Proceedings Volume 7439

Astronomical and Space Optical Systems

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Proceedings Volume 7439

Astronomical and Space Optical Systems

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Volume Details

Date Published: 20 August 2009
Contents: 11 Sessions, 40 Papers, 0 Presentations
Conference: SPIE Optical Engineering + Applications 2009
Volume Number: 7439

Table of Contents

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Table of Contents

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  • Session 1
  • Session 2
  • Session 3
  • Poster Session: Focal Plane Arrays for Space Telescopes IV
  • Design and System Status
  • Novel System Optimization and Control
  • Poster Session: Astronomical Adaptive Optics Systems and Applications IV
  • Cryogenic Mechanisms and Systems
  • Cryogenic Telescope Technology
  • Cryogenic Optical Instruments I
  • Cryogenic Optical Instruments II
Session 1
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Improving radiation tolerance in e2v CCD sensors
D. Burt, J. Endicott, P. Jerram, et al.
e2v have been developing new approaches to mitigate against the effects of radiation damage in CCD sensors. The first of these is our "rad-hard" device technology, primarily developed to reduce the flat-band voltage shift following ionising radiation. With this a very significant improvement has been demonstrated, the flat-band shift reducing from typically 100-200 mV/kRad(Si) with standard devices to only 6 mV/kRad(Si), plus an associated reduction in the increase in surface dark signal. The rad-hard process thereby allows devices to be operated in environments with up to at least 500kRad total dose and/or with reduced shielding. Developments aimed at reducing the impact of proton radiation have included the manufacture of p-channel devices. Our initial data indicates that at -50°C the increase in charge transfer inefficiency is reduced by a factor of two times for parallel transfer and five times for serial transfer.
CCD radiation damage in ESA Cosmic Visions missions: assessment and mitigation
David H. Lumb
Charge Coupled Device (CCD) imagers have been widely used in space-borne astronomical instruments. A frequent concern has been the radiation damage effects on the CCD charge transfer properties. We review some methods for assessing the Charge Transfer Inefficiency (CTI) in CCDs. Techniques to minimise degradation using background charge injection and p-channel CCD architectures are discussed. A critical review of the claims for p-channel architectures is presented. The performance advantage for p-channel CCD performance is shown to be lower than claimed previously. Finally we present some projections for the performance in the context of some future ESA missions.
Modelling electron distributions within ESA's Gaia satellite CCD pixels to mitigate radiation damage
G. M. Seabroke, A. D. Holland, D. Burt, et al.
The Gaia satellite is a high-precision astrometry, photometry and spectroscopic ESA cornerstone mission, currently scheduled for launch in 2012. Its primary science drivers are the composition, formation and evolution of the Galaxy. Gaia will achieve its unprecedented positional accuracy requirements with detailed calibration and correction for radiation damage. At L2, protons cause displacement damage in the silicon of CCDs. The resulting traps capture and emit electrons from passing charge packets in the CCD pixel, distorting the image PSF and biasing its centroid. Microscopic models of Gaia's CCDs are being developed to simulate this effect. The key to calculating the probability of an electron being captured by a trap is the 3D electron density within each CCD pixel. However, this has not been physically modelled for the Gaia CCD pixels. In Seabroke, Holland & Cropper (2008), the first paper of this series, we motivated the need for such specialised 3D device modelling and outlined how its future results will fit into Gaia's overall radiation calibration strategy. In this paper, the second of the series, we present our first results using Silvaco's physics-based, engineering software: the ATLAS device simulation framework. Inputting a doping profile, pixel geometry and materials into ATLAS and comparing the results to other simulations reveals that ATLAS has a free parameter, fixed oxide charge, that needs to be calibrated. ATLAS is successfully benchmarked against other simulations and measurements of a test device, identifying how to use it to model Gaia pixels and highlighting the affect of different doping approximations.
Session 2
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Fundamental performance differences between CMOS and CCD imagers: part III
James Janesick, Jeff Pinter, Robert Potter, et al.
This paper is a status report on recent scientific CMOS imager developments since when previous publications were written. Focus today is being given on CMOS design and process optimization because fundamental problems affecting performance are now reasonably well understood. Topics found in this paper include discussions on a low cost custom scientific CMOS fabrication approach, substrate bias for deep depletion imagers, near IR and x-ray point-spread performance, custom fabricated high resisitivity epitaxial and SOI silicon wafers for backside illuminated imagers, buried channel MOSFETs for ultra low noise performance, 1 e- charge transfer imagers, high speed transfer pixels, RTS/ flicker noise versus MOSFET geometry, pixel offset and gain non uniformity measurements, high S/N dCDS/aCDS signal processors, pixel thermal dark current sources, radiation damage topics, CCDs fabricated in CMOS and future large CMOS imagers planned at Sarnoff.
