A study was undertaken, early in 1992, to determine if a SIRTF class mission could be developed to allow the Observatory to be launched on an Atlas IIAS rather than the Titan IV/Centaur previously planned. The substantial mass reduction required to make that change resulted in a new set of requirements for science and a heightened interest in light weight optics. This paper addresses the rationale and plan for the Atlas SIRTF cryo-optics technology plan.

A 46 cm diameter, lightweight, Amersil TO8E, fused-natural-quartz mirror with a single-arch cross section was tested at the NASA-Ames Research Center Cryogenic Optical Test Facility to measure its cryogenic distortion at 6.5 K. Then the mirror was refigured with the inverse of the measured cryogenic distortion to compensate for this figure defect. The mirror was retested at 6.5 K and found to have a significantly improved figure. The compensation for cryogenic distortion was not complete, but preliminary analysis indicates that the compensation was better than 0.25 waves P-V if edge effects are ignored. The feasibility of compensating for cryogenic distortion by refiguring has thus been verified.

We describe the design and analysis of a high precision 1-m class fused quartz mirror for cryogenic applications. This design is based on requirements from the 1992 Space Infrared Telescope Facility mission. We present primary mirror derived requirements, comparisons of mirror substrate materials, and mirror geometry trades and analyses. We also address the particular impact on the mirror design due to telescope integration and cryogenic testing in a 1-g environment. We discuss numerous aspects of our proposed mirror design. We also show compliance of this proposed design in meeting the top level optical performance as well as structural and verification requirements in a simple and cost effective manner compared with other geometries.

Space Dynamics Laboratory of Utah State University has built the SPIRIT III sensor that will be flown aboard the Midcourse Space Experiment (MSX) spacecraft and will operate for about 20 months. The MSX mission objective is to measure the spectral, spatial, and radiometric parameters of various orbital and suborbital targets; the earth's airglow, aurora, and other upper atmospheric phenomena; and the celestial background. This paper discusses the development of the SPIRIT III sensor - the primary instrument for collecting long-wave infrared data during the MSX mission. SPIRIT III consists of a sensor system and 19 electronic units distributed near the sensor and in the electronics section. The sensor assembly consists of an extremely high off-axis-rejection telescope, a radiometer, and an interferometer, all of which are cooled to cryogenic temperatures by a solid-hydrogen-filled dewar/heat exchanger. In addition to these, there are a number of ancillary/diagnostic instruments.

This paper documents the development of the SPIRIT III telescope from the design through its test activities. The SPIRIT III Instrument is the primary infrared instrument on the Mid-Course Space Experiment (MSX). The telescope is an all reflective optical system consisting of twelve mirrors. The nominal collecting apertures is 14 inches. It was designed and built to integrate with a multicolor radiometer and a Michelson interferometer built by the Space Dynamics Laboratory at Utah State University. Key performance features are discussed, and measured test data is presented. The structural/thermal trade-off issues of a satellite-based cryogenic instrument are presented along with a review of the test techniques and test equipment.

The paper discusses the design, analysis, and testing of a rapidly cooled beryllium cryogenic mirror, which is the primary mirror in the four-element optical system for the Long Wavelength Infrared Advanced Technology Seeker. The mirror is shown to meet the requirement of five minutes for cooling to cryogenic operating temperature; it also maintains its optical figure and vacuum integrity and meets the nuclear specification. Results of a detailed thermal analysis on the mirror showed that, using nitrogen gas at 80 K as coolant, the front face of the mirror can be cooled from an initial temperature of 300 K to less than 90 K within five minutes. In a vacuum chamber, using liquid nitrogen as coolant, the mirror can be cooled to 80 K within 1.5 min. The mirror is well thermally insulated, so that it can be maintained at less than its operating temperature for a long time without active cooling.

The Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) instrument, launched on the Shuttle Discovery (STS-39) on 28 April 1991, was developed to characterize the phenomenology and dynamics of ionospheric processes. The primary objective of the CIRRIS 1A mission was to obtain spectral and spatial measurements of infrared atmospheric emissions in the spectral region between 2.5 and 25 microns over altitudes ranging from the Earth's surface to 260 km. The primary sensors are a Michelson interferometer/spectrometer and a multi-spectral radiometer, which share a common high off-axis rejection telescope. The sensor/telescope complex is enclosed in a cyogenic dewar. Excellent data were obtained from this mission, and preliminary analysis shows that all sensors performed well. This paper describes the experiment hardware, summarizes instrument performance during flight, and presents examples of significant results.

