, using an absolute method that can also measure fiber throughput. This
paper describes the metrology system and shows measurements of Polymicro’s fiber FBP129168190, FBP127165190
and Fujikura fiber 128170190. Although the FRD of the two fibers are low and similar to one another, it is very
important to know the exact characteristics of these fibers since both will be used in the construction of FOCCoS (Fiber
Optical Cable and Connectors System) for PFS (Prime Focus Spectrograph) to be installed at the Subaru telescope.
Development of deployable fibre integral-field-units for the E-ELT
Show abstract
The use of deployable fibre-bundles plays an increasing role in the design of future Multi-Object-Spectrographs (MOS).
Within a research and development project for "Enabling Technologies for the E-ELT", various miniaturized, fibrebundles
were designed, built and tested for their suitability for a proposed ELT-MOS instrument.
The paper describes the opto-mechanical designs of the bundles and the different manufacture approaches, using glued,
stacked and fused optical fibre bundles. The fibre bundles are characterized for performance, using dedicated testbenches
in the laboratory and at a telescope simulator. Their performance is measured with respect to geometric
accuracy, throughput, FRD behavior and cross-talk between channels.
CARMENES in SPIE 2014. Building a fibre link for CARMENES
Show abstract
Optical fibres have successfully been used to couple high-resolution spectrographs to telescopes for many years. As they allow the instrument to be placed in a stable and isolated location, they decouple the spectrograph from environmental influences. Fibres also provide a substantial increase in stability of the input illumination of the spectrograph, which makes them a key optical element of the two high-resolution spectrographs of CARMENES. The optical properties of appropriate fibres are investigated, especially their scrambling and focal ratio degradation (FRD) behaviour. In the laboratory the output illumination of various fibres is characterized and different methods to increase the scrambling of the fibre link are tested and compared. In particular, a combination of fibres with different core shapes shows a very good scrambling performance. The near-field (NF) shows an extremely low sensitivity to the exact coupling conditions of the fibre. However, small changes in the far-field (FF) can still be seen. Related optical simulations of the stability performance of the two spectrographs are presented. The simulations focus on the influence of the non-perfect illumination stabilization in the far-field of the fibre on the radial velocity stability of the spectrographs. We use ZEMAX models of the spectrographs to simulate how the barycentres of the spots move depending on the FF illumination pattern and therefore how the radial velocity is affected by a variation of the spectrograph illumination. This method allows to establish a quantitative link between the results of the measurements of the optical properties of fibres on the one hand and the radial velocity precision on the other. The results provide a strong indication that 1ms1 precision can be reached using a circular-octagonal fibre link even without the use of an optical double scrambler, which has successfully been used in other high-resolution spectrographs. Given the typical throughput of an optical double scrambler of about 75% to 85 %, our solution allows for a substantially higher throughput of the system.
Design of the opto-mechanical mounts of the ESPRESSO spectograph
Show abstract
ESPRESSO is the next generation European exoplanet hunter, combining the efficiency of a modern echelle
spectrograph with extreme radial velocity and spectroscopic precision. The instrumental radial velocity precision will be
improved to reach 10 cm/s level, to achieve a gain of two magnitudes with respect to its predecessor HARPS.
The fiber-fed, non-rotating instrument will be installed in the Combined Coudé Laboratory of the VLT (Very Large
Telescope), which is situated in the Paranal Observatory (Chile).
The main challenge in the design of the optical mounts of the instrument has been the extreme long-term stability of big
rectangular optical components in a seismic environment.
This paper describes the requirements and the adopted solution for the opto-mechanical design of the collimator mirrors,
dichroic lens, field lens and cross dispersers of the instrument.
MULEC: multiple lenses connectors for optical fibers
Show abstract
Astronomical instruments with hundreds of optical fibers are increasingly common in the setup of the modern telescopes. Multi-fibers connectors assure precise connection among several optical fibers, providing flexibility for instrument exchanges. In fact, highly multiplexed instruments require a fiber connector system that can deliver excellent optical performance and reliability. In this paper, we present a multi-fiber connector developed to assure strong and accurate connection. MULEC is a multi-fibers connector where each fiber end, suitably polished, is coupled at a microlens such that the beam of light from one end of the optical fiber can be collimated and then, focused by another microlens coupled with another optical fiber end. Given the optical magnification inferred by the microlens, the optical accuracy of the coupling is significantly increased.