Advanced staring Si PIN visible sensor chip assembly for Bepi-Colombo mission to Mercury
The planet Mercury, by its near proximity to the sun, has always posed a formidable challenge to spacecraft. The Bepi-Colombo mission, coordinated by the European Space Agency, will be a pioneering effort in the investigation of this planet. Raytheon Vision Systems (RVS) has been given the opportunity to develop the radiation hardened, high operability, high SNR, advanced staring focal plane array (FPA) for the spacecraft destined (Fig. 1) to explore the planet Mercury. This mission will launch in 2013 on a journey lasting approximately 6 years. When it arrives at Mercury in August 2019, it will endure temperatures as high as 350°C as well as relatively high radiation environments during its 1 year data collection period from September 2019 until September 2020. To support this challenging goal, RVS has designed and produced a custom visible sensor based on a 2048 x 2048 (2k2) format with a 10 μm unit cell. This sensor will support both the High Resolution Imaging Camera (HRIC) and the Stereo Camera (STC) instruments. This dual purpose sensor was designed to achieve high sensitivity as well as low input noise (<100 e-) for space-based, low light conditions. It also must maintain performance parameters in a total ionizing dose environment up to 70 kRad (Si) as well as immunity to latch-up and singe event upset. This paper will show full sensor chip assembly data highlighting the performance parameters prior to irradiation. Radiation testing performance will be reported by an independent source in a subsequent paper.
Wide Field Camera 3 CCD quantum efficiency hysteresis: characterization and mitigation
Nicholas R. Collins, Nicholas Boehm, Gregory Delo, et al.
In ground testing of the Hubble Space Telescope Wide Field Camera 3 (HST/WFC3), the CCDs of its UV/visible channel exhibited an unanticipated quantum efficiency hysteresis (QEH) behavior. The QEH first manifested itself as an occasionally observed contrast in response across the format of the CCDs, with an amplitude of typically 0.1-0.2% or less at the nominal -83°C operating temperature, but with contrasts of up to 3-5% observed at warmer temperatures. The behavior has been replicated in the laboratory using flight spare detectors and has been found to be related to an initial response deficiency of ~5% amplitude when the CCDs are cooled with no illumination. A visible light flat-field (540nm) with a several times full-well signal level is found to pin the detector response at both optical (600nm) and near-UV (230nm) wavelengths, suppressing the QEH behavior. We have characterized the timescale for the detectors to become unpinned (days for significant response loss at -83°C and have developed a protocol to stabilize the response in flight by flashing the WFC3 CCDs with the instrument's internal calibration system.
Session 3
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Telescope guiding with a HyViSI H2RG used in guide mode
We report on long exposure results obtained with a Teledyne HyViSI H2RG detector operating in guide mode. The sensor simultaneously obtained nearly seeing-limited data while also guiding the Kitt Peak 2.1 m telescope. Results from unguided and guided operation are presented and used to place lower limits on flux/fluence values for accurate centroid measurements. We also report on significant noise reduction obtained in recent laboratory measurements that should further improve guiding capability with higher magnitude stars.
Initial laboratory and sky testing results for the second generation H4RG-10 4k x 4k, 10 micron visible CMOS-Hybrid detector
Bryan N. Dorland, Rachel P. Dudik, Dan Veillette, et al.
We present the initial performance test results for the H4RG-10 (A2), the second generation of the H4RG-10 visible CMOS-Hybrid Sensor Chip Assembly (SCA). The first science grade H4RG-10 (A2), delivered in 2009, is an evolution of the first generation A1, first delivered and tested in 2007. The H4RG-10 is primarily intended for ground- and space-based astronomical applications. Our evaluation focused on the performance parameters as they are related to astrometric applications. We find that the A2 SCA shows high pixel interconnect (99.6%), and low read noise (10-15 e- RMS) when operated at high speeds, consistent with A1 results. Most importantly, the H4RG-10 (A2) shows a dramatic improvement in dark current vs. the A1, with a two order of magnitude reduction in mean dark level and significantly reduced hot pixel population below 200 K.
