Aluminum mirrors are in increasingly common use for low background cryogenic radiometers, spectrometers, telescopes, collimators, scene generators, and black body reference targets used in the contexts of research in astronomy, and sensor system calibration and simulation. Reflective aluminum systems are relatively low cost, provide broadband performance, high thermal conductivity, and exceptionally isotropic thermal contraction. Aluminum optics may be net diamond point machined or nickel plated and post polished to yield more refined tolerances for surface figure and surface finish. Nickel plated mirrors must neutralize the influences of bi-metal interfaces over a large reduction in temperature. General design and manufacturing guidelines applied to three actual, and widely varied cryogenic reflective assemblies are presented. These applications were shown respectively to exhibit diffraction-limited performance in the range of 0.80-1.5 micron at cryogenic temperatures ranging from 20°K to 80°K.

A lightweight athermal optical system has been demonstrated using silicon lightweight mirrors (SLMS) manufactured by Schafer Corporation, mounted in carbon fiber reinforced silicon carbide mounts (C/SiC or Cesic) produced by IABG and ECM. SLMS are quickly and inexpensively super-polishable (figure of 0.021 waves rms @ 633 nm, 4 Å rms surface finish), stiff (first mode greater than 1500 Hz), lightweight (areal density <10 kg/m²), have superior thermal properties at cryogenic temperatures, and do not out-gas. Under a 1998 NASA Phase I SBIR, Schafer and IABG engineered the so-called A-3 formulation of Cesic, which has a near-perfect CTE match with silicon over the temperature range of 8-300 K, making it the ideal material to athermally mount SLMS. This paper presents results of the cryogenic testing of a 6-inch diameter flat SLMS-Cesic mount assembly at NASA GSFC from room temperature to 80K and back again.

The excellent polishability, low density and relatively high stiffness of silicon make it an attractive candidate for optical applications that require superior performance. Assembly of silicon details by means of glass frit bonding permits significant weight reduction thus enhancing the benefit of silicon mirrors. To demonstrate the performance potential, a small lightweight glass frit bonded silicon mirror was fabricated and tested for cryostability. The test mirror was 12.5cm in diameter with a 60cm spherical radius and a maximum thickness, at the perimeter, of 2.5cm. A machined silicon core was used to stiffen the two face sheets of the silicon sandwich. These three elements were assembled, by glass frit bonding, to form the substrate that was polished. The experimental evaluation, in a liquid nitrogen cryostat, demonstrated cryostability performance significantly better than required by the mirror specification.

Deep Impact is a NASA Discovery Mission to impact and observe the nucleus of Comet Temple 1. The instrumentation includes a 300 mm aperture telescope that will operate at 130K once in deep space. It is critical that the telescope mirrors maintain their figure at the operational temperature. We report on measurements of the surface figure changes of three Zerodur primary mirrors from room temperature to 130K, using a PhaseCam interferometer from 4D Vision Technologies, Inc. Although the mirror substrates were taken from the same melt and annealing, they did not perform equally, with differential surface figures ranging from 0.014 waves RMS at 633 nm to 0.082 waves.

We present the results of index of refraction and absorption coefficient measurements of high-quality optical grade sapphire over the 1850 cm^-1 to 10000 cm^-1 wavenumber range and temperatures from 10 K to 295 K. The refractive index is determined by a combination of room-temperature minimum-deviation prism measurements at 2950 cm^-1 and temperature-dependent high-resolution transmission measurements of a 1 mm thick etalon sample from the same batch of material. A Brewster-angle polarizer with an extinction ratio of <10^-5 is used for polarization selection. The uncertainties in the fringe-counting method are analyzed. The temperature dependence of the absorption coefficient is compared with the predictions of a multi-phonon model for sapphire.

The Infrared Multi-Object Spectrograph is a facility instrument for the KPNO Mayall Telescope. IRMOS is a low- to mid-resolution, near-IR (0.8-2.5 um) spectrograph that produces simultaneous spectra of ~100 objects in its 2.8 × 2.0 arcmin field of view. The instrument operating temperature is ~80 K and the design is athermal. The bench and mirrors are machined from Al 6061-T651. In spite of its baseline mechanical stress relief, Al 6061-T651 harbors residual stress, which, unless relieved during fabrication, may distort mirror figure to unacceptable levels at the operating temperature (~80 K). Other cryogenic, astronomy instruments using Al mirrors have employed a variety of heat treatment formulae, with mixed results. We present the results of a test program designed to empirically determine the best stress relief procedure for the IRMOS mirrors. Identical test mirrors are processed with six different stress relief formulae from the literature and institutional heritage. After figuring via diamond turning, the mirrors are tested for figure error at room temperature and at ~80 K for three thermal cycles. The heat treatment procedure for the mirrors that yielded the least and most repeatable change in figure error is applied to the IRMOS mirror blanks. We correlate the results of our optical testing with heat treatment and metallographic data.

There is a current trend in space-based remote sensing toward long duration missions that produce hyper-spectral imaging data. One instrument that is uniquely suited to hyperspectral imaging is the infrared Michelson spectrometer. Michelson spectrometers use a translating mirror stage to vary the optical path length of one leg of the interferometer. Infrared applications often require cooling of this stage to achieve optimum performance. This cryogenic mirror stage is a critical spectrometer component that must be designed and constructed to achieve high reliability and performance during long duration missions. This paper concentrates on three specific areas of optimization. First, an accelerated lifetime test was performed on the mirror stage, with particular attention to the flexural pivots in the joints of the structure. There was no change seen over 22 million translation cycles. Second, a vibration model was created to predict the stage's response to launch and operational accelerations. The model's results closely matched measured values obtained during shake tests of the mirror stage. Third, a cryogenic mirror design was improved to decrease its weight and increase its stability over a wide temperature range. The improved design offers excellent performance for cryogenic operation.