MULEC is easy to coupling using powerful micro magnets and also, has devices for adjustment in x, y, z and rotation. The optical fiber arrangements on both sides of the connector are constructed with a special dark composite, made with refractory oxide, which is able to sustain its polishing with minimum quantities of abrasives during the polishing process. In other words, when in polishing, the detachment of the refractory oxide nanoparticles reinforces gently the polishing process and increases the efficiency of this procedure.
The bench tests with these connector systems will be implemented in a near future and the chosen fibers should measure the throughput of light and the stability after many connections and disconnections. In this paper, we describe some optical features and mechanical details.
New multicore low mode noise scrambling fiber for applications in high-resolution spectroscopy
Show abstract
We present a new type of multicore fiber (MCF) and photonic lantern that consists of 511 individual cores designed to operate over a broadband visible wavelength range (380-860nm). It combines the coupling efficiency of a multimode fiber with modal stability intrinsic to a single mode fibre. It is designed to provide phase and amplitude scrambling to achieve a stable near field and far field illumination pattern during input coupling variations; it also has low modal noise for increased photometric stability. Preliminary results are presented for the new MCF as well as current state of the art octagonal fiber for comparison.
An economic Fabry-Perot wavelength reference
Gábor Fűrész,
Alex Glenday,
Christian Latham
Show abstract
Precision radial velocity (PRV) measurements are key in studying exoplanets, and so are wavelength calibrators in PRV instruments. ThAr lamps offer an affordable but somewhat limited solution for the visible passband. Laser frequency combs are ideal calibrators, except the (still) narrow wavelength coverage and large price tag. White light Fabry-Perot (FP) calibrators offer frequency-comb like properties in a more affordable and less complicated package1. Using a commercial solid FP etalon and off-the shelf components we have constructed an economic FP calibrator suitable for observatories on a smaller budget.
Accurate astronomical atmospheric dispersion models in ZEMAX
Show abstract
ZEMAX provides a standard built-in atmospheric model to simulate atmospheric refraction and dispersion. This model has been compared with other ones to assess its intrinsic accuracy, critical for very demanding application like ADCs for AO-assisted extremely large telescopes. A revised simple model, based on updated published data of the air refractivity, is proposed by using the “Gradient 5” surface of Zemax. At large zenith angles (65 deg), discrepancies up to 100 mas in the differential refraction are expected near the UV atmospheric transmission cutoff. When high-accuracy modeling is required, the latter model should be preferred.
Space qualification of an antireflection coating on the surface of a ruled grating prism: increasing the throughput of the single-object slitless spectroscopy mode of NIRISS onboard JWST
Show abstract
Grating prisms (grism) designed for near-infrared spectroscopy typically make use of high-refractive index materials such as zinc selenide (ZnSe), at the expense of large Fresnel losses ( 18%). Part of the loss can be recovered by using anti-reflection (AR) coatings. The technique is however considered risky when applied on the ruled surface of a grating, especially for a space application at cryogenic temperature. Such a grism, made of ZnSe and machined at Lawrence Livermore National Laboratory (LLNL) is mounted in the Near-Infrared Slitless Spectrograph (NIRISS) onboard the James Webb Space Telescope (JWST). Its Single Object Slitless Spectrograph (SOSS) observing mode uses the ZnSe grism and a cross-dispersing prism to produce R=700 spectra in orders 1 and 2 to cover the 0.6 to 2.5 microns spectral domain. The ZnSe grism is blazed at 1.23 microns, has a density of 54 lines/mm and its triangular grooves have a depth of 700 nm, a base of 18 microns, with facets angled at 1.9 degrees. Here, an AR coating produced by Thin Film Lab (TFL) and deposited on the ruled surface of a ZnSe grism sample was space qualified. Atomic force microscopy (AFM) showed no groove profile change pre/post coating despite the large relative thickness of the AR coating to that of the groove depth ( 35%). Also, the wavefront error map remained almost unchanged at lambda/8 (peak-to-valley at 632 nm) and survived unscathed through a series of three cryogenic cycles to 20 K. Finally, the transmission gain across our operating spectral range was almost as high as that for a unruled surface covered with the same AR coating (10-15%).
Diamond machining of ZnSe grisms for the Near Infrared Imager and Slitless Spectrograph (NIRISS) onboard JWST
Show abstract
LLNL diamond machined a ZnSe grism for spectroscopy of transiting exoplanets on NIRISS, a Canadian instrument that
will fly on the James Webb Space Telescope. The grism operates over the wavelength range of 0.6 to 2.5 μm. It is cross-dispersed
by a ZnS prism and has a resolving power in first order of ~700. The surface error over the full 29 x 30 mm
grating aperture is 0.03 wave rms at 633nm. We measured a diffraction efficiency at 633 nm of 56% (nearly 88% after
accounting for Fresnel reflection). The diffraction pattern is clean with no discernible ghosts.