Poster Session: Focal Plane Arrays for Space Telescopes IV
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Gaia: operational aspects and tests of Gaia Flight Model CCDs
ESA's cornerstone mission Gaia is planning to map 1% of the stellar population of our galaxy, around one thousand million objects, to micro-arcsecond accuracy. In addition to high precision astrometric information, prism dispersion optics will be used to provide multi-band photometry and a spectroscopic instrument provides information for deriving radial velocities. Gaia's focal plane will be the largest ever flown to space comprising an almost Giga-pixel mosaic of 106 specially designed CCDs, the e2v technologies CCD91-72, operated synchronously in TDI mode. This paper will address some operational aspects of these detectors in the Gaia focal plane array and report on recent test results with respect to calibration needs.
Design and System Status
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NFIRAOS: the optical design of an adaptive optics system for the Thirty Meter Telescope
NFIRAOS (Narrow Field InfraRed Adaptive Optics System, pronounced nefarious) is the first light adaptive optics system for the Thirty Meter Telescope (TMT). It is a near-IR, diffraction limited, multi-conjugate adaptive optics (MCAO) system that uses two deformable mirrors to correct aberrations due to atmospheric turbulence. The two arcminute field of view f/15 beam delivered by the telescope is relayed to one of three client instrument ports. Wavefront sensing is accomplished with six high order sodium laser guide star (LGS) wavefront sensors (WFSs) and three visible natural guide star (NGS) wavefront sensors. In this paper, we describe the general layout and design drivers of each optical system in NFIRAOS. The primary subsystems are the science path optics, the LGS wavefront sensors, the visible NGS truth WFSs, the IR acquisition camera and the calibration unit. Particular attention is given to the design of the LGS system, which uses all spherical components and a zoom system to compensate for aberrations and changes in distance to the sodium layer.
Status of the PALM-3000 high-order adaptive optics system
Antonin H. Bouchez, Richard G. Dekany, John R. Angione, et al.
The PALM-3000 upgrade to the Palomar Adaptive Optics system on the 5.1 meter Hale telescope will deliver extreme adaptive optics correction in near-infrared wavelengths and diffraction-limited images in visible wavelengths. PALM-3000 will use a 3388-actuator tweeter and a 241-actuator woofer deformable mirror, a Shack-Hartmann wavefront sensor with selectable pupil sampling, and an innovative wavefront control computer based on a cluster of 17 graphics processing units to correct wavefront aberrations at scales as fine as 8.1 cm at the telescope pupil using natural guide stars. The system is currently undergoing integration and testing, with deployment at Palomar Observatory planned in early 2011. We present the detailed design of key aspects of the adaptive optics system, and the current status of the deformable mirror characterization, wavefront sensor performance, and testbed activities.
The plenoptic camera as a wavefront sensor for the European Solar Telescope (EST)
The plenoptic wavefront sensor combines measurements at pupil and image planes in order to obtain wavefront information from different points of view simultaneously, being capable to sample the volume above the telescope to extract the tomographic information of the atmospheric turbulence. After describing the working principle, a laboratory setup has been used for the verification of the capability of measuring the pupil plane wavefront. A comparative discussion with respect to other wavefront sensors is also included.
EAGLE: an MOAO fed multi-IFU working in the NIR on the E-ELT
Jean-Gabriel Cuby, Simon Morris, Philip Parr-Burman, et al.
EAGLE is an instrument for the European Extremely Large Telescope (E-ELT). EAGLE will be installed at the Gravity Invariant Focal Station of the E-ELT, covering a field of view of 50 square arcminutes. Its main scientific drivers are the physics and evolution of high-redshift galaxies, the detection and characterization of first-light objects and the physics of galaxy evolution from stellar archaeology. These key science programs, generic to all ELT projects and highly complementary to JWST, require 3D spectroscopy on a limited (~20) number of targets, full near IR coverage up to 2.4 micron and an image quality significantly sharper than the atmospheric seeing. The EAGLE design achieves these requirements with innovative, yet simple, solutions and technologies already available or under the final stages of development. EAGLE relies on Multi-Object Adaptive Optics (MOAO) which is being demonstrated in the laboratory and on sky. This paper provides a summary of the phase A study instrument design.
Performance of a modal zone wavefront recovery algorithm (Hudgin) implemented on FPGAs for its use in ELTs
José Javier Díaz-García, Alberto Dávila-González, Luis-Fernando Rodríguez-Ramos, et al.
