Proceedings Volume 5494

Optical Fabrication, Metrology, and Material Advancements for Telescopes

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Proceedings Volume 5494

Optical Fabrication, Metrology, and Material Advancements for Telescopes

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Volume Details

Date Published: 24 September 2004
Contents: 9 Sessions, 61 Papers, 0 Presentations
Conference: SPIE Astronomical Telescopes + Instrumentation 2004
Volume Number: 5494

Table of Contents

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Table of Contents

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  • Optical Fabrication I
  • Optical Fabrication II
  • Optical Metrology
  • IFU/Image Slicers I
  • IFU/Image Slicers II
  • VPH/Coatings/Fibres
  • Poster Session a: Optical Fabrication
  • Poster Session b: Optical Metrology
  • Poster Session c: VPH/Coatings/Fibers
  • Poster Session b: Optical Metrology
  • Poster Session c: VPH/Coatings/Fibers
Optical Fabrication I
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Corning's approach to segment blank manufacturing for an extremely large telescope
It is expected that the next generation of large ground based astronomical telescopes will be built using a segmented primary and secondary mirror versus monolithic blanks. Corning has proposed a process to be used in the manufacturing of segment mirror blanks from Corning ULE titania-silica glass in segment sizes ranging from 1.0 to 1.8 meters flat to flat. This paper will describe the facilities, equipment, resources and process required by Corning to produce several hundred to several thousand mirror segment blanks for extremely large telescope mirror blanks.
The SiC technology is ready for the next generation of extremely large telescopes
Michel Bougoin, Pierre Deny
Beside the baseline glass-ceramics, the sintered silicon carbide (SSiC) technology appears as a valid and very attractive alternative for the mass production of the next generation extremely large mirror segments. The specific stiffness of SSiC ceramic is about four times better than the one of glass-ceramics. Furthermore, the SSiC manufacturing process allows lightweighing the blanks at very cost effective condition. The spectacular expected gain in mass will dramatically relax the requirements of the moving structure, which supports the mirror segments. Boostec team has got two different and very useful experiences in SSiC components that are serial production of low cost parts and the manufacturing of the largest space telescopes particularly through Herschel ESA program. A paper study has clearly shown that the thousands of OWL segments could be made of SSiC at competitive costs. The needed investments are easily affordable and they do not require any technological development. On the other hand, four hexagonal segments have been designed (1 m flat to flat, only 44 kg/m2) and successfully manufactured in order to qualify this technology.
Cesic: manufacturing study for next generation telescopes
Matthias R. Kroedel, J. Lichtscheindl
Under ESO -- European Southern Observatory -- contract ECM has performed a feasibility study for the manufacturing of Cesic primary and secondary mirror segments for the OWL-Telescope. The main issues of this study were to demonstrate the feasibility of the serial production (~ 2550 segments) of Cesic mirror segments under a certain schedule and cost optimisation aspect for the segments. Part of this study was also a pre-design of a manufacturing facility for this big amount of mirror segments. This study is limited only up to the manufacturing of a polishable surface, the feasibility of the polishing capability is not part of this study.
Fabrication of blanks, figuring, polishing, and testing of segmented astronomic mirrors for SALT and LAMOST projects
Alexandr P. Semenov, Magomed A. Abdulkadyrov, Alexandr N. Ignatov, et al.
The technical capabilities of LZOS allow optics manufacturing from the phase of melting and annealing of Astrositall blanks to their final figuring. Now LZOS is working on the several large contracts, among them are manufacturing of 96 hexagonal segment blanks of 1019 mm x 55 mm of Sitall CO-115M for the SALT (Southern African Large Telescope) primary segmented mirror, and 40 sub-mirrors of Zerodur for the 6.7 m x 6 m segmented MB mirror for the LAMOST (Large Sky Area Multi-Object Spectroscopic Telescope) project.
JWST beryllium telescope: material and substrate fabrication
A new grade of beryllium, O-30, has been chosen for the primary, secondary, and tertiary cryogenic optics for the James Webb Space Telescope (JWST) program. This paper will describe the characterization of O-30 beryllium for this cryogenic space telescope, including cryogenic material properties. It will also show the cryogenic performance data that resulted in the selection of the O-30 beryllium for the JWST primary mirror, as compared to the other material candidate ULE. The paper will also describe the consolidation process of this 1.315-meter hexagon segment to produce a highly isotropic mirror segment. In addition, this paper will describe a technology effort called near net shape (NNS), to significantly reduce the cost of beryllium substrates for future cryogenic telescopes like SPICA, SAFIR, and GMST.
Highlights of the ASPE 2004 Winter Topical Meeting on Free-Form Optics: Design, Fabrication, Metrology, Assembly
Raymond G. Ohl IV, Thomas A. Dow, Alex Sohn, et al.
We present highlights from the American Society for Precision Engineering’s 2004 Winter Topical Meeting entitled "Free-Form Optics: Design, Fabrication, Metrology, Assembly." We emphasize those papers that are most relevant to astronomical optics. Optical surfaces that transcend the bounds of rotational symmetry have been implemented in novel optical systems with fantastic results since the release of Polaroid’s first instant camera. Despite these successes, free-form optics have found only a few niche applications and have yet to enter the mainstream. The purpose of this meeting was to identify the state of the art of free-form optics design, fabrication, metrology and assembly and to identify the technical and logistical challenges that inhibit their widespread use. Issues that were addressed include: What are free-form optics? How can optical systems be made better with free-form optics? How can designers use free-form optics? How can free-form optics be fabricated? How can they be measured? How are free-form optical systems assembled?
Gran Telescopio Canarias optics manufacture: progress report no. 3
Roland Geyl, Marc Cayrel, Michel Tarreau
This paper is intended to establish, after delivery of the first batch of segments, the third progress report of the manufacturing and testing of the Gran Telescopio Canarias optics.
Manufacture of 8.4-m off-axis segments: a 1/5-scale demonstration
Hubert M. Martin, James H. Burge, Brian Cuerden, et al.
We describe the requirements for manufacturing and maintaining alignment of the 8.4 m off-axis segments of the Giant Magellan Telescope’s primary mirror, and a demonstration of the manufacturing techniques on the 1.7 m off-axis primary mirror of the New Solar Telescope. This mirror is approximately a 1/5 scale model of a GMT segment. We show that the stressed lap polishing system developed for highly aspheric primary and secondary mirrors is capable of figuring the GMT segments and the NST mirror. We describe an optical test with a null corrector consisting of a tilted spherical mirror and a computer-generated hologram, and derive accuracy requirements for the test. The criterion for accuracy of low-order aberrations is that the active support system can correct any figure errors due to the laboratory measurement, with acceptably small forces and residual errors.
New results from the Precessions polishing process scaled to larger sizes
David D. Walker, Anthony T. H. Beaucamp, David Brooks, et al.