Cleaning of extremely sensitive optical surfaces
Show abstract
Most of the optical surfaces are sensitive; the cleaning should be done with great cares and using well characterized
technique and equipment. In this paper we are reporting about cleaning of some of the most critical optical surfaces,
surfaces which are generally known as not allowing any cleaning. The first section reports about the cleaning of the
silver coating on an echelle grating. Silver becomes oxidized, or tarnished, from exposure to sulphates in the air. With
time this can lead to dramatic loss of the efficiency of the grating. The second section reports about the cleaning of a
CCD chip which has been contaminated with grease. A special “contactless” cleaning procedure has been developed and
used successfully on real components. In both cases the full procedure is described and a set of measurements document
the results and shows how the original performance have been recovered.
On-sky tests of an A/R coated silicon grism on board NICS at TNG
Show abstract
We present the results of our project for the design and construction and on-sky test of silicon
grisms. The fabrication of such devices is a complex and critical process involving litho-masking,
anisotropic etching and direct bonding techniques. After the successful fabrication of the silicon
grating, we have optimized the bonding of the grating onto the hypotenuse of a silicon prism to get
the final prototype. After some critical phases during the experimentation a silicon grism with 363
grooves/mm and a blaze angle of 14 degrees has been eventually fabricated. The application of an
A/R coating on both the surfaces has been the last step: this procedure is critical because of the
groove geometry of the diffraction grating, whose performace might be compromised by the
coating. Then, the grism was inserted in the filter wheel of the Near Infrared camera NICS, at the
focal plane of the National Galileo Telescope (TNG), the 3.5 m Italian facility in the Canary Islands
(E). The result of the on-sky tests are given in detail.
Birefringence Bragg Binary (3B) grating, quasi-Bragg grating and immersion gratings
Show abstract
A volume phase holographic (VPH) grating achieves high angular dispersion and very high diffraction efficiency for the
first diffraction order and for S or P polarization. However the VPH grating could not achieve high diffraction efficiency
for non-polarized light at a large diffraction angle because properties of diffraction efficiencies for S and P polarizations
are different. Furthermore diffraction efficiency of the VPH grating extinguishes toward a higher diffraction order. A
birefringence binary Bragg (3B) grating is a thick transmission grating with optically anisotropic material such as lithium
niobate or liquid crystal. The 3B grating achieves diffraction efficiency up to 100% for non-polarized light by tuning of
refractive indices for S and P polarizations, even in higher diffraction orders. We fabricated 3B grating with liquid crystal
and evaluated the performance of the liquid crystal grating. A quasi-Bragg (QB) grating, which consists long rectangle
mirrors aligned in parallel precisely such as a window shade, also achieves high diffraction efficiency toward higher orders.
We fabricated QB grating by laminating of silica glass substrates and glued by pressure fusion of gold films. A
quasi-Bragg immersion (QBI) grating has smooth mirror hypotenuse and reflector array inside the hypotenuse, instead of
step-like grooves of a conventional immersion grating. An incident beam of the QBI grating reflects obliquely at a
reflector, then reflects vertically at the mirror surface and reflects again at the same reflector. We are going to fabricate
QBI gratings by laminating of mirror plates as similar to fabrication of the QB grating. We will also fabricate silicon and
germanium immersion gratings with conventional step-like grooves by means of the latest diamond machining methods.
We introduce characteristics and performance of these gratings.
Additional narrow bandpass steep edge optical filters for the JAST/T80 telescope instrumentation
Show abstract
The Observatorio Astrofisico de Javalambre in Spain observes with its telescope galaxies in the Local Universe plans to
perform a multi-band survey, where optical filters are needed. Different filters with a full width half maximum between
10-20 nm with central wavelengths at 395 nm, 410 nm, 430 nm, 515 nm, and an average transmission Tave larger than
85 % in the passband and blocking from 250 nm to 1050 nm of OD5 (T < 10-5) will be shown. The edges are steep for a
narrow transition from 5 % to 80 % and the transmitted wavefront error of the optical filter are less than λ/2 over an
aperture > 100 mm.