The use of AO in Extremely Large Telescopes, used to improve performances in smaller telescopes, becomes now mandatory to achieve diffraction limited images according to the large apertures. On the other hand, the new dimensions push the specifications of the AO systems to new frontiers where the order of magnitude in terms of computation power, time response and the required numbers of actuators impose new challenges to the technology. In some aspects implementation methods used in the past result no longer applicable. This paper examines the real dimension of the problem imposed by ELTs and shows the results obtained in the laboratory for a real modal wavefront recovery algorithm (Hudgin) implemented in FPGAs. Some approximations are studied and the performances in terms of configuration parameters are compared. Also a preferred configuration will be justified.
LGS adaptive optics and integral field spectroscopy
Ground based astronomical telescopes have limited resolutions due to wavefront distortions caused by constantly changing atmospheric turbulence. Resolution can be improved using adaptive optics. Creating an artificial guidestar in the mesospheric sodium layer using a laser has been a major quest in the astronomical community. The sodium guidestar lasers of five major observatories are compared: AFRL/SOR (solid state, resonant sum-frequence generator), Palomar/Caltech (solid state sum-frequency mode-locked), Lick/LLNL (tunable dye), Keck (tunable dye), and VLT (tunable dye). Increased sky coverage provided by laser guide stars has made possible integral field spectrographs such as OSIRIS and Keck and SINFONI at VLT to study high red shift galaxies. The first results obtained with OSIRIS was diffraction-limitied spectroscopic observations of an infrared flare associated with the radio source Sgr A*. Increased resolution will be defined and the impact on integral field spectroscopy projected.
Novel System Optimization and Control
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Development of hot slumping technique and last optical performances obtained on a 500mm diameter slumped segment prototype for adaptive optics
In the framework of the E-ELT Design Study financed by the European Community under OPTICON-FP6, the INAF Astronomical Observatory of Brera (INAF-OAB) has developed a technique for the manufacturing of thin optical segments. Thin glass segments are produced by mean of an hot slumping technique that makes use of an optical quality ceramic mould and a precise thermal circle to impart the desired shape to a glass sheet. In the present paper we summarize the results obtained during this study and report the last results of the effort in scaling-up the procedure: in particular the overall process has been refined in order to optimize the parameters (such as time, maximum temperature and amount of pressure) used to slump a 500 mm diameter glass segment. The thickness of these glass segments is of about 1.7 mm, making the optical surface very floppy and easy to be deformed. For this reason optical tests have been performed using a astatic support implemented into a vertical optical bench.
Control of deformable mirrors in MOAO using H∞ optimization
We present a new approach for the control of a deformable mirror (DM), part of a Multi Object Adaptive Optics (MOAO) on-sky demonstrator. The control is based on H synthesis methods, achieving better performance than classical Proportional-Integral methods, while offering other appealing advantages such as an optimized design based on the temporal spectra of the wavefront and vibration rejection capabilities. We describe laboratory results obtained with a 97 actuator Xinetics DM, using a high-resolution Shack-Hartmann wavefront sensor for measuring DM surface. A connection between the turbulence dynamics represented in a Zernike basis and the controller requirements is studied, showing that the controller parameters and structure can be easily optimized for each Zernike mode according to their particular temporal spectra.
Improvement of science throughput of 1m class telescopes thanks to COTS adaptive optics
We present the improvement of science throughput of 1m class telescopes that can be obtained using COTS adaptive optics. It is based on a new architecture of adaptive optics system using a new kind of magnetic deformable mirrors, a highly sensitive EMCCD wavefront sensor and a novel real time architecture called ACE and working on a standard workstation. It will be shown the dramatically increase of performances that can be achieved using small adaptive optics (typically 8x8 actuators) with 1m to 2m class telescopes and in particularly, we will focus our presentation of the improvement of the science throughput thanks to this simple and efficient A.O. system
Poster Session: Astronomical Adaptive Optics Systems and Applications IV
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Advances in solar adaptive optics system at the domeless solar telescope of the Hida Observatory
Noriaki Miura, Yuuki Noto, Shuusuke Kato, et al.
A solar adaptive optics system for the 60 cm domeless solar telescope of the Hida Observatory in Japan is developed. A high-speed deformable mirror with 52 electromagnetic actuators is newly used in an experimental adaptive optics system. The use of the mirror resulted in the improvement of Strehl ratios in laboratory experiments. In solar observations, the system worked well when solar granulation was used as a target for wavefront sensing. An adaptive optics system being developed for a vertical spectrograph of the domeless solar telescope is described.