A helium-cooled interferometer was flown aboard shuttle flight STS-39. This interferometer, along with its sister radiometer, set new benchmarks for the quantity and quality of data collected. The interferometer generated approximately 150,000 interferograms during the course of the flight. Data was collected at tangent heights from the earth's surface to celestial targets. The interferograms encoded spectral data from aurora, earth limb, and earth terminator scenes. The interferometer collected data at resolutions of 8, 4, and 1 wavenumbers over a spectral range of 2 to 25 micrometers. The interferometer's optics, detectors and preamps, laser reference system, realignment system, and eight-position optical filter wheel are described as they performed on-orbit.

We describe the status of the development and testing program for the inertial reference system for the Gravity Probe B gyroscopes. The gyroscope housings are attached to a cryogenic telescope with a 14 cm aperture that continuously points at a guide star. The star image is split to provide quadrant pointing information which is used to steer the spacecraft. This data is also combined with the gyro readout data to provide an absolute precession measurement. Motion of the guide star is independently checked by reference to background galaxies. Room temperature testing of a prototype telescope has been completed and preparations are being made for low temperature tests.

The Simultaneous Quad-Color Infrared Imaging Device (SQIID) is the first of a new generation of infrared instruments to be put into service at the Kitt Peak National Observatory (KPNO). The camera has been configured to be modular in design and to accept new innovations in detector format as they become available. Currently the camera is equipped with four 256 x 256 platinum silicide arrays with 30 micron pixels for each of the four bands J (1.1-1.4 microns), H (1.5-1.8 microns), K (2.0-2.4 microns), and L' (3.52-4.12 microns). The optics of the instrument have been designed to accept detector arrays as large as 512 x 512, or an equivalent field size of 12.4 mm x 12.4 mm. The instrument is cooled with a pair of closed cycle cryogenic coolers, which are mechanically aligned and electrically phased to eliminate vibration. In addition, a transputer based electronics system has been incorporated to facilitate fast frame rates, co-add frames, and ease the data handling burden.

The Radiation Measurement System (RAMS) is a project at NASA Ames Research Center to develop infrared radiometers for use in studying radiative transfer within the earth's atmosphere. These radiometers will use a liquid nitrogen cooled reference source and thermal stabilization of the optics. One of the radiometer designs calls for a hemispherical field of view, which has stimulated development of a new optical system. The system developed uses Winston cones, some of them in the reverse of the usual direction. This optical system has been optimized for the RAMS project needs and simulated on a computer for verification.

This paper will present transmittance data, in the 1- to 1000-micron wavelength region, at temperatures from 300K down to near 4K, for a selection of filters composed of multilayer thin films on transmitting substrates, reststrahlen crystals, mesh-grid elements, and hybrids of these types. Polyethylene laminates and vapor deposited parylene will be compared as antireflection layers for high refractive index infrared crystals at long wavelengths.

Experimental results on mild lead telluride (PbTe) material and vacuum-deposited multilayers are reported. Optical absorption of mild lead telluride is determined by envelope methods and found to be different from that of stoichiometric lead telluride on cooling. Generally, the performance of multilayer coatings which are composed of mild PbTe/ZnSe is deteriorated in middle infrared band when temperature decreases. The wavelength shift of a multilayer filter is essentially a linear function of filter temperature.

Results are presented of the first liquid helium temperature testing of the Space Infrared Telescope Facility (SIRTF) prototype Secondary Mirror Assembly (PSMA). Attention is given to the key requirements of PSMA, its design, and the test instrumentation and procedures. Results of preliminary tests completed to date showed that the design of the PSMA forms a very strong basis for meeting the SIRTF requirements.

The performance parameters, the implemented design, and the testing processes used to develop a variable profile scan mirror mechanism for operations in a cryovacuum environment are described. Particular attention is given to the mechanical design of the mirror mechanism, the electrical control system, and the synchronization pulse generation. The results of scan mirror performance tests are presented, showing that the mirror has met or exceeded all performance requirements. The scan mirror has operated at 30 K with a high reliability in positional accuracy and constant velocity scanning for an estimated 700,000 cycles.