Design, development, and test of a grism prototype for Euclid-NISP mission
Show abstract
The ESA mission Euclid is designed to map the geometry of the dark Universe by investigating the distance-redshift
relationship and the evolution of cosmic structures. In the Euclid design of the NISP instrument, the spectroscopic
channel uses four slitless low resolution grisms in NIR wavelength with four different orientations. Euclid grisms
combine two optical functions: a grism function (ie dispersion without deviation at a specific wavelength) done by the
grating associated with the prism and a spectral filter function done by a multilayer filter deposited on the entrance
surface of the prism. After a successful development of a prototype of a grating realized by a photolithography process,
we have begun a new phase of the prototype to manufacture a complete component, with a grism and a filter, and to
validate its performance. Its development is very challenging as it requires manufacturing of the component in several
steps which involve three different companies. We will present first the main optical requirements for the grism defined
for the phase B and how the efficiency and wavefront specifications are split into the different components of the grism
(mechanical mount, grating and filter). Then, we will describe the manufacturing process chosen for the NISP grism.
Finally, we will present the first results of the optical characterisation of the prototype of the grism: global efficiency
measurement, shape of the groove, wavefront contribution, and the trade-off made to achieve the final performance.
Core-to-core uniformity improvement in multi-core fiber Bragg gratings
Show abstract
Multi-core fiber Bragg gratings (MCFBGs) will be a valuable tool not only in communications but also various
astronomical, sensing and industry applications. In this paper we address some of the technical challenges of fabricating
effective multi-core gratings by simulating improvements to the writing method. These methods allow a system designed
for inscribing single-core fibers to cope with MCFBG fabrication with only minor, passive changes to the writing
process. Using a capillary tube that was polished on one side, the field entering the fiber was flattened which improved
the coverage and uniformity of all cores.
Development of high-throughput silicon lens and grism with moth-eye anti-reflection structure
Show abstract
Anti-reflection (AR) is very important for high-throughput optical elements. The durability against cooling is
required for the AR structure in the cryogenic optics used for mid-infrared astronomical instruments. Moth-eye structure is a promising AR technique strong against cooling. The silicon lens and grism with the moth-eye structure are being developed to make high-throughput elements for long-wavelength mid-infrared instruments. A double-sided moth-eye plano-convex lens (Effective diameter: 33 mm, Focal length: 188 mm) was fabricated. By the transmittance measurement, it was confirmed that its total throughput is 1.7± 0.1 times higher than bare silicon lenses in a wide wavelength range of 20{45 μm. It suggests that the lens can achieve 83±5% throughput in the cryogenic temperature. It was also confirmed that the moth-eye processing on the lens does not modify the focal length. As for the grism, the homogeneous moth-eye processing on blaze pattern was realized by employing spray coating for the resist coating in EB lithography. The silicon grism with good surface roughness was also developed. The required techniques for completing moth-eye grisms have been established.
Coating and surface finishing definition for the Solar Orbiter/METIS inverted external occulter
Show abstract
The METIS coronagraph aboard the Solar Orbiter mission will undergo extreme environmental conditions (e.g.,
a thermal excursion of about 350 degrees throughout the various mission phases), due to the peculiar spacecraft
trajectory that will reach a perihelion of 0.28 AUs. METIS is characterized by an innovative design for the
occultation system that allows to halve the thermal load inside the instrument while guaranteeing the stray light
reduction that is required for a solar coronagraph. The Inverted External Occulter (IEO) concept revolutionizes
the classical scheme, by exchanging the usual positions of the entrance aperture (that is now the outermost element of the instrument facing the Sun) with the actual occulter (that is a spherical mirror inside the coronagraph
boom). The chosen material for the IEO manufacturing is Titanium, as a trade o_ between light weight, strength
and low thermal expansion coefficient. A 2 years long test campaign has been run to define the IEO geometry,
and its results are addressed in previous dedicated papers. This work describes the results of a further campaign
aimed at defining the IEO surface and edge finishing, the support flange geometry and the Titanium coating.
Various edge finishing were installed on a prototype of the instrument occulting system and their performance
in stray light reduction were compared. The support flange geometry was designed in order to reduce the overall
weight, to control the thermal load and to accentuate its stray light suppression performance. The coating is a
particularly delicate issue. A black coating is necessary in order to assess the stray light issues, typically critical
for visible coronagraphs. Black coating of Titanium is not a standard process, thus several space qualified black
coatings were experimented on Titanium and characterized. The impact of the IEO coatings was evaluated, the
reflectivity and the BRDFs were measured and are addressed in the paper.
Fluoride damage to substrates during stripping of mirrors
Show abstract
Fluorides are useful low-index materials that can be used to enhance reflectivity of over-coated metallic films. In particular, YF3 has been suggested as a useful low-stress low-index material in the IR where film layers must be thicker, and it has also been found to enhance durability in silver-based mirrors. However, if these mirrors need to be stripped for recoating, care must be taken with the stripping process to avoid damaging a silica-based substrate through production of hydrofluoric acid. We present data that such damage can occur, and discuss empirically-derived alternative stripping processes in place of the normal acid-based approach to mitigate the danger.