The Precessions process uses an inflated membrane-tool that delivers near-Gaussian polishing spots. The tool-motion over the part can be constructed to preserve an aspheric form whilst removing damage from preceding processes, or control the form through a tool-path prescribed by numerical optimization. The process has previously been validated on surfaces up to 200mm diameter and used extensively in industrial environments. In this paper we report the first trials on a substantially larger part - a 500mm diameter f/1 ellipsoidal mirror - as part of the UK’s technology-development for Extremely Large Telescopes. We draw attention to subtle problems that have arisen along the way. We also report on developing the process for free-form surfaces, in contrast to the axially-symmetric parts worked hitherto. The paper concludes with an assessment of the lessons learnt from the experiments, as they may impact on realization in a practical ELT segment fabrication facility.
Magnetorheological finishing and subaperture stitching interferometry of large and lightweight optics
Significant challenges are faced in the manufacturing of the complex optics for the next generation of astronomical telescopes. Process improvements are required to establish cost effective techniques to finish the optics to the tight specification required in a timely manner. An added complication is realized when the optics are lightweight. The non-uniform support of the face-sheet in this case requires special efforts to avoid a print-through of the cell structure due to fabrication processes, gravity and/or cryogenic effects. Magnetorheological finishing (MRF) is a deterministic, sub-aperture polishing process that has been a revolutionary success in the fabrication of optics in the size range of 10-1000 mm. This production proven process is capable of polishing flats, spheres, aspheres and cylinders to a surface figure accuracy of better than 30 nm peak-to-valley (better than 5 nm rms), and microroughness better than 1 nm rms on a variety of glasses, glass ceramics and single crystal materials. Unique characteristics of MRF such as a high, stable removal rate, conformal nature of the sub-aperture tool and shear mode of material removal give it advantages in the finishing of large and lightweight optics. These qualities provide for a cost-effective process with a high rate of convergence that requires few iterations. Such a technology is ideally complemented by a system for the stitching of interferometric sub-aperture data. Stitching inherently enables the testing of larger apertures with higher resolution and, thanks to the built-in calibration, even to higher accuracy in many situations. While this approach enables the non-null testing of parts with greater aspheric departure and can lead to a significantly reduced non-common air path in the testing of long-radius concave parts, it is especially effective for convex optics. That is, stitching is particularly well suited to the testing of secondary mirrors and, alongside the testing of the off-axis primary segments.
New perspectives in hydrodynamic radial polishing techniques for optical surfaces
In order to overcome classic polishing techniques, a novel hydrodynamic radial polishing tool (HyDRa) is presented; it is useful for the corrective lapping and fine polishing of diverse materials by means of a low-cost abrasive flux and a hydrostatic suspension system that avoids contact of the tool with the working surface. This tool enables the work on flat or curved surfaces of currently up to two and a half meters in diameter. It has the advantage of avoiding fallen edges during the polishing process as well as reducing tool wear out and deformation. The functioning principle is based on the generation of a high-velocity, high-pressure, abrasive emulsion flux with radial geometry. The polishing process is repeatable by means of the control of the tool operational parameters, achieving high degrees of precision and accuracy on optical and semiconductor surfaces, with removal rates of up to 9 mm3/hour and promising excellent surface polishing qualities. An additional advantage of this new tool is the possibility to perform interferometric measurements during the polishing process without the need of dismounting the working surface. A series of advantages of this method, numerical simulations and experimental results are described.
Optical Fabrication II
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Active optics and the non-axisymmetric case: multimode deformable mirrors aspherized from vase and meniscus forms
The realizations of non-axisymmetric aspherical surfaces is of rapid growing interest for the development of segmented ELTs and astronomical instrumentation. Active Optics methods have already proved useful for the aspherization of off-axis segments of a paraboloid. The analogy between optical aberration modes and elastic deformation modes leads to consider Clebsch-Zernike modes that belongs to a subclass of aberration modes and are available up to high-order optical corrections. The meniscus form as well as the vase form allow the coaddition of these modes. Several loading configurations associated to bending moments are presented including interferometric results for coadditions with multimode deformable mirrors -- MDMs -- and for some monomode mirrors with bending moments geenrated by a single force distribution. Active Optics methods are also under development for the Axisymmetric Case with vases and meniscuses (cf. this Conference).
Criterion to appreciate difficulties of aspherical polishing
The increase of optical resolution and size of astronomical telescopes needs to: (1) Improve surface quality of optical components. (2) Use more and more complex aspherical shape for lenses and mirrors. Due to these 2 constraints, the optical manufacturers had to improve theirs equipments such as the Computer Controlled Polishing and to appreciate the feasibility of very complex aspherical shape. Concerning this second point, we propose a new criterion based on surface definition, on quality specification for the polishing and on limitations of equipments. Manufacturing examples show that this criterion is well representative, and particularly for direct off-axis polishing.
Optical Metrology
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High-accuracy surface figure measurement of silicon mirrors at 80 K
Peter N. Blake, Ronald G. Mink, John Chambers, et al.
This report describes the equipment, experimental methods, and first results at a new facility at the Goddard Space Flight Center Optics Branch for interferometric measurement of cryogenically-cooled spherical mirrors. A mirror is cooled to 80 K and 20 K within a cryostat; and its surface figure error is measured through a fused-silica window using standard phase-shifting interferometry. The first mirror tested was a concave spherical silicon foam-core mirror with a clear aperture of 120 mm. The optic surface was measured at room temperature outside the dewar using standard "absolute" techniques; and then the change in surface figure error within the dewar from room temperature to 80 K was measured, and the two measurements added to create a representation of the two-dimensional surface figure error at 80 K, with a combined standard uncertainty of 3.4 nm rms. The facility and techniques will be used to measure the surface figure error at 20K of prototype lightweight silicon carbide and Cesic mirrors developed by Galileo Avionica (Italy) for the European Space Agency (ESA).
High-precision metrology on the ultra-lightweight UV 51-cm f/1.25 parabolic SHARPI primary mirror using a CGH null lens
Scott Antonille, David A. Content, Doug Rabin, et al.
For potential use on the SHARPI mission, Eastman Kodak has delivered a 50.8cm CA f/1.25 ultra-lightweight UV parabolic mirror with a surface figure error requirement of 6nm RMS. We address the challenges involved in verifying and mapping the surface error of this large lightweight mirror to 3nm RMS using a diffractive CGH null lens. Of main concern is removal of large systematic errors resulting from surface deflections of the mirror due to gravity as well as smaller contributions from system misalignment and reference optic errors. We present our efforts to characterize these errors and remove their wavefront error contribution in post-processing as well as minimizing the uncertainty these calculations introduce. Data from Kodak and preliminary measurements from NASA Goddard will be included.
Current concepts for cryogenic optical testing of the JWST secondary mirror
Koby Z. Smith, John P. Schwenker, Robert J. Brown, et al.