Wave-front sensing and correction for 4-meter LAMOST
LAMOST is a 4m spectroscopic telescope recently operational at Xinglong, China. Several active optics are being used to remove optical aberration of the telescope, but large residual aberration exists since the active optics actuators on the telescope's segmented mirrors cannot provide enough precision. We proposed a wave-front sensing system and the corresponding algorithm to measure this low frequency residual aberration. We developed a compact Shack-Hartmann wave-front sensor that can use point source as well as extended structure images for wave-front sensing and can achieve good measurement accuracy. The wave-front sensing algorithm is realized by LabVIEW that is based on block-diagram programming and is suitable for rapid prototype development. Combined with deformable mirrors, the system will be able to provide a fine wave-front correction and therefore eventually remove the residual aberration for LAMOST. The wave-front sensor and the DMs will also be used for our high-contrast imaging coronagraph to remove speckle noise for the direct imaging of exoplanets.
Image reconstruction of Io from the bispectrum using building block method
Yuuki Yoshinoya, Susumu Kuwamura, Noriaki Miura, et al.
The building block method provides a promising algorithm to reconstruct an astronomical object image from its bispectrum. While the building block method has been well applied on stellar objects, in the present study we examine the applications to extended objects such as planets and satellites. We have obtained the visible light specklegrams of Io (a Jupiter's satellite) at 515nm using the 2m telescope in Nishi-Harima Astronomical Observatory. We report a preliminary imaging result of Io using the building block method. The result is compared with the image as previously restored by the shift-and-add method with a deconvolution post-processing.
Testbed for the multi-conjugate adaptive optics system of the solar telescope GREGOR
Dirk Schmidt, T. Berkefeld, F. Heidecke, et al.
We present the latest concept of the multi-conjugate adaptive optics system for the 1.5-meter solar telescope Gregor. This system will employ three deformable mirrors in order to compensate for seeing introduced by the ground layer, and by shear winds in 5 and 15 km above the telescope ground. Thus, the compensated field of view will grow compared to ground layer compensation only. We describe the design and the used components and present a testbed which is used to improve control algorithms and to test all the components before installing them at the Gregor telescope.
Enabling technologies for visible adaptive optics: the Magellan adaptive secondary VisAO camera
Since its beginnings, diffraction-limited ground-based adaptive optics (AO) imaging has been limited to wavelengths in the near IR (λ>1μm) and longer. Visible AO (λ>1μm) has proven to be difficult because shorter wavelengths require wavefront correction on very short spatial and temporal scales. The pupil must be sampled very finely, which requires dense actuator spacing and fine wavefront sampling with large dynamic range. In addition, atmospheric dispersion is much more significant in the visible than in the near-IR. Imaging over a broad visible band requires a very good Atmospheric Dispersion Corrector (ADC). Even with these technologies, our AO simulations using the CAOS code, combined with the optical and site parameters for the 6.5m Magellan telescope, demonstrate a large temporal variability of visible (λ=0.7μm) Strehl on timescales of 50 ms. Over several hundred milliseconds, the visible Strehl can be as high at 50% and as low as 10%. Taking advantage of periods of high Strehl requires either the ability to read out the CCD very fast, thereby introducing significant amounts of read-noise, or the use of a fast asynchronous shutter that can block the low-Strehl light. Our Magellan VisAO camera will use an advanced ADC, a high-speed shutter, and our 585 actuator adaptive secondary to achieve broadband (0.5-1.0 μm) diffraction limited images on the 6.5m Magellan Clay telescope in Chile at Las Campanas Observatory. These will be the sharpest and deepest visible direct images taken to date with a resolution of 17 mas, a factor of 2.7 better than the diffraction limit of the Hubble Space Telescope.
Cryogenic Mechanisms and Systems
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Cryogenic piezoelectric actuators
Xiaoning Jiang, William B. Cook, Wesley S. Hackenberger
In this paper, PMN-PT single crystal piezoelectric stack actuators and flextensional actuators were designed, prototyped and characterized for space optics applications. Single crystal stack actuators with footprint of 10 mm x 10 mm and the height of 50 mm were assembled using 10 mm x 10 mm x 0.15 mm PMN-PT plates. These actuators showed stroke of 65 - 85 μm at 150 V at room temperature, and > 30 μm stroke at 77 K. Flextensional actuators with dimension of 10 mm x 5 mm x 7.6 mm showed stroke of > 50 μm at room temperature at driving voltage of 150 V. A flextensional stack actuator with dimension of 10 mm x 5 mm x 47 mm showed stroke of ~ 285 μm at 150 V at room temperature, and > 100 μm at 77K under driving of 150 V should be expected. The large cryogenic stroke and high precision of these actuators are promising for cryogenic optics applications.