High reflectivity large scale telescope mirror coatings via long throw sputtering
A. J. Bourque,
J. H. Gurian
Show abstract
Here we present reflectivity measurements of long throw sputtered aluminum coatings at normal and oblique incidence
angles on glass test chips. Experimental results indicate that water vapor partial pressure, not incident angle, is the dominant
factor for front surface reflectivity of sputtered aluminum. These results highlight the importance of water vapor pumping in
telescope mirror coating vacuum chambers and indicate that long-throw sputtering is an economical and reliable technique
for producing high uniformity, high reflectivity mirror coatings across large substrates.
High performance Si immersion gratings patterned with electron beam lithography
Show abstract
Infrared spectrographs employing silicon immersion gratings can be significantly more compact than spectro-
graphs using front-surface gratings. The Si gratings can also offer continuous wavelength coverage at high
spectral resolution. The grooves in Si gratings are made with semiconductor lithography techniques, to date
almost entirely using contact mask photolithography. Planned near-infrared astronomical spectrographs require
either finer groove pitches or higher positional accuracy than standard UV contact mask photolithography can
reach. A collaboration between the University of Texas at Austin Silicon Diffractive Optics Group and the Jet
Propulsion Laboratory Microdevices Laboratory has experimented with direct writing silicon immersion grating
grooves with electron beam lithography. The patterning process involves depositing positive e-beam resist on
1 to 30 mm thick, 100 mm diameter monolithic crystalline silicon substrates. We then use the facility JEOL
9300FS e-beam writer at JPL to produce the linear pattern that defines the gratings.
There are three key challenges to produce high-performance e-beam written silicon immersion gratings. (1) E-
beam field and subfield stitching boundaries cause periodic cross-hatch structures along the grating grooves. The
structures manifest themselves as spectral and spatial dimension ghosts in the diffraction limited point spread
function (PSF) of the diffraction grating. In this paper, we show that the effects of e-beam field boundaries must
be mitigated. We have significantly reduced ghost power with only minor increases in write time by using four or
more field sizes of less than 500 μm. (2) The finite e-beam stage drift and run-out error cause large-scale structure
in the wavefront error. We deal with this problem by applying a mark detection loop to check for and correct out
minuscule stage drifts. We measure the level and direction of stage drift and show that mark detection reduces
peak-to-valley wavefront error by a factor of 5. (3) The serial write process for typical gratings yields write times
of about 24 hours- this makes prototyping costly. We discuss work with negative e-beam resist to reduce the
fill factor of exposure, and therefore limit the exposure time. We also discuss the tradeoffs of long write-time
serial write processes like e-beam with UV photomask lithography. We show the results of experiments on small
pattern size prototypes on silicon wafers. Current prototypes now exceed 30 dB of suppression on spectral and
spatial dimension ghosts compared to monochromatic spectral purity measurements of the backside of Si echelle
gratings in reflection at 632 nm. We perform interferometry at 632 nm in reflection with a 25 mm circular beam
on a grating with a blaze angle of 71.6°. The measured wavefront error is 0.09 waves peak to valley.
A new facility for manufacturing and testing very large narrow bandpass filters and other high performance optical coatings
Show abstract
Optical coatings are key elements of any optical system. They can reduce surface reflection loss, isolate spectral bands, re-direct the light path and split light beams by wavelength. For decades, astronomers have made use of these special characteristics embodied in Anti-Reflection (AR) coatings, Band Pass (BP) filters, mirrors and Dichroic Beamsplitters (DBS). In the last several years, a need has arisen for much larger high performance filters and coatings. This is being driven by the ever increasing size of new and planned telescopes with their correspondingly larger focal planes.
Typical Broadband filters require modest wavelength uniformity and can be produced in legacy (existing) coating chambers, even in fairly large formats. However, some new instruments require narrow BP (NBP) filters of 60 cm or greater diameter in order to perform efficiently. Some planned systems will even require filters in the 75 cm diameter range. The implications for coating such large, very expensive optics are that the equipment must not only accommodate a large optic, but the process must achieve excellent uniformity over broad areas. It must also exhibit excellent performance, reproducibility and reliability in depositions consisting of well over one hundred layers and many hours duration. And finally, the spectral performance must be verifiable, not through an indirect method, but directly of the science optic itself. To address these challenges, Materion designed, built, tested and put into production a purposebuilt laboratory. This paper will describe in detail the elements of the lab creation and initial achievements.