The James Webb Space Telescope (JWST) Secondary Mirror (SM) is a 738 mm edge-diameter convex hyperbola that will be operating at 30K. Due to JWST’s science and technical requirements, the requirements on the SM are relatively tight. Therefore highly accurate, rigorous cryogenic testing of the surface figure as well as the prescription is required. The optical testing of a convex mirror of this size has not been performed before at cryogenic temperatures. This paper discusses the testing approaches and configurations that are under consideration at Ball Aerospace & Technologies Corp. (BATC) for testing the JWST SM at cryogenic temperatures.
Instantaneous phase-shifted speckle interferometer for measurement of large optical structures
Digital Speckle Pattern Interferometry (DSPI) is a well-established method for the measurement of diffuse objects in experimental mechanics. DSPIs are phase shifting interferometers. Three or four bucket temporal phase shifting algorithms are commonly used to provide phase shifting. These algorithms are sensitive to vibrations and can not be used to measure large optical structures far away from the interferometer. In this research a simultaneous phase shifted interferometer, PhaseCam product of 4D Technology Corporation in Tucson Arizona, is modified to be a Simultaneous phase shifted Digital Speckle Pattern Interferometer (SDSPI). Repeatability, dynamic range, and accuracy of the SDSPI are characterized by measuring a 5 cm x 5 cm carbon fiber coupon.
IFU/Image Slicers I
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Freeform diamond machining of complex monolithic metal optics for integral field systems
Implementation of the optical designs of image slicing Integral Field Systems requires accurate alignment of a large number of small (and therefore difficult to manipulate) optical components. In order to facilitate the integration of these complex systems, the Astronomical Instrumentation Group (AIG) of the University of Durham, in collaboration with the Labor für Mikrozerspanung (Laboratory for Precision Machining - LFM) of the University of Bremen, have developed a technique for fabricating monolithic multi-faceted mirror arrays using freeform diamond machining. Using this technique, the inherent accuracy of the diamond machining equipment is exploited to achieve the required relative alignment accuracy of the facets, as well as an excellent optical surface quality for each individual facet. Monolithic arrays manufactured using this freeform diamond machining technique were successfully applied in the Integral Field Unit for the GEMINI Near-InfraRed Spectrograph (GNIRS IFU), which was recently installed at GEMINI South. Details of their fabrication process and optical performance are presented in this paper. In addition, the direction of current development work, conducted under the auspices of the Durham Instrumentation R&D Program supported by the UK Particle Physics and Astronomy Research Council (PPARC), will be discussed. The main emphasis of this research is to improve further the optical performance of diamond machined components, as well as to streamline the production and quality control processes with a view to making this technique suitable for multi-IFU instruments such as KMOS etc., which require series production of large quantities of optical components.
Image slicers: design for manufacturability
David Lee, Colin J. Dickson, Peter R. Hastings, et al.
This paper describes our approach to the design of reflective, image-slicing Integral-Field Units. The first IFU built at the UK ATC (Ramsay-Howat et al 2004) has been successfully commissioned and is in routine use on the UKIRT. This IFU uses all diamond-turned optics and close attention was paid in the design to the manufacturability of the individual components, their alignment into groups and the overall alignment and integration of the IFU into the spectrometer. Building on this experience we are designing and making four IFUs for the MIRI on JWST where constraints on mass and launch survivability have led us to use monolithic arrays of slicing and re-imaging mirrors. This paper describes the ways in which the optical design and the manufacturing methods have each been adapted to: (a) achieve image quality, pupil alignment and mass requirements (b) ensure components can be reliably manufactured to the tolerances needed so that the IFUs can be aligned without adjustment and (c) ensure that performance is maintained after launch and cooling to 7 K. We also show that, although designed specifically for a space instrument, such a compact and reproducible IFU is well suited to instruments where multiple IFUs are required.
Image slicer manufacturing: from space application to mass production
Christophe Bonneville, Jean-Francois Cagnat, Florence Laurent, et al.
This presentation aims to show technical and industrial inputs to be taking into account for Image Slicer systems design and development for different types of projects from space application to mass production for multi-IFU instruments. Cybernetix has a strong experience of precision optics assembled thanks to molecular adhesion and have already manufactured 6 prototypes of image slicer subsystem (prototypes of NIRSPEC-IFU, IFS for JWST, MUSE ...) in collaboration with the Laboratoire d’Astrophysique de Marseille (LAM) and the Centre de Recherche Astronomique de Lyon (CRAL). After a brief presentation of the principle of manufacturing and assembly, we will focus on the different performances achieved in our prototypes of slicer mirrors, pupil and slit mirrors lines: an accuracy on centre of curvature position better than 15 arsec has been obtained for a stack of 30 slices. The contribution of the slice stacking to this error is lower than 4 arcsec. In spite of very thin surfaces (~ 0.9 x 40 mm for instance), a special process allows to guarantee a surface roughness about 5 nm and very few digs on the slice borders. The WFE of the mini-mirror can also be measured at a stage of the manufacturing. Different environmental tests have shown the withstanding of these assemblies to cryogenic temperature (30 K). Then, we will describe the different solutions (spherical, flat, cylindrical surfaces) and characteristics of an image slicer that can influence difficulties of manufacturing and metrology, cost, schedule and risks with regard to fabrication. Finally, the study of a mass production plan for MUSE (CRAL) composed of 24 Image Slicers of 38 slices, that’s to say 912 slices, will be exposed as an example of what can be do for multi-module instruments.
IFU/Image Slicers II
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Designing, manufacturing, and testing of an advanced image slicer prototype for the James Webb Space Telescope
Florence Laurent, Charles Macaire, Pierre-Eric Blanc, et al.
In the frame of an European Space Agency (ESA) contract, a consortium of three European research institutes (Laboratoire d'Astrophysiqu de Marseille, Centre de Recherche Astronomique de Lyon and the University of Durham) and the Cybernetix company have designed, manufactured and tested a prototype of an Integral Field Unit (IFU) for the NIRSpec instrument of the future James Webb Space Telescope (JWST). After a brief presentation of the optical design of this prototype, which is based on the advanced slicer concept, we will focus on the optical tests of this prototype. We will first present the tests peformed at LAM on the individual optical elements prior to their integration in the mechanical structure, as well as the alignment tests conducted as part of the integration procedure. We will then describe the tests and their results in the visible of the complete IFU system both at room temperature (tests performed at CRAL) and at operating temperature (30 K, tests peformed at the University of Durham). Briefly, these tests included: measurements of the characteristics (position, shape, size ...) of the pseudo-slit of the IFU prototype; measurement of the point-spread-function at different locations within its field of view; and measurement of the position, shape and size of the exit pupils. Last, we will conclude on the TRL6 readiness of the advanced image slicer technique and we will provide a glimpse of how wide-spread this technique is becoming both for ground- and space-based applications.
