Hughes cryogenic holographic test facility allows for the rapid characterization of optical components and mechanical structures at elevated and reduced temperatures. The facility consists of a 1.6 meter diameter thermal vacuum chamber, vibration isolated experiment test platform, and a holographic camera assembly. Temperatures as low as 12 Kelvin and as high as 350 Kelvin have been demonstrated. Complex aspheric mirrors are tested without the need for auxiliary null lenses and may be tested in either the polished or unpolished state. Structural elements such as optical benches, solar array panels, and spacecraft antennas have been tested. Types of materials tested include beryllium, silicon carbide, aluminum, graphite epoxy, silicon/aluminum matrix material and injection molded plastics. Sizes have ranged from 7 cm X 15 cm to 825 cm X 1125 cm and have weighed as little as 0.2 Kg and as much as 130 Kg. Surface figure changes as little as (lambda) /10 peak-to-valley ((lambda) equals .514 micrometers ) are routinely measured.

A silicon carbide mirror has been holographically tested at cryogenic temperatures in an attempt to determine the level of surface distortion as a function of temperature. The test was conducted using the 1M thermal vacuum holographic test chamber, located at the Hughes El Segundo facility. The mirror was initially tested in February 1993, using full thermal strapping of the mirror blank, this allowed the mirror to reach a temperature of 48 Kelvin (K). Digitization of the holograms showed a distinct parallel pattern on the surface which corresponded to the webs located on the back of the mirror. A second test was conducted in June 1993, using only the thermal shrouds to cool the mirror. Without the thermal straps attached the mirror exhibited significantly different performance. With thermal strapping the mirror figure distortion was 2.15 waves peak-to-valley (P-V), 0.5 waves rms at a temperature of 184 K, when the test was repeated without straps the mirror figure distortion averaged 0.69 waves P-V, 0.15 waves rms at a temperature of 181 K and a measurement wavelength of 0.514 micrometers.

One of two flight qualified beryllium 0.5 m diameter F/6 Cassegrain telescopes from the Modified InfraRed Interferometer Spectrometer (MIRIS) project is baselined to be flown on the Cassini mission as part of Composite InfraRed Spectrometer (CIRS) instrument. The imaging requirement for the CIRS telescope, 80% ensquared energy within 500 microns, is more stringent than that for MIRIS (95% encircled energy within 0.9 mm). Because the MIRIS telescopes had never been tested at cryogenic temperatures, only ambient data existed for these subsystems. A 0.864 meter liquid nitrogen dewar is being modified and will be used to perform in-house double pass and single pass ensquared energy tests of the beryllium telescopes at 170 K. This paper summarizes the ambient and cryogenic optical tests performed, the results and the status of the new cryogenic facility.

The Instituto de Astrofisica de Canarias infrared camera is intended as a common user instrument for the Cassegrain focus of the Carlos Sanchez Telescope at the Observatorio del Teide. The camera will be equipped with a 256 X 256 InSb array, two filter wheels and a centered diamond turned aluminum optical system working at liquid nitrogen (LN2) temperature. This paper presents the mechanical design, tolerancing and development of the mirrors system, as well as the fabrication and testing of the optical elements.

A dither mirror has been developed which uses three piezo-electric translators for mirror positioning. Three voltages, generated by a strain gauge in each translator and proportional to translator position, are used as position feedback in the closed-loop controller. The nominal mirror scan pattern, a 33 Hz square, is produced by commanding azimuth and elevation angles. This paper describes the three-channel electronic controller under development. Each channel is a closed-loop controller for one actuator/strain gauge pair. Controller circuitry is described and the results of bandwidth optimization are presented. Controller performance is further enhanced through the application of a Digital Error Correction System (DECS). DECS is a second control loop which measures the controller response and alters the command signal to achieve the desired response. Controller performance is presented and compared with the significant improvement achieved using DECS.

We present an astronomical cooled grating spectrometer (LonGSp--Longslit Gornergrat Spectrometer) designed to work at Telescopio Infrarosso del Gornergrat. The covered spectral range is 0.9 divided by 2.5 micrometers and the resolving power is between 300 and 4000 depending on the order and slit width. The spectrometer employs a NICMOS 3 (256 X 256) engineering array of which a subsection of 70 X 40 is used.