A control system is presented for precise and rapid positioning of spectral filters in the multispectral infrared optical system used for the MUlti-Spectral Infrared Camera (MUSIC). A hybrid system employing a stepper motor as a direct-drive brushless servomotor presents an ideal solution, applying high torque at low speed for the optimum response as limited by available torque. As the end of the repositioning transient is approached, closed-loop proportional control of torque provides quick settling to a positioning accuracy of 0.02 degrees. The use of a stepper motor avoids the problems of a brushed DC servomotor, such as brush and bearing failures at high speeds in a cryogenic vacuum, and backlash in reduction gears. The analog servo loop with commutation hardware to optimally switch the current to the stepper motor windings, avoids problems typical of stepper motors, such as limited positioning precision and high power dissipation while holding position.

The paper describes the electromechanical design of the drive system for the actively controlled chopping secondary mirror of the W.M. Keck Ten Meter Telescope, which performs two-axis spatial chopping. Also described are the reaction cancellation system, the special packaging and cooling features, and the operation of the adaptive drive system. Laboratory test data are presented showing the performance of the drive and control system for chop amplitudes up to 54 arcmin (mirror motion), duty cycles of up to 98 percent, and frequencies from 1 to 50 Hz.

Progress made in the development of a range of small compressors and gas cleaning equipment designated High Pressure Pure Air Generators (HiPPAG) is described together with the HiPPAG design considerations and the choice of operation conditions. Particular attention is given to the characteristics of several HiPPAG systems, including a three-stage open-cycle compressor, HiPPAG 100; a four-stage open-cycle compressor, HiPPAG 320; and a three-stage closed-cycle compressor, HiPPAG 340. Design diagrams of HiPPAG 100 and HiPPAG 320 system layouts are presented along with some performance data.

The Cryogenic InfraRed Radiance Instrumentation for Shuttle (CIRRIS 1A) instrument, successfully flown and operated on the Shuttle Discovery from 28 April to 6 May 1991, was designed to operate at supercritical helium temperatures. During flight, the focal plane temperature control and telescope contamination purge systems performed as designed and 36 hours of excellent data was obtained; however, the parasitic helium flow rate was higher than expected. This paper reviews thermal data obtained for the CIRRIS 1A cooling system during both ground and flight operations. The temperature control and purge systems are discussed, along with helium flow rates, dewar helium pressure, and thermal stratification. In addition, possible reasons for the high on-orbit parasitic flow rate are presented.

An existing operational low-earth-orbit radiative cooler is being upgraded for use on the Mars Observer polar-orbiting mission, using the Landsat Thematic Mapper cooler as a basis for upgrading. Significant improvements were achieved in the design of the thermal performance and vibration endurance, and in the mass minimization design. The cooler provides about 18 mW of useful refrigeration power in Mars orbit at its equilibrium operating temperature. The detector operating temperature for the worst-case orbital environment is about 73 K.

The paper describes the Cryogenic Limb Array Etalon Spectrometer (CLAES) launched on September 12, 1991 aboard the NASA Goddard's Upper Atmosphere Research Satellite the purpose of which is to measure the global concentrations of stratospheric species and their temperature, as a function of altitude. Particular attention is given to the design-level thermal predictions and their correlation to the results of ground tests, and to the on-orbit performance of CLAES. Also presented are data on the cryostat's thermal performance during ground operations, at spacecraft integration and during launch preparations. The CLAES functional block diagram and the cryostat schematic diagram are included.

An effort was made to increase the predicted lifetime of the SIRTF dewar by lowering the exterior shell temperature, increasing the radiated energy from the vapor cooled shields and reconfiguring the vapor cooled shields. The lifetime increases can be used to increase the scientific return from the mission and as a trade-off against mass and cost. This paper describes the configurations studied, the steady state thermal model used, the analytical methods and the results of the analysis. Much of the heat input to the outside dewar shell is radiative heat transfer from the solar panel. To lower the shell temperature, radiative cooled shields were placed between the solar panel and the dewar shell and between the bus and the dewar shell. Analysis showed that placing a radiator on the outer vapor cooled shield had a significant effect on lifetime. Lengthening the distance between the outer shell and the point where the vapor cooled shields are attached to the support straps also improved lifetime.