VPH/Coatings/Fibres
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Mosaiced and high line frequency VPH gratings for astronomy
To increase the size of the volume phase holographic gratings the Centre Spatial de Liege can produce, mosaic technic has been tested and characterized. This method consists of assembling VPH gratings recorded and processed independently into one larger grating. By this way, the final grating size becomes virtually unlimited and dispersive elements can accommodate the largest telescope beams. The second research line about VPH gratings was the high line frequency domain: ν > 3000 lp/mm. Actually, for these frequencies, diffraction according to TE and TM modes is maximum for different wavelengths. However, it is possible to tune the index modulation to three times what is usually required to use the first diffraction TE peak. In this case, the second TE maximum matches the first TM maximum and unpolarized light is so entirely diffracted. This article also summarizes our prospects in the field of very high index modulation gratings where Δn as high as 0.14 has been reached; cryogenic temperature operation for which we have demonstrated our VPH gratings stand -180°C without any Blaze modification; and wavefront correction by post-polishing to minimize diffracted beam aberrations. With this latter technique, λ/6 wavefront over 10 cm diameter has been obtained in the first trial.
Optimal design and fabrication of high dispersion VPH grism for Subaru Telescope
Masako Kashiwagi, Keiko Oka, Misako Irisawa, et al.
We fabricated a large size (110x106mm2) Volume Phase Holographic (VPH) grism which was extremely high efficiency and dispersion. In June 2003, the VPH grism with straight wavelength of 450nm and the resolution 2500, installed in Subaru telescope used for the Faint Object Camera And Spectroscopic (FOCAS) for a test observation. As a result of this performance assessment, it was verified that our optimal design and fabrication process was valid. Liquid photopolymer is used for VPH grating as photosensitive materials. We design the optimal parameters such as grating thickness and strength of the refractive index modulation, which is crucial in obtaining a well performing grism, by reference to Rigorous Coupled Wave Analysis (RCWA). Here, the optimal design and fabrication procedure, and the evaluation of high performance VPH grism (wavelength:520nm, resolution:3000, grating period:1μm) will be discussed.
Where have our VPHGs gone?
Emilio Molinari, Andrea Bianco, Chiara Bertarelli, et al.
The new possibilities offered since few years by the use of Volume Phase Holographic Gratings (VPHG) in astronomy have not yet yielded a new generation of instrumentation. New larger telescopes await their light analyzing devices and the higher throughput, higher versatility and lower cost of VPHG still do not award their position. We seems to have reached a stability in gratings manufacturing but their use resides mainly in R&D sector. Our group in Italy does not walk too far off this road and proposes a series of non-conventional uses of VPHG aside of refurbishing of classical, existing instrumentation.
Coating the 8-m Gemini telescopes with protected silver
Maxime Boccas, Tomislav Vucina, Claudio Araya, et al.
The Gemini telescopes were designed to be infrared-optimized. Among the features specified for optimal performance is the use of silver-based coatings on the mirrors. The feasibility study contracted by Gemini in 1994-1995 provided both techniques and recipes to apply these high-reflectivity and low-emissivity films. All this effort is now being implemented in our coating plants. At the time of the study, sputtering experiments showed that a reflectivity of 99.1% at 10μm was achievable. We have now produced bare and protected silver sputtered films in our coating plants and conducted environmental testing, both accelerated and in real-life conditions, to assess the durability. We have also already applied, for the first time ever, protected-silver coatings on the main optical elements (M1, M2 and M3) of an 8-m telescope. We report here the progress to date, the performance of the films, and our long-term plans for mirror coatings and maintenance.
Durable coatings for large segmented mirrors
The reflective coatings applied to telescope mirrors affect not only the optical performance, but also affect significantly the telescope operation. Replacement of a primary segment of a large segmented mirror is expected to be a major event. An increased service life span of such segment is of enormous value. The optics community is currently aggressively pursuing development in broadband high reflective durable coatings. We are undertaking research with the goal of a high, broadband reflective coatings that, with appropriate cleaning and in situ maintenance, will provide a service life time of more than seven years. Based on the VLOT (Very Large Optical Telescope) project requirements, we conducted a literature search on available materials, thin film deposition and cleaning processes to get as much information as possible. The results of this survey will be presented as the starting point of our study. Different thin film processes have been identified but energetic processes such as Reactive Low Voltage Ion Plating (RLVIP), Magnetron Sputtering and Ion Beam Assisted Deposition (IBAD) will be of great interest for durable coating fabrication. Regarding the cleaning process, we have concentrated our effort on laser cleaning processes.
New improvements in reflecting-absorbing coatings for astronomy
More and more complex telescopes and astronomical instruments require: (1) Mechanics with low diffusion/reflection properties in order to decrease the level of scattered light, especially for use in infrared. (2) Mirrors with high reflectivities, in order to improve the level of useful light and/or extend the useful spectrum, especially to the near UV(320 nm - 400 nm), SESO will present simultaneously here some results of its current developments concerning: (1) Solutions to increase the signal/noise ratio for systems operating in the infrared and needing a cryogenic cryostat assembly for the detection system, (2) A high efficiency reflective UV-coating.
Cryogenic VPH gratings for the CELT/TMT
Sebastien Blais-Ouellette, Dani Guzman, Amal Elgamil, et al.
Characterization of Volume Phase Holographic gratings at cryogenic temperatures have been conducted using a new test facility at Caltech. The new test bench includes a cryostat that allows large angles for incident and diffracted light. Gratings under tests are shielded from thermal background, and precisely and uniformly temperature controlled. Preliminary results are presented and show little temperature dependence of the efficiency function.
Poster Session a: Optical Fabrication
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CeSiC a new technology for lightweight and cost-effective space instrument structures and mirrors
Since years Alcatel Space is interested in the development of a new material to produce lightweight, stable and cost effective structures and mirrors for space instrument. CeSiC from ECM has been selected for its intrinsic properties (high specific modulus, high conductivity, low thermal expansion coefficient and high fracture toughness), added to ample manufacturing capabilities. Based on first results on a full scale demonstration model made under own R&D project funding, Alcatel Space is now in position to propose for space project this technology mastered in common with ECM. In the frame of NIRSPEC project, instrument of JWST, an optical bench breadboard made of CeSiC has been designed, manufactured and tested under ESA contract. The aim of this breadboard was to confirm the high lightweight capabilities of CeSiC technology on such large and complex structure, its strength and stability under vibration and cryogenic environment, and therefore its capability to be used for the NIRSPEC optical bench. In addition, a database of this material has been built, thanks tests performed by Alcatel Space and by ECM, with the support of an additional specific ESA contract. Other immediate application is the use of CeSiC for large and lightweight space telescopes mirrors which shall be cladded for polishing and reach roughness up to visible range applications needs. CeSiC is also foreseen for the future generation of focal plane offering a very stable structure holding all the detectors thanks to CeSiC rigidity and stability performances, and its high thermal conductivity.
Cesic: engineering material for optics and structures
Cesic -- a trademark of ECM for carbon-fiber reinforced silicon carbide -- allows relatively quick and cheap manufacturing of components with good repeatability and reproducibility due to the implemented product assurance (PA) system. Through a joining process and our development of optical surfaces, Cesic allows for a direct up-scaling of structures and optical surfaces to large-size applications and systems. The size of the structures and mirrors that can be manufactured is limited only by the scale of the available furnaces facilities, the largest of which currently is 2.4 m in diameter. Under ESA contract ECM performed a measurement program to get reliable material properties data on Cesic for space and other future programs.