A broadband scalar optical vortex coronagraph
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In recent years, new coronagraphic schemes have been proposed, the most promising being the optical vortex
phase mask coronagraphs. In our work, a new scheme of broadband optical scalar vortex coronagraph is proposed
and characterized experimentally in the laboratory. Our setup employs a pair of computer generated phase
gratings (one of them containing a singularity) to control the chromatic dispersion of phase plates and achieves a
constant peak-to-peak attenuation below 1:1000 over a bandwidth of 120 nm centered at 700 nm. An inner working
angle of λ/D is demonstrated along with a raw contrast of 11.5magnitudes at 2λ/D. A more compact setup
achieves a peak-to-peak attenuation below 1:1000 over a bandwidth of 60 nm with the other results remaining
the same.
Laboratory demonstration of the Savart-plate lateral-shearing interferometric nuller for exoplanets (SPLINE)
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We have developed the Savart-Plate Lateral-shearing Interferometric Nuller for Exoplanets (SPLINE), which is a kind of
a nulling interferometer, for directly imaging exoplanets. The SPLINE consists of two polarizers and a Savart plate
between them. The SPLINE can theoretically obtain fully achromatic and stable nulled output. However, a drawback of
the SPLINE is its low system throughput due to the polarizers. For improving the system throughput, we propose a dualchannel
SPLINE using polarization beam splitters instead of the polarizers. We have carried out laboratory
demonstration of the dual-channel SPLINE. The achievable contrast of the SPLINE is limited by residual speckles
caused by surface roughness of optical elements. For improving the achievable contrast, we propose a method of
wavefront correction using a liquid-crystal spatial light modulator (LCSLM). We have carried out preliminary laboratory
demonstration using a liquid-crystal variable retarder (LCVR), instead of the LCSLM, for simulating the proposed
wavefront correction method. We report the laboratory demonstration in this paper.
Performance of an achromatic focal plane mask for exoplanet imaging coronagraphy
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Coronagraph technology combined with wavefront control is close to achieving the contrast and inner working angle
requirements in the lab necessary to observe the faint signal of an Earth-like exoplanet in monochromatic light. An
important remaining technological challenge is to achieve high contrast in broadband light. Coronagraph bandwidth is
largely limited by chromaticity of the focal plane mask, which is responsible for blocking the stellar PSF. The size of a
stellar PSF scales linearly with wavelength; ideally, the size of the focal plane mask would also scale with wavelength. A
conventional hard-edge focal plane mask has a fixed size, normally sized for the longest wavelength in the observational
band to avoid starlight leakage. The conventional mask is oversized for shorter wavelengths and blocks useful discovery
space. Recently we presented a solution to the size chromaticity challenge with a focal plane mask designed to scale its
effective size with wavelength. In this paper, we analyze performance of the achromatic size-scaling focal plane mask
within a Phase Induced Amplitude Apodization (PIAA) coronagraph.
We present results from wavefront control around the achromatic focal plane mask, and demonstrate the size-scaling
effect of the mask with wavelength. The edge of the dark zone, and therefore the inner working angle of the
coronagraph, scale with wavelength. The achromatic mask enables operation in a wider band of wavelengths compared
with a conventional hard-edge occulter.
The VORTEX coronagraphic test bench
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In this paper, we present the infrared coronagraphic test bench of the University of Liège named VODCA (Vortex
Optical Demonstrator for Coronagraphic Applications). The goal of the bench is to assess the performances of the
Annular Groove Phase Masks (AGPMs) at near- to mid-infrared wavelengths. The AGPM is a subwavelength grating
vortex coronagraph of charge two (SGVC2) made out of diamond. The bench is designed to be completely achromatic
and will be composed of a super continuum laser source emitting in the near to mid-infrared, several parabolas,
diaphragms and an infrared camera. This way, we will be able to test the different AGPMs in the M, L, K and H bands.
Eventually, the bench will also allow the computation of the incident wavefront aberrations on the coronagraph. A
reflective Lyot stop will send most of the stellar light to a second camera to perform low-order wavefront sensing. This
second system coupled with a deformable mirror will allow the correction of the wavefront aberrations. We also aim to
test other pre- and/or post-coronagraphic concepts such as optimal apodization.