Cryogenic bonding for lens mounts
Craig L. Hom, Howard C. Holmes, Dennis N. Lapicz, et al.
Lockheed Martin Space Systems Company (LMSSC) has performed a feasibility study for bonded cryogenic optical mounts. That investigation represents a combined effort of design, experiments and analysis with the goal to develop and validate a working cryogenic mount system for refractive lens elements. The mount design incorporates thermal expansion matched bond pads and radial flexures to reduce bondline stress and induced optical distortion. Test coupons were constructed from lens and selected mount materials and bonded with candidate adhesives to simulate the design's bond pads. Thermal cycling of those coupons to 35K demonstrated both the system's survivability and the bond's structural integrity. Finally, a companion finite element study determined the bonded system's sensitivity to bondline thickness, adhesive modulus and adhesive CTE. The design team used those results to tailor the bondline parameters to minimize stress transmitted into the optic.
Athermalized embedding of actuators in cryogenic mirrors
Embedding solid-state ceramic actuators in a bending style deformable mirror presents unique athermalization challenges when operated at cryogenic temperatures. Approaches to athermally embed actuators in a substrate are presented in this study. Each approach is rated according to established design criteria: unmatched displacement, range, compliance ratio, bondline stress, design, and manufacturability. We show the results of our design that allows a large thermal range of operation for the actuators.
Low emittance, semi-transparent coating for cryogenic window applications
A warm window surface with a relatively high (>50%) surface emittance can add significant undesired heat loading into a cryogenic test chamber. However, a front surface coating that consists of a very thin adherent layer of evaporated Cr that is overcoated with about 7nm of evaporated Au has been demonstrated to reduce the inherently high emittance of a glass or sapphire window surface down to about 14%, while maintaining a visible transmittance in excess of 55%. The coating possesses reasonably good adhesion and cleaning durability when deposited onto glass or sapphire substrates and has survived multiple temperature cycles between 316K and 20K. The addition of a single layer anti-reflection coating, such as reactively evaporated SiOx, to the otherwise uncoated exterior surface of a cryogenic window produced a further increase in visible wavelength transmittance without altering window emittance. This paper will present measured reflectance, transmittance, and emittance data for the Cr + Au window surface coating relevant to a cryogenic window application.
Cryogenic Telescope Technology
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Cryogenic stray light testing of the James Webb Space Telescope: an easy approach
A practical method for introducing stray light for testing the James Webb Space Telescope (JWST) at cryogenic temperatures using hollow shell spherical reflectors is described. Several alternate approaches to stray light testing are compared, including fiber sources, diffuse panels, and curved specular reflectors. Alignment of the sources can pose special difficulties when cooling to cryogenic temperatures, since the shape of the reflector, mounts, and support structure can all change. The hollow shell spherical reflectors do not have any of these difficulties, and can be mounted so that they are automatically aligned by gravity. This also makes them insensitive to vibration, so they can be used with long detector integration times to provide adequate stray light signal to noise without interfering with other optical tests. The reflectors are positioned so as to work with test source(s) already in the cryo-vac chamber. The spherical reflectors operate at all wavelengths, reducing the number of reflectors required and providing operational flexibility in reflector placement. Electroplated stainless steel hollow shell reflectors are inherently compatible with cryogenic and vacuum environments. The reflectors are passive and have no thermal dissipation, eliminating impact on sensitive thermal tests. Their light weight and single point suspension mounting minimize the dynamic and static loads. Finally, the reflector's simple geometry is inherently compatible with optical alignment metrology (e.g. LIDAR), making position measurements both more accurate and simpler to document.
Wavefront sensor for the ESA-GAIA mission
L. L. A. Vosteen, F. Draaisma, W. P. van Werkhoven, et al.
TNO developed a Wave Front Sensor (WFS) instrument for the GAIA mission. This Wave Front Sensor will be used to monitor the wave front errors of the two main telescopes mounted on the GAIA satellite, which may be corrected by a 5-degree of freedom (DOF) mechanism during operation. The GAIA-WFS will operate over a broad wavelength (450 to 900 nm) and under cryogenic conditions (130 to 200 K operation temperature). The WFS uses an all reflective, a-thermal design and is of the type of Shack-Hartmann. The boundary condition for the design is that the focal plane of the WFS is the same plane as the focal plane of the GAIA telescopes. The spot pattern generated after a micro lens array (MLA) by a star is compared to the pattern of one of the three calibration sources that is included in the WFS, allowing in flight calibration. We show the robust and lightweight opto mechanical design that is optimised for launch and cryogenic operation. Furthermore we give details on its alignment and commissioning. The WFS can measure wave front distortions in the order of lambda/1000, and determines the focal plane with an accuracy of 50 μm.