Development of sintered-SiC and C/SiC mirrors for cryogenic telescope
Giuseppe Mondello, Andrea Novi, Christophe Devilliers
Mirror prototypes in cold-pressed sintered SiC-54 and in Carbon-SiC (Cesic) have been designed, manufactured and optically tested. The scope of the work was the development of materials, technologies and manufacturing processes to get high quality optics very stable at cryotemperatures. The activity has been performed under ESA funding in the frame of the technology development for the JWST/NIRSpec program. A description of the polishing performance and final testing results are discussed.
Low-thermal expansion material for telescope mirror substrate application
Kousuke Nakajima, Nobuo Kawasaki, Toshihide Nakajima
The material property and processability of the low thermal expansion glass-ceramics product by Ohara Inc. called CLEARCERAM-Z were studied for telescope mirror substrate application. For material property, numbers of the key properties for the application, such as Coefficient of Thermal Expansion (CTE) characteristic in wide temperature range, Stress Birefringence and Mechanical strengths were intensively investigated focusing on the blank uniformity. The mean CTE of +0.15x10-7/degree C in wide temperature range (-50 to +150degree C) with the standard deviation (Std.) of 0.03x10-7/degree C and Young’s Modulus & Poisson Ratio data with the coefficient of variation less than 1% were obtained for the blanks with the size up to Dia.670mm. The maximum Stress Birefringence was 3nm/cm within a 400mm square blank. For processability, the surface finish data of AFM Rms 0.15nm, the Power Spectral Density profile in the same level of low expansion amorphous glass and three dimensional structured samples were demonstrated. The comparison of the obtained data with known blanks specification for past and future telescope projects revealed that CLEARCERAM-Z has capability to meet material property requirements for telescope mirror substrate application in the size up to Dia.670mm. Also for the precision metrology to support the material technology, CTE measurement system developed at OHARA was described.
Strategies of primary mirror segment fabrication for CFGT
An extremely large telescope named Chinese Future Giant Telescope (CFGT) has been presented. The primary mirror of CFGT is a 30-meter diameter hyperboloid with a focal ratio F/1.2 and it consists of over one thousand of sector-shaped segments with the size about 1.1-meter in diagonal. Based on the optical design concept and the experience of existing large segmented primary mirror, we explore the segment fabrication and testing issues in this paper. The relationship between external contour, the size and the asphericity of sub-mirror is studied. Two potential segment fabrication approaches for mass-production-scale are discussed. One is the optical replication. The other is stressed-mirror polishing. Both of two processes are tightly combined with several key techniques and devices, the ion-beam figuring, large annular polisher, and the stressed lap. Some preliminary concepts for testing of l-meter class convex/concave off-axis aspheric surface are discussed.
Production of the 4.1-m Zerodur mirror blank for the VISTA Telescope
Thorsten Doehring, Ralf Jedamzik, Volker Wittmer, et al.
VISTA (Visible and Infrared Survey Telescope for Astronomy) is designed to be the world's largest wide field telescope. After finishing of the construction the telescope will be part of ESO and located in Chile close to the VLT observatory at Cerro Paranal. In November 2001 SCHOTT was selected by the VISTA project office at the Royal Observatory of Edinburgh to deliver the 4.1 m diameter primary mirror blank. The manufacturing of the mirror blank made from the zero expansion material Zerodur was challenging especially due to the f/1 design. Several tons of the glass ceramic material were removed during the grinding operation. A meniscus blank with a diameter of 4100 mm and a thickness of 171.5 mm was generated, having a large central hole of 1200 mm and an aspherical shape of the concave surface. Also the handling and turning operations needed special effort and were performed by a skilled team. This paper presents details and pictures of the corresponding production and inspection sequence at SCHOTT. The geometrical parameters were measured during manufacturing by help of a laser tracker system and the achieved parameters were compared with the initial technical specification. The final quality inspection verified the excellent quality of the mirror blank. The close co-operation between the astronomers and industry resulted in a project management without problems. In April 2003 the VISTA blank was delivered successfully within a ceremony dedicated to the anniversary of "100 years of astronomical mirror blanks from SCHOTT."
Selection of a mirror technology for the 1.8-m Terrestrial Planet Finder demonstrator mission
Detection of extrasolar planets should be possible with a telescope that has the required resolution and a coronagraph to block the starlight. The resolution that is needed suggests that the diameter of the primary mirror be at least 6 m. For use in space, the mirror would need to be moderately lightweighted, with an areal density of roughly 50 kg/m2 or lower. Most important is the surface quality of the mirror over the spatial frequency range of roughly 10 cm to 4 m. A ripple in the surface of the mirror, with a spatial scale in this range, would cause starlight to diffract onto the region where a planet may be located. In terms of an rms surface error the mirror would need to be better than 5 nm rms in this range. The Terrestrial Planet Finder (TPF) project realized that to demonstrate that a coronagraphic telescope concept could be used for terrestrial planet detection there needs to be a demonstration that a mirror of the required technology could be built. There are several concepts that could be used for designing and fabricating such a mirror but in order to select the most promising technology a survey of the best mirror concepts from the best large mirror builders was needed. This paper describes what was learned from this study and the rational for the mirror concept that was selected.
Mechanical and thermal performance of C/SiC composites for SPICA mirror
Tsuyoshi Ozaki, Masami Kume, Takeharu Oshima, et al.
One of the key technologies for next generation space telescope with a large-scale reflector is a material having high specific strength, high specific stiffness, low coefficient of thermal expansion and high coefficient of thermal conductivity. Several candidates such as fused silica, beryllium, silicon carbide and carbon fiber reinforced composites have been evaluated. Pitch-based carbon fiber reinforced SiC composites were developed for the SPICA space telescope mirror to comply with such requirements. Mechanical performance such as bending stiffness, bending strength and fracture toughness was significantly improved. Evaluation procedures of thermal expansion and thermal conductivity behavior at cryogenic temperatures (as low as 4.5K) were established and excellent performance for the SPICA mirror was demonstrated.
M1 and M2 mirrors manufacturing for VISTA telescope
Magomed A. Abdulkadyrov, Alexandr N. Ignatov, Vladimir E. Patrikeev, et al.
Nowadays LZOS is carrying out work on the manufacturing of the M1 Mirror and M2 Mirror for the VISTA project (Visible and Infrared Survey Telescope for Astronomy) with the 4100 mm diameter primary hyperbolic mirror with asphericity about 800 μmi and the 1241 mm diameter secondary hyperbolic mirror with asphericity about 350 μm. The current status of the work carried out is presented in the manuscript.