The National Institute of Standards and Technology (NIST) is developing a facility to provide absolute responsivity calibrations of detectors in the 2 to 20 micrometers spectral region. The goal is to tie the measurements of NIST's primary radiometric scale with a 1 (sigma) uncertainty of approximately 5%. The system uses chopped thermal IR sources, a room temperature prism/grating spectrometer for spectral selectivity with 1 - 2% spectral resolution and a cryogenic bolometer as a transfer standard detector. The composite design bolometer has an absorbing surface 5 mm in diameter, a nonlinearity < 1% over 5 decades and a responsivity of approximately 1 X 10⁴ V/W without amplification. This bolometer is being calibrated against the High Accuracy Cryogenic Radiometer, the nation's primary standard for optical radiometric measurements. The bolometer's spectral response will also be measured using the Low Background Infrared facility at NIST, the nation's primary blackbody calibration facility.

The effect of condensates on optical surfaces is a continuing concern for space-based optical systems such as the Midcourse Space Experiment. Many such systems contain cryogenic optical surfaces that operate on low temperatures where gases such as nitrogen, oxygen, carbon dioxide, and water will condense. This study presents the effects of these gases on mirror surfaces at temperatures as low as 15 K under high vacuum conditions. The bidirectional reflectance distribution function was determined for these condensates in various film thicknesses up to 8 mm. Optical scatter, thickness, and density measurements were obtained simultaneously with the superpolished quartz crystal microbalance (SPQCM). Correlations between thin film deposition, as determined by the SPQCM, and the expected increase in optical scatter are shown. These correlations are important in determining launch decisions in cases where various degrees of condensation may have occurred on cryogenic optical systems during ground processing.

A cooled Fabry-Perot interferometer system is described in which the mirror parallelism is servo-stabilized and the mirror spacing is electronically tuned using an existing commercially available etalon control system. Capacitance sensors monitor the mirror spacing at points around the periphery, and piezoelectric actuators are used to tune the mirror spacing. The etalon operates at liquid nitrogen temperature. It has a clear aperture of 50 mm and a nominal mirror spacing of between 5 and 60 microns. It is coated for the 3 - 5 micrometers spectral region, although coatings are also available for the 2 - 2.5 micrometers and 9 - 13 micrometers regions. Under servo-control at operating temperature the etalon has a response time of 30 msec and a minimum cavity tuning range of +/- 3 micrometers about the nominal cavity length, corresponding to approximately 3 orders of interference at the midrange wavelength of 4 micrometers .

SD/US has built the SPIRIT III sensor that will be flown aboard the MSX spacecraft experiment and will operate for about 18 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 design and 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 several ancillary/diagnostic instruments, including an autocollimator for alignment corrections, a cryogenic quartz-crystal microbalance to monitor contamination, an onboard signal data processor, various internal stimulation sources, and associated monitors, controls, and telemetry.

The Composite Infrared Spectrometer (CIRS) instrument is scheduled to fly on NASA's Cassini mission to Saturn. CIRS operates at 170 Kelvin and utilizes two Michelson interferometers to measure the infrared spectrum between 7.1 and 1000 microns. The Mid-InfraRed interferometer (MIR) is a classical Michelson design operating in the 7.1 to 16.7 micron band. The Far-InfraRed interferometer (FIR) is a polarizing Michelson design measuring the 16.7 to 1000 micron band. Both the MIR and FIR use retroreflector elements rather than flat mirrors. The MIR requires hollow cube corner style retroreflectors while the FIR polarizing nature requires roof-top mirror style retroreflectors. Initial testing of available technology indicated that interferometric quality retroreflectors do exist in ambient temperatures. Tests were performed using commercially available mounted and unmounted cube corners and commercial cube corners mounted to GSFC designed mounts to characterize their cryogenic, interferometric performance. The Goddard Space Flight Center's ambient and cryogenic test and results are presented here.