Picometer metrology for the GAIA Mission
To measure the relative motions of GAIA's telescopes, the angle between the telescopes is monitored by an all Silicon Carbide Basic Angle Monitoring subsystem (BAM OMA). TNO is developing this metrology system. The stability requirements for this metrology system go into the pico meter and pico radian range. Such accuracies require extreme measures and extreme stability. Specific topics addressed are mountings of opto-mechanical components, gravity deformation, materials and tests that were necessary to prove that the requirements are feasible. Especially mounting glass components on Silicon Carbide and mastering the Silicon Carbide material proved to be a challenge.
Cryogenic radius of curvature matching for the JWST primary mirror segments
The JWST (James Webb Space Telescope) primary mirror consists of 18 hexagonal mirror segments. Each segment is approximately 1.5 meters point to point and is constructed from a lightweight beryllium substrate. In order for the 18 segments to act as a single 6.5 meter diameter mirror each one must be capable of 6 degrees of freedom motion relative to the mirror backplane and be able to change its radius of curvature to closely match those of the other segments. As it would be nearly impossible to manufacture the 18 individual segments with the same radius, a RoC (radius of curvature) actuation mechanism is attached to each mirror allowing RoC fine tuning post manufacturing. The RoC actuation system consists of a single actuator and six struts attached to the back of the mirror. The radius of curvature is matched by closely manufacturing the radius of each segment relative to the nominal value and then, during cryogenic testing, actuating the RoC of each mirror. This cryogenic actuation reduces polishing times and allows for compensation of radius changes measured during other manufacturing steps. Presented here is a high-level overview of the method used to set the mirror's radius of curvature at cryogenic temperature, disassemble the mirror system for additional polishing and processing, and perform final cryogenic verification.
Cryogenic Optical Instruments I
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Wide-field Infrared Survey Explorer cryogenic support system status and thermal test results
The Wide-Field Infrared Survey Explorer (WISE) is a MIDEX mission that is being developed by the Jet Propulsion Laboratory (JPL) to address several of NASA's Astronomical Search of Origins (ASO) objectives. Space Dynamics Laboratory/ Utah State University is providing the cryogenically cooled infrared instrument. Cooling for the instrument optics and focal planes is provided by a dual-stage solid hydrogen cryostat. Driving requirements for the cryogenic subsystem are: a seven-month lifetime and operating temperatures of less than 17 K for the optics, 32 K for the HgCdTe focal planes, and 7.8 K for the Si:As focal planes. This paper provides an overview of the dual-stage hydrogen cryogenic subsystem status and discusses the results of the thermal performance testing.
Application of infrared optical projection systems in the cryogenic test environment using AEDC's 7V and 10V space sensor test chambers
H. S. Lowry, R. A. Nicholson, S. L. Steely, et al.
The application of infrared scene projection systems in the cryovacuum ground-test environment is a very challenging process. Work performed at Arnold Engineering Development Center (AEDC) in the past few years can offer lessons learned from its experience in projection technologies, optical system design, optical material characteristics and measurement (including cryodeposition), positioning systems, and pertinent analytical tools involved in performing ground testing of a sensor system under flight conditions. Such testing is fundamental to characterizing its performance, and should be accomplished early and often in order to manage operational uncertainty and reduce system life-cycle cost. AEDC provides a comprehensive capability that strives to ensure system performance evaluations that are not limited by test infrastructure. For over 30 years, the space chambers at AEDC have performed space sensor characterization, calibration, and mission simulation testing on space-based, interceptor, and airborne sensors. This paper describes recent efforts at AEDC to enhance this cryovacuum test capability.
A low-cost, low-temperature radiometer for thermal measurements
Michael DiPirro, Thomas Hait, James Tuttle, et al.
We have developed an inexpensive, compact radiometer for in situ measurement of low levels of flux in the far infrared. It utilizes a Winston cone fabricated cheaply using a boring tool and standard commercial thermometers as sensors. Its form is similar to bolometers which have long been used for sensitive astronomical infrared measurements. By relaxing the sensitivity and response times the radiometers can be made much less costly. The sensitivity varies with the operating temperature, but at 20 K the measured resolution is better than 0.01 microwatt per cm2, equivalent to the heat from a low emissivity blanket at 30 K or a high emissivity surface at just 1 K hotter than the radiometer held at 20 K. The Winston cone provides a directional signal, excluding sources that are outside the acceptance cone (11 degree halfangle in this case) by more than a factor of 1000). The intended use is to measure the in-situ properties of aluminized Kapton at low T and to look for heat leaks and reflected flux in low temperature thermal vacuum systems.