Large optical glass blanks for astronomy
At present extremely large telescopes are planned with primary mirrors from 20 m up to 100 m. Such telescopes need not only huge mirror arrays but also downstream refractive optics like atmospheric dispersion correctors, color correctors for imaging and beam shapers for spectrometers. For classical boro-crown and lead-flint glass types blanks have been made in the past up to about 1 m. Now there is an increasing demand to be expected for optical glass blanks with diameters up to 1.5 m for use as lenses or prisms. Additionally optics designers ask for glass types like the low dispersion fluoro-phosphate glasses. The production of high quality blanks of such glasses has been a challenge even for diameters around 200 mm. This presentation shall give information about the feasibility of large glass blanks and recommendations how to specify the quality balancing the requirements of the application on one side and the possibilities and conditions of the production and the measurement for inspection on the other side.
Large lenses, mirrors, and filters for astronomy
This manuscript is intended to report some recent astronomical optics achievement in term of large lenses, large area filters, off-axis aspheric mirrors and the SALT Spherical Aberration Corrector (SAC).
GRANTECAN telescope M3 mirror manufacturing
Magomed A. Abdulkadyrov, Alexandr N. Ignatov, Vladimir E. Patrikeev, et al.
LZOS manufactured the flat surface of the M3 mirror for the GRANTECAN Project. The mirror has an elliptical configuration and axial dimensions of 1521 mm x 1073 mm. The results of the M3 mirror figuring and final polishing, testing in the Ritchey-Common layout at two 30° and 60° angles of incidence of a main beam are presented in the manuscript.
The DART cylindrical infrared 1-meter membrane reflector
Rhonda M. Morgan, Greg A. Agnes, Dan Barber, et al.
The Dual Anamorphic Reflector Telescope (DART) is an architecture for large aperture space telescopes that enables the use of membranes. A membrance can be readily shaped in one direction of curvature using a combination of boundary control and tensioning, yielding a cylindrical reflector. Two cylindrical reflectors (orthogonal and confocal) comprise the 'primary mirror' of the telescope system. The aperture is completely unobstructed and ideal for infrared and high contrast observations. The DART high precision testbed researches fabrication, assembly, adjustment and characterization of 1 meter cylindrical membrane reflectors made of copper foil or kapton. We have implemented two metrology instruments: a non-contacting, scanning profilometer and an infrared interferometer. The profilometer is a laser confocal displacement measuring unit on an XYZ scanning stage. The infrared interferometer used a cylindrical null lens that tests a subaperture of the membrane at center of curvature. Current surface figure achieved is 25 μm rms over a 50 cm diameter aperture.
Poster Session b: Optical Metrology
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Laser metrology for distance and displacement measurement: application to large optical systems
Olivier Gry, Bertran Liagre, Anne Poupinet, et al.
In the frame of the CNES R&T activity related to laser metrology, SAGEIS-CSO is actually developing a new smart interferometric sensor, last to arrive in the "MOUSE" family. In this article, we give at first a brief oversight of the MOUSE interferometric concept as well as its performances for two kinds of applications. Secondly, some interesting other set-ups for space applications are mentioned and finally we present modifications that the system will undergo in order to be compliant with long distance measurements. These different developments point out the ability of the MOUSE/IASI interferometric system to be a powerful metrological instrument for space environment.
Active optics and the axisymmetric case: MINITRUST wide-field three-reflection telescopes with mirrors aspherized from tulip and vase forms
Gerard R. Lemaitre, Pierre Montiel, Patrice Joulie, et al.
Wide-field astronomy requires larger size telescopes. Compared to the catadioptric Schmidt, the optical properties of a three mirror telescope provides significant advantages. (1) The flat field design is anastigmatic at any wavelength, (2) the system is extremely compact -- four times shorter than a Schmidt -- and, (3) compared to a Schmidt with refractive corrector -- requiring the polishing of three optical surfaces --, the presently proposed Modified-Rumsey design uses all of eight available free parameters of a flat fielded anastigmatic three mirror telescope for mirrors generated by active optics methods. Compared to a Rumsey design, these parameters include the additional slope continuity condition at the primary-tertiary link for in-situ stressing and aspherization from a common sphere. Then, active optics allows the polishing of only two spherical surfaces: the combined primary-tertiary mirror and the secondary mirror. All mirrors are spheroids of the hyperboloid type. This compact system is of interest for space and ground-based astronomy and allows to built larger wide-field telescopes such as demonstrated by the design and construction of identical telescopes MINITRUST-1 and -2, f/5 - 2° FOV, consisting of an in-situ stressed double vase form primary-tertiary and of a stress polished tulip form secondary. Optical tests of these telescopes, showing diffraction limited images, are presented.
Reflectometer shoot-out: comparing the performance and accuracy of hand-held reflectometers
The reflectivity of mirror surfaces is a critical aspect of telescope performance. This is especially true of large adaptive optics systems and interferometers where twenty or more reflective surfaces may be encountered prior to reaching the final detector. The ability to accurately measure the reflectivity of mirrors in-situ allows not only for the determination of system throughput, but also helps to determine when optical elements need cleaning or recoating. We compare the performance of three commercially available, hand-held reflectometers by measuring calibrated reference surfaces of varying reflectivity and scatter characteristics. The reference surfaces are precisely calibrated for absolute reflectivity against NIST traceable standards using a state-of-the-art laboratory reflectometer. Through analysis of the reflectometer measurements of the precisely calibrated surfaces, we determine the relative ability of each reflectometer to measure reflectivity and differentiate scattered light from specular reflection.
Incoming metrology of segmented x-ray mandrels at MSFC
Mikhail V. Gubarev, Steve L. O'Dell, Thomas J. Kester, et al.
The baseline design of the Constellation-X Spectroscopy X-ray telescope (SXT) employs segmented x-ray mirrors, to be replicated from precision mandrels. Thus far, the Constellation-X Project has procured and received three (3) flight-scale mandrels, for use in development of mirror technologies. Complementary to 30° sections of 10-m-focal-length Wolter-1 optics of diameters 1.6, 1.2, and 1.0 m, the mandrels’ primary (parabolic) and secondary (hyperbolic) optical surfaces are each 0.5-m long. In order to generate surface maps for x-ray performance predictions, NASA’s Marshall Space Flight Center (MSFC) is conducting incoming metrology. Using a combination of instruments, this metrology measures axial-slope deviations and axial profiles, slope differences, roundness, absolute radius, and micro-roughness. This paper describes the mandrels, the metrology requirements, and MSFC’s metrology instrumentation and procedures.
Surface roughness results using a hydrodynamic polishing tool (HyDra)
Manuel Nunez, Javier Salinas, Esteban Luna, et al.
HyDRa is a hydrodynamic radial polishing tool ideal for the corrective lapping and fine polishing of diverse materials by means of an accelerated abrasive flux. The roughness of an optical surface is analysed for a continuous manufacturing process, beginning with the basic generation steps up to a finished optical surface. These results were obtained using a Linnik interferometer.