The future of rapid cooldown cryogenic IR systems lies in lightweight, compact and inexpensive designs that use state-of-the-art configurations, materials and mass-production manufacturing processes. Miniature JT cryocoolers are ideally suited for cooling applications that require long storage periods (10 - 15 yr), fast cooldown times (< 30 s) and short operating missions (< 3 min). Most of the applications cool seeker sensors on missiles, projectiles, or `smart' bombs where environments conditions of temperature, acceleration, shock and vibration are harsh, where volume and weight must be minimized, and where low cost is required for large quantities. The Lockheed Palo Alto Research Laboratories are currently developing the Low Cost Sensor Integrated Dewar Assembly (LCS IDA) using an APD Cryogenics Inc. JT cryocooler. This paper presents the design fundamentals of rapid cooldown dewars and JT cryocoolers with the LCS IDA as a case study. Specific topics include: the performance trade-offs of the many thermodynamic parameters; the systems approach to the design process, and; recent test results.

This paper will present an overview of the Joule-Thomson cryocooler technology, the COOLLAR program, the Engineering Development Model design and supporting test results.

This paper discusses the concept of a mechanical refrigerator integrated with a phase-change cryogen for cooling space borne sensors. Also described is the proposed integration of a small tactical mechanical refrigerator with a small amount of methanol to cool an orbiting sensor imaging atmospheric gravity waves. Included are the results of ground tests of the methanol melting/freezing process. Results indicate that a constant temperature during freezing and melting is possible if the heat loads into the system are low enough to maintain the phase-change substance near equilibrium. Plans for the next laboratory experiment are also outlined. The paper further presents a comparison of other substances that may be considered for use as a phase- change material.

This paper describes the analysis, development, and laboratory testing of a closed loop control system that maintains temperature over a range of 50 to 80 K to +/- 0.01 K. The control loop uses position, velocity, and integrated error information to achieve excellent short-term and long-term temperature stability by modulating the length of the compressor stroke. The resulting control system is compatible with adaptive vibration cancellation techniques. The control algorithm is implemented on a personal computer using a 386 processor, but it is simple enough to run as one of many tasks on a less powerful microprocessor.

This paper describes an active multi-axis vibration cancellation system designed for the expander of a Hughes split Stirling cryocooler. The axial force and the two lateral torques generated by the expander are cancelled by an active counterbalance driven by electrodynamic forcing elements. The counterbalance drive signals are generated by an narrowband adaptive feedforward controller; load cells provide the error signals. Multi-axis cancellation results are presented in which 30 dB of cancellation was achieved at the first five axial harmonics while simultaneously the fundamental harmonic of both lateral torques was reduced by 20 dB. The system demonstrates that effective multi-axis cancellation can be implemented with compact hardware and simple analog electronics.

A thermoelectric Peltier device for cooling HgCdTe 2D focal plane arrays was developed. Reliability, accurate temperature control and absence of vibration while operating make the thermoelectric cooler very attractive for cooling IR components.

Internal temperatures in filled cryostats must be continuously monitored to preserve the health and safety of hardware and personnel. The accidental response of cryogenic gases into the atmosphere pose a health threat and, if the gases are flammable, may lead to an explosion. One indication of an imminent cryogen release is the sudden increase in cryogen temperature. Although there are many data acquisition systems and temperature monitoring products commercially available, these systems lack the portability and safety features required during cryostat qualification tests and transport. This paper describes a temperature monitor and alarm circuit developed for the Spirit II solid hydrogen cryostat program. The instrument is battery-operated, accurate, portable, and intrinsically safe in an explosive atmosphere.

This paper presents the on-orbit performance of the Cryogenic Limb Array Etalon Spectrometer (CLAES) cryostat through the mission, and compares its performance to pre-launch predictions for lifetime and for temperature vs. time for the CLAES cryogenic subsystems. Absolute temperatures of various subsystems in the instrument are presented along with temperature gradient discussions and their correlation to predictions of mission lifetime. The cryostat's thermal performance during ground operations, at spacecraft integration and during launch preparations at the Kennedy Space Center are also presented.

Wide-Field Infrared Explorer, an instrument proposed for a NASA Small Explorer mission, is designed to measure the 12 to 25 mm radiation of starburst