High-contrast imaging in wide spectral band with a self-coherent camera and achromatic coronagraphs
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Direct imaging of exoplanets is very attractive but challenging. It requires high angular resolution and very high-contrast imaging. One solution is the use of coronagraphs behind the adaptive optics of large telescopes. Unfortunately, optics of space telescope and ground telescope introduce quasi-static aberrations which strongly limit the quality of the final image and a dedicated stage of adaptive optics is required. We proposed a self- coherent camera (SCC) in 2006 and we obtained contrast levels of (approximately equal to) 2 10-8 at a few λ0=D at 638 nm in laboratory. In this paper, we explain how to achromatize the SCC. We present laboratory performance in wide spectral band (approximately equal to) 5 - 10 % bandpass.
Kinematic modeling and path planning for MIRADAS arms
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The Mid-resolution InfRAreD Astronomical Spectrograph (MIRADAS) is a near-infrared (NIR) multi-object
spectrograph for the Gran Telescopio Canarias (GTC). It can simultaneously observe multiple targets selected by
20 identical deployable probe arms with pickoff mirror optics. The bases of the arms are fixed to the multiplexing
system (MXS) plate, a circular platform, and arranged in a circular layout with minimum separation between
elements of the arms. This document presents the MXS prototype P2a, a full-scale, fully operational prototype
of a MIRADAS probe arm. This planar closed-loop mechanism compared to other previous designs offers some
advantages specially in terms of stability and from the point of view of optics. Unfortunately, these benefits come
at the expense of a more complicated kinematics and an unintuitive arm motion. Furthermore, the cryogenic
motor controllers used in prototyping impose severe restrictions in path planing. They negatively impact in the
slice of pie approach, a collision-avoidance patrolling strategy that can gives good results in other scenarios. This
study is a starting point to define collision-free trajectory algorithms for the 20 probe arms of MIRADAS.
Conservation of étendue in a primary objective grating telescope
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A novel telescope design that uses a primary objective grating has been shown to have valuable performance features for
multiple object spectroscopy as well as a flat collector with relaxed figure tolerances suitable for membrane substrates.
The commonly applied figure of merit called étendue takes on new meaning here when compared to telescopes which
employ a parabolic mirror In this new telescope design, a mirror is in the secondary. One dimension of the mirror
conforms to the familiar metrics for measuring field-of-view and surface area. However, in the other, an anamorphic
magnification feature of plane gratings takes effect at angles of diffraction as subtended at grazing off the grating plane
normal. When the secondary parabolic mirror is placed at a grazing angle, compression increases exponentially,
permitting unprecedented fields-of-view, collection surface areas and aperture in one of the two dimensions. We provide
an analysis that calculates the compression, resolution, and field-of-view. More importantly, we study how these
parameters are conserved by a secondary parabolic mirror notwithstanding that the mirror diameter can be significantly
less than the length of the primary objective grating. Zemax models are used to show that along the dimension of the
grating, étendue is conserved in this telescope architecture.
Development of a universal tunable filter for future solar observations
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We have developed a new narrowband tunable filter to perform imaging spectroscopy of the solar chromosphere.
Using Liquid Crystal Variable Retarders (LCVRs) as the tuning elements for wavelength, wide-band polarizers
and super achromatic half-wave plates, it is possible to make high speed tuning (about 0.1Sec), to exclude
mechanical drives (and oil tank), and to cover a wide wavelength range (510-100nm). This filter builds up
with seven stages each consisting of a pair of calcites, LCVR, half-wave plates and linear polarizer. The full
width at half maximum (FWHM) of the filter transmission is about 0.025nm at 656.3nm.We demonstrate that
the concept of the universal tunable filter using the LCVR's as tuning elements is highly promising for future
application to space mission and ground based observations.
JWST ASPA fiber optic development for testing at 2.12 µm
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The James Webb Space Telescope (JWST) requires testing of the full optical system in a cryogenic vacuum environment
prior to launch. Challenges with the telescope architecture and the test environment led to placing removable optical test
sources at the intermediate focus of this off-axis Three Mirror Anastigmat (TMA) to provide test sources in the Science
Instruments. Fiber optic cables will route light whose wavelength is between 600 and 2120 nm from photonics external
to the cryo-vacuum chamber to intermediate focus locations. These fiber optic cables will be greater than 50 meters in
length and transition from ambient (293K) temperatures outside the chamber to cryogenic (30K) temperatures at the
intermediate focus. Presented here are results from risk reduction testing conducted to ensure the fiber optic cables
selected will 1) transmit 2.12μm light over long fiber runs, 2) show strong macro bending performance, and 3) perform
at cryogenic temperatures.