Telescope simulator using CGH and deformable mirror
Test and characterization of the optical performance for the novel and advanced sub-optical-systems, as designed for large astronomical optical telescopes, often need to be carried out with the simulated telescope wavefront in the labs before working with the telescopes. Computer generated hologram (CGH) is a simple solution for monochromatic wavefront generation. Severely off-axis aberrated wavefront coming from the one-mirror or two-mirrors telescope can be generated by CGH with symmetric and un-symmetric terms of the Zernike polynomials. The chrome mask and/or the binary CGHs have been made as the CFH telescope simulator. The unused diffraction orders of the CGH can be completely eliminated by adding the carrier frequency on the phase function of the designed CGHs. The test results that are given with the fabricated CGH simulator have a good agreement with that of the simulation. The aberrated wavefront of the simulated telescope can be also generated by the ferrofluid deformable mirror (FDM) at low cost. A 100-mm FDM is designed and built with 271 actuators for performance evaluation. The revolution symmetric aberrated wavefront has been experimentally generated and proven by the FDM prototyping. The simulation and test results for the prototyping FDM are both given in this paper.
Static and dynamic micro deformable mirror characterization by phase-shifting and time-averaged interferometry
Since micro deformable mirrors based on Micro-Opto-Electronico-Mechanical Systems (MOEMS) technology would be essential in next generation adaptive optics system, we are designing, realizing and characterizing blocks of this key-component. An in-house designed tiltable mirror (170*100 μm2) has been processed by surface micromachining in the Cronos foundry, and a dedicated characterization bench has been developed for the complete analysis of building blocks as well as operational deformable mirrors. This modular Twyman-Green interferometer allows high in-plane resolution (4μm) or large field of view (40mm). Out-of-plane measurements are performed with phase-shifting interferometry showing highly repeatable results (standard deviation<5nm). Features such as optical quality or electro-mechanical behavior are extracted from these high precision three-dimensional component maps. Range is increased without loosing accuracy by using two-wavelength phase-shifting interferometry authorizing large steps measurements such as 590 nm print-through steps caused by the Cronos process. Dynamic analysis like vibration mode and cut-off frequency is realized with time-averaged interferometry. Rotation mode frequency of 31±3kHz of the micro tiltable mirror, and a resonance with a tuned damping at 1.1kHz of the commercial OKO deformable mirror are revealed.
Poster Session c: VPH/Coatings/Fibers
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Advances in telescope mirror cleaning
Maarten F. Blanken, Alan K. Chopping, Kevin M. Dee
Metrology and cleaning techniques for telescope mirrors are generally well established. CO2 cleaning and water washing are mainly used. Water washing has proven to be the best method of removing oil and water stains and restoring the aluminium to nearly fresh values. The risk of water getting to unwanted places such as electronics or other optics prevents this method from being employed more often. Recently the Isaac Newton Group introduced a new cleaning technique for their telescope mirrors, which reduces the risks discussed above. This technique uses water vapour instead of water to wash the mirror. The advantage of this method is that the amount of water needed is drastically reduced. In addition the pressure of the vapour will blow away any large dust particles on the mirror and the temperature shock between the vapour and the mirror will help to de-bond the dust particles. Adding a soapy solution will help to clean oil and watermarks of the mirror. This paper describes the vapour cleaning method, tests that have been done and the overall findings.
Fabrication of high-precision solar filters: variety and limitations
Yakov Sidorin, Andrew G. Lunt, Jeff Bergthold, et al.
Using H-alpha etalon-based solar filter as an example, we address the issues of practical realization of etalon-based filters used for solar astronomy and other related applications. The design rationale as well as the choice among two conventional formats of etalons is described, depending on the requirements on the devised solar filter.
Designing ultra-narrowband interference filters
To understand the factors involved in the cost and risk of manufacture of ultra-narrowband imaging filters in the near IR, we have carried out detailed design work for a range of different filter specifications. This work was essential to the success of the DAZLE near-IR imaging spectrograph, a project led by the Institute of Astronomy at the University of Cambridge. The necessity for very narrow passbands (approximately 1 nm), for observing in the windows between the OH emission lines in the near-IR bands, requires coatings of more than 200 layers and greater than 30 μm thickness. Broadband filters to block OH and thermal emission over wide near-IR spectral ranges (1.4 - 2.8 μm) also require many thin film layers in the filter coatings. In most cases, the large number of layers is the most significant factor in determining manufacturing cost, as such coatings require long periods of time for deposition (days to weeks), with a resultant high risk of failure during manufacture.
Optical coating behavior under y irradiation for space applications
Stefania Baccaro, Angelica Cecilia, Ilaria Di Sarcina, et al.
Optical coatings are widely used in optical instrumentation for astronomical and space applications. The required optical components are similar to the ones necessary for ground-based instrumentation: antireflection coatings, mirrors, transmission filters. In addition, depending on the specific application, resistance to the radiation induced damage is needed. The analysis here is limited to optical components for instruments to be used for Earth observation from the polar sun-synchronous orbit, where they will withstand a total radiation dose of 50 Gy accumulated during a typical lifetime of 5 years. In this work a set of optical coating materials were submitted to γ irradiation at the 60Co radioisotope source (ENEA Research Center) in order to simulate the hostile radiation environment in which they will be employed. The selected coatings are made with a multilayer structure that contains oxide layers and/or metal layers. Before the coating irradiation, the behavior of single-layer materials was investigated by comparing their reflectance and transmittance before and after the γ-ray exposure. Results are reported for a number of oxide single layers (SiO2, HfO2, Y2O3), metal layers (Ag, Al) and multilayer coatings made with these materials, by the physical vapor deposition (PVD) technique. The behavior of different substrates (glass, quartz) is also compared.
Silicon immersion gratings for very high-resolution infrared spectroscopy
Daniel Ludlow McDavitt, Jian Ge, Shane Miller, et al.
In our group, development of large format silicon immersion gratings with sizes up to 4 inches in diameter is a routine practice. The first silicon anamorphic immersion grating has an 80x50 mm2 etched grating area, a 63.5° blazed angle and a 5.4 l/mm groove density (or 185 μm period) on a 30mm thick silicon substrate. The groove density is about 4 times coarser than any existing commercial echelle grating, allowing a complete coverage of a cross-dispersed echelle spectrum on a 1k x 1k IR array at R = 220,000 in the K band. The optical measurements show the grating has a high quality wavefront and surface. The rms wavefront error is 0.125 waves and the integrated scattered light is ~1% at 0.6328 nm. A silicon immersion grating with an 85x50 mm2 etched area, a 54.7° blazed angle and 16.1 l/mm groove density on a 40 mm thickness allows for complete wavelength coverage of 1.2-2.4 μm on a 2kx2k IR array. We are in the middle of processing a silicon disk with a 6 inch diameter and 2.5 inch thickness to make a large format silicon immersion grating for the Gemini next generation Advanced Cryogenic Echelle Spectrograph (ACES) and space applications.
Rewriting optical elements
Andrea Bianco, Emilio Molinari, Chiara Bertarelli, et al.