Cryogenic Optical Instruments II
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NIRCam mechanical support structure: from concept to qualification
Two optical modules, mounted back to back, comprise JWSTs NIRCam (Near Infrared Camera) instrument. Each module contains a short wavelength (SW) and long wavelength (LW) path. The instrument will be mounted to the ISIM (Integrated Science Instrument Module) of the spacecraft via a mechanical support structure. Within a fourteen month timeframe this aerospace structure was conceived, designed, analyzed, manufactured, integrated, tested and qualified for flight. This paper describes the technical product and its fast, affordable, and successful evolution from concept design to flight qualification, including critical decision points and lessons learned.
NIRCam pupil imaging lens actuator assembly
The near infrared camera (NIRCam) is one of four science instruments installed on the integrated science instrument module (ISIM) of NASA's James Webb Space Telescope (JWST) which is intended to conduct scientific observations over a five-year mission lifetime. NIRCam's requirements include operation at 37 Kelvin to produce high-resolution images in two-wave bands encompassing the range from 0.6 to 5 microns. The NIRCam instrument is also required to provide a means of imaging the primary mirror for ground testing, instrument commissioning, and diagnostics which have resulted in the development of the pupil imaging lens actuator assembly. This paper discusses the development of the pupil imaging lens (PIL) assembly, including the driving requirements for the PIL assembly, and how the design supports these conditions. Some of the design features included in the PIL assembly are the titanium isothermal optical flexure mounts with multi-axis alignment flexures, a counterbalanced direct drive rotary actuator, and a fail-safe retraction system with magnetic stowage stop. The paper also discusses how the PIL assembly was successfully tested to the demanding requirements typical for cryogenic instruments.
Cryogenic spectral performance of bandpass filters for the NIRCam instrument
Yalan Mao, David Harrison, Todd Richardson, et al.
The Bandpass Filters in the NIRCam instrument are required to have high throughput in bandpass spectral region and excellent out-of-band blocking over the entire region of detector spectral response. The high throughput is needed for the instrument to have high sensitivity for detecting distant galaxies, and the out-of-band blocking is needed for accurate calibration on James Webb Space Telescope. The operating temperature of the instrument is at cryogenic temperature from 32 Kelvin to 39.5 Kelvin. We have performed spectral measurement of NIRCam bandpass filters at cryogenic temperature after three cryo-to-ambient cycles. We will report the experiment and results in this paper. This work was performed and funded by NASA Goddard Space Flight Center under Prime Contract NAS5-02105.
NIRCam filter wheel optic mount design
The mechanical design of any optic mount requires an understanding of the sensitivities of the optical design. The design of the filter optic mounts used on the James Webb Space Telescope - NIRCam filter wheel assemblies have been designed to support the optics in a manner that does not compromise optical performance, while coping with several environmental conditions. We will review the design of the NIRCam filter optic assemblies and confirm the merits of the approach chosen to mount the optics, considering thermal, vibration and stress effects.
Results of environmental testing of the focus and alignment mechanism of the near-infrared camera on the James Webb Space Telescope
The Focus and Alignment Mechanism (FAM) is an opto-mechanical, cryogenic mechanism that positions the Pick-Off Mirror (POM) for the Near Infrared Camera of the James Webb Space Telescope. The POM is used to direct the light collected by the telescope into the Near Infrared Camera. The POM is a spherical, fused silica mirror. In order to retain high surface quality at cryogenic temperatures, the POM is attached to the mechanism via a titanium flexure-mount assembly. Three linear actuators are employed to position the POM in tip, tilt and piston. These linear actuators are stepper motor driven, with harmonic drive gear reduction. In this paper, we will summarize the design and role of this opto-mechanical mechanism and present the results of the environmental testing of the Engineering Test Unit. The tests performed were thermal-vacuum cryogenic cycling, and vibration testing.
Cold alignment prediction of the NIRCam instrument components
The Near Infrared Camera for the James Webb Space Telescope is designed to operate at a temperature of 37K. The instrument must be assembled and aligned at room temperature. The optical design is refractive and incorporates several different lens materials in addition to several mirrors which make an athermal design very difficult. All of the instrument components are designed so that the instrument can come into alignment at 37K after assembly at room temperature. The methods to predict alignment shifts are presented in this paper.