Volume phase holographic gratings for astronomy based on solid photopolymers
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Volume Phase Holographic Gratings (VPHG) are gaining more and more interest as optical dispersing elements
in new astronomical spectrographs at low and medium resolution. A key point is the development of new
photosensitive materials suitable to produce VPHGs that match the stringent requirements of the astronomical
environment. Here we report on the performances of VPHG based on Bayfol®HX solid photopolymer films
developed by Bayer MaterialScience AG. Parameters affecting the grating efficiency (e. g. photopolymer film
thickness and refractive index modulation) are measured and linked to the performances of VPHG working in the
VIS-NIR region. Moreover, the behavior at low temperature and the aging properties of such materials/gratings
are reported. Substantial efficiency gains on a new VPHG installed at the Asiago's spectrograph are shown and
proven on the observation of a standard star (hr5501).
WEAVE MOS fibre bundle test plan
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WEAVE is the next-generation wide-field optical spectroscopy facility for the William Herschel Telescope (WHT) in La
Palma, Canary Islands, Spain. WEAVE mainly aims at spectroscopic follow-up of ground-based (e.g. LOFAR) and
space-based (GAIA) surveys. The facility consists of 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 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 2017 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 MOS fibre bundle, we described the proposed test plan and the test bench of the 2x1000
WEAVE MOS fibres. The test bench allows us to evaluate the Focal Ratio Degradation and the throughput of the fibers
fitted with their buttons and slitlets.
Multi-fibers connectors systems for FOCCoS-PFS-Subaru
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The Fiber Optical Cable and Connector System (FOCCoS), provides optical connection between 2400
positioners and a set of spectrographs through optical fibers cables as part of PFS instrument for Subaru telescope. The
optical fiber cable will be segmented in 3 parts along the route, cable A, cable B and cable C, connected by a set of
multi-fiber connectors. The company USCONEC produces the multi-fiber connector under study. The USCONEC 32F
model can connect 32 optical fibers in a 4 x 8 matrix arrangement. The ferrules are made of a durable composite,
Polyphenylene Sulfide (PPS) based thermoplastic. The connections are held in place by a push-on/pull-off latch, and the
connector can also be distinguished by a pair of metal guide pins that protrude from the front of the connector. Two
fibers per connector will be used for monitoring the connection procedure. It was found to be easy to polish and it is
small enough to be mounted in groups.
Highly multiplexed instruments like PFS require a fiber connector system that can deliver excellent optical
performance and reliability. PFS requires two different types of structures to organize the connectors. The Tower
Connector system, with 80 multi-fiber connectors, will be a group of connectors for connecting cable B (Telescope
Structure) with cable C (Positioners Plate). The Gang Connector system is a group of 8 gang connectors, each one with
12 multi-fibers connectors, for connecting cable B (Telescope Structure) with cable A (Spectrograph). The bench tests
with these connector systems and the chosen fibers should measure the throughput of light and the stability after many
connections and disconnections. In this paper we describe tests and procedures to evaluate the throughput and FRD
increment. The lifetime of the ferrules is also in evaluation.
A wide field corrector with loss-less and purely passive atmospheric dispersion correction
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A 2.5 degree field diameter corrector lens design for the Cassegrain focus of the VISTA 4 meter telescope is presented.
It comprises four single elements of glasses with high UV transmission, all axi-symmetric for operation at the zenith.
One element is displaced laterally to provide atmospheric dispersion correction. A key feature, especially beneficial for
the VISTA application, is that the ADC element can be mounted so it is driven simply by gravity; thus its operation
needs no motors, encoders, cabling, or software control. A simple mechanical design to achieve this and the optical
performance details are described.
MEMS accelerometers in accurate mount positioning systems
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In order to attain precise, accurate and stateless positioning of telescope mounts we apply microelectromechanical
accelerometer systems (also known as MEMS accelerometers). In common practice, feedback from the mount
position is provided by electronic, optical or magneto-mechanical systems or via real-time astrometric solution
based on the acquired images. Hence, MEMS-based systems are completely independent from these mechanisms.
Our goal is to investigate the advantages and challenges of applying such devices and to reach the sub-arcminute
range { that is well smaller than the field-of-view of conventional imaging telescope systems. We present how
this sub-arcminute accuracy can be achieved with very cheap MEMS sensors. Basically, these sensors yield raw
output within an accuracy of a few degrees. We show what kind of calibration procedures could exploit spherical
and cylindrical constraints between accelerometer output channels in order to achieve the previously mentioned
accuracy level. We also demonstrate how can our implementation be inserted in a telescope control system.
Although this attainable precision is less than both the resolution of telescope mount drive mechanics and the
accuracy of astrometric solutions, the independent nature of attitude determination could significantly increase
the reliability of autonomous or remotely operated astronomical observations.