The observatories of tomorrow will necessarily be equipped with robotic facilities which will ease the use and allow a full time schedule optimization. Herein we present the application of organic materials with photochromic properties in astronomical spectrographs. Photochromism is the reversible transformation of the optical (in particular the color) and spectral properties of a material by suitable light. Organic photochromic materials can be cast in thin layers (tens to hundredths of microns) where a chosen pattern can be written leaving the layer with two areas showing different refractive index and absorption/transmission spectra. We used the opaque/transparent property of such photochromic materials to build rewritable focal plane mask for MOS spectroscopy. They were tested in laboratory and then sky tests were successfully carried out by using the AFOSC camera (Asiago Faint Object Spectrograph and Camera) of the 1.8 m telescope at Asiago Observatory. The results obtained are encouraging and the first MOS writing device is now in use at the astronomical observatory of Asiago.
Multi-notch holographic filters for atmospheric lines suppression
Sebastien Blais-Ouellette, Etienne Artigau, Frank Havermeyer, et al.
Near-infrared emission from atmospheric OH radicals is known to severely affect astronomical observations. Until now, only complex dispersive instruments were partially capable of removing this unwanted background, which is composed of hundreds of narrow emission lines. Recent development in photosensitive glass and holographic recording technologies now allow the elaboration of filters with a large number of narrow reflecting bands well matched to OH lines. This technology shows promise for removing many tens of lines in the J, H, and K bands. That would result in a many fold increase in imaging and low resolution signal-to-noise ratio. Filters with 10 lines have been tested and show the appealing possibilities of these new devices.
Novel coating and photofabrication techniques for astronomy
Ian J. Lewis, Gavin B. Dalton, Cyril Band, et al.
In this paper we describe two coating and fabrication techniques we have developed applicable to astronomical instrumentation with particular emphasis on the FMOS instrument for the Subaru telescope.
Progress in the optical maintenance of the VLT
Since February 2000, the Washing & Coating Unit of Paranal Observatory, one of the master piece in the coating and re-coating of the VLT mirrors, has been continuously improved. Details of those works will be described as well as the washing process revealed. Measured data will be presented and the efforts made at ESO since 1998 to define suitable on-line monitoring and preventive maintenance will be detailed. In-situ techniques, existing equipment and procedures will be reviewed. Emphasis will be put on the CO2 snow-flake technique and the re-enable of the peelable lacque (XL Clean 5) will be described. The plans for the long-term optical maintenance of the VLT and VLTI mirrors will be exposed.
Mirror coating 2003 in Subaru Telescope
Tomio Kurakami, Masami Yutani, Tomio Kanzawa, et al.
The SUBARU Telescope has four focal positions to allow different types of astronomical instrument. At present, there are four different Top Units; three types of secondary mirrors and one primary focus unit. IR secondary mirror which is one of the three units, has silver coated surface. Other secondary mirrors are coated by aluminum for observations at visible wavelength. The silver coating for IR secondary mirror was first carried out in 1999 at the medium size (1.6 m) vacuum evaporation chamber in Mitaka campus of NAOJ at Tokyo JAPAN. Since then the reflectivity had deteriorated over the years. Then, we made a plan to recoat IR secondary mirror in 2003 using the SUBARU’s large-size vacuum evaporation chamber at the summit facility on Mauna Kea, Hawaii. Some tests were performed for silver vacuum evaporation at the base facility, and then the IR secondary mirror was recoated at the summit. The reflectivity achieves 97.6% and 99.3% at the wavelength of 500 nm and 2000 nm, respectively. Degradation of the coat has not been seen 8 months after recoating. We also performed the recoating of the aluminum surface of the primary mirror in 2003. This year we made effort to simplify the procedure. The reflectivity is 91.2% and 97.4% at the wavelength of 500 nm and 2000 nm, respectively.
New age fibers: the children of the photonic revolution
The Photonics Crystal Fibers (PCFs) also known as "holey" or "micro-structured" fibers herald a new age for optical fibers. The astronomical applications could revolutionize instrument design through: broadband performance combined with excellent UV transmission, extremely large numerical aperture fibers, and aperture transforming fibers improving input coupling/sampling while maintaining a good match between the fiber outputs and the detector pixels. The Photonic Crystal effects can provide unprecedented non-linear effects in materials and when combined with micro-photonics it is expected that a photonic chip will be realized in which optical switching, wavelength dispersion and even wavelength conversion could take place. However, conventional optical fibers are currently the benchmark for many astronomical applications and improvements in the performance of silica fibers are also being made. In this paper we present a brief review of the current status of photonic crystal fibers with particular focus on the astronomical applications. In addition we present the optical characterization of a new silica/silica broadband fiber that delivers very good transmission from 300nm to 1100nm and beyond.
Poster Session b: Optical Metrology
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Cryogenic High-Accuracy Refraction Measuring System (CHARMS): a new facility for cryogenic infrared through far-ultraviolet refractive index measurements
The infrared optics community continually expresses its desire for high accuracy, cryogenic, infrared, refractive index data for a wide variety of materials, as well as knowledge of index variability for each species. The Near-Infrared Camera (NIRCam) for the James Webb Space Telescope (JWST) will not only provide astronomical scientific imaging in the 0.6 to 5 micron wavelength range but it will also serve as the observatory’s wavefront control sensor. The provision of accurate, infrared (IR) cryogenic index data for optimizing the refractive optical design of NIRCam will be an important component in mission risk reduction. To this end, we have built the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS). This paper is the fourth covering the design and construction of CHARMS and highlights the significant design changes culminating in the finished refractometer. Aspects which make CHARMS a high accuracy system, one which will measure refractive index more accurately than previous refractometers, are presented along with sub-system performance data. Finally, we present our "first light," cryogenic, infrared refractive index data for lithium fluoride (LiF) for temperatures down to 25 K, relating it to previously published ambient data for this material.
Poster Session c: VPH/Coatings/Fibers
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All-silica fiber with low or medium OH-content for broadband applications in astronomy
Saleh Ferwana, Hanns-Simon Eckhardt, Thorsten Simon, et al.
For astronomical applications, different types of step-index all-silica fibers with high-transparency in the whole spectral region from UV (300 nm) to NIR (1100 nm) will be introduced. The light-guiding core-material consists of high-purity silica, especially with low or medium OH-content. In UV region, the losses are mainly influenced by Rayleigh scattering, while the losses in the IR region are limited by traces of OH-groups (in the order of approx. 2 ppm) and fundamental vibration-bands. Due to processing, typical UV-defects below 280 nm can be suppressed significantly within fibers with medium or low OH-content. Especially, one fiber-type with low-OH content in the core possess high resistance against UV radiation in the DUV-region down to 200 nm, which is comparable to high-OH all-silica fibers specially developed for UV-application below 250 nm. In addition, a medium-OH will be presented. The properties of these new fibers in respect to basic attenuation and spectral damage in the UV-region will be discussed, in comparison to high-OH fibers, based on the same measurement-technique. In addition, first results on focal ratio degradation (FRD) and additional loss related to higher propagation angles will be shown, in comparison to standard high-OH fibers.