The 10V Chamber Test Facility at the Arnold Engineering Development Center (AEDC) is being upgraded to provide a closed-loop capability to assess multi-band electro-optical sensor performance under realistic operational scenarios against evolving threats. This test facility will leverage existing facilities and expertise from several Government agencies including AEDC, Army/AMCOM, and USAF/KHILS to investigate performance issues during ground testing at cryogenic conditions. Radiometrically accurate simulated scenes will be presented to the test article using dual-band infrared point sources, a dual-band infrared emitter array projector, and a visible array projector. Various optical assemblies will be required to project the images from these radiometric source systems onto the sensor aperture. The infrared point sources will be positioned in the XY plane using two-stage linear translators, which must meet stringent spatial coverage and position accuracy requirements to create realistic closed-loop target motion. A large two-axis steering mirror will simulate sensor line of sight movements for the blackbody sources. A high-speed jitter mirror will simulate high frequency image motion for the emitter arrays. These mirror systems must be vibrationally isolated to minimize the jitter induced in other optical elements. Narcissus and ghost image effects will be minimized using appropriate fabrication, shielding, and calibration techniques. A multi-spectral calibration and alignment system will be integrated into the facility to ensure proper radiometric and goniometric operation of the various target sources. The target and optical systems must all meet performance specifications at cryo-vacuum conditions. Code V will be the primary tool used to evaluate wave front error and distortion coating performance for ghosting/polarization/transmission effects, optical manufacturing errors, and energy-on-detector (EOD). Finite element models of the facility will be used to analyze the structural rigidity and dynamics of the components due to the cryogenic environment.

Multi Layer Insulation (MLI) performance at cold boundary temperatures above 80K to 100K have been well characterized. Further, Lockheed Martin has found that at these temperatures, a blanket’s performance can be well represented by analytic expressions, so long as edge effects, local compressions, and blanket layer density are controlled during the blanket installation. Testing a cold boundary temperatures of 4.2K for the superfluid helium Gravity Probe-B program also found that these same relationships tend to deviate considerably from measurements by a significant margin. Between 4.2K and 80K, no data has been published that quantifies the point where this deviation becomes significant. This lack of data has the potential for a major impact in the thermal subsystem performance for systems that operate at 30K to 80K cold boundary temperatures. The Next Generation Space Telescope (NGST), Terrestrial Planet Finder (TPF), and Constellation-X are systems that benefit from this data, as they all require passive cooling of critical observatory components to between 30K and 40K. To meet each system’s cooling requirement with low performance risk, large design margins must currently be incorporated in the baseline design. This paper summarizes testing performed with cold boundary temperatures at liquid nitrogen (77K), solid nitrogen (47K). These measurements fill the gap between 4.2K and 80K and provide MLI design data at low boundary temperatures that are relevant for future NASA initiatives. Commentary is also provided on some limited cold boundary testing with liquid neon (25K) and solid neon (17K).

The repeatability of positioning of wheels carrying optical components can be critical for the performance of optical instruments. We present the cryogenic worm driven wheel positioning mechanism designed for Anglo-Australian Observatory's Infrared Imager and Spectrograph IRIS2. The mechanism, which was designed for a high vacuum environment and working temperature of 70-90K, utilized an aluminum worm and gearwheel, stepper motor and an encoding system based on infrared sensors. The mechanism has demonstrated repeatability of 1 arcmin.

Baseline configurations for NASA's Next Generation Space Telescope (NGST) include a multi-module science instrument package with near-infrared (near-IR) detectors passively cooled to below 30 K. This integrated science instrument model (ISIM) will also house mid-infrared (mid-IR) detectors that are cooled to 6-7 K with a mechanical cooler or stored cryogen. These complex cooling requirements, combined with the NGST concept of a large deployed aperture optical telescope passively cooled to below 40 K, makes NGST one of the most unique and thermally challenging missions flown to date. This paper describes the current status and baseline thermal/cryogenic systems design and analysis approach for the ISIM. The extreme thermal challenges facing the ISIM are presented along with supporting heat maps and analysis results.

An engineering test unit Fabry-Perot interferometer has been designed and built to operate in the 1.5-1.7 um regime from room temperature to 30 K°. The Fabry-Perot interferometer is tuned by controlling the gap spacing between the two highly reflecting mirrors. Capacitance sensors are used to control the gap spacing and maintain parallelism of the mirrors. An overview of the optical, mechanical, electrical, and control designs of the instrument are described. Some early results at cryogenic temperature indicative of the performance of the instruments are presented.

Utah State University/Space Dynamics Laboratory, teaming with NASA Langley, has built, tested, integrated, and launched the SABER instrument. This instrument is orbiting the Earth on board the TIMED satellite, which resides in a 600 km circular orbit. SABER utilizes a pulse tube cryocooler to cool the focal plane assembly to 75K and passive radiators to cool the remaining components of the instrument. This paper will document the thermal design and modeling of the SABER instrument and compare the modeling results with acceptance testing and on orbit performance data. Preliminary on orbit data indicates that SABER is performing as modeled and is meeting all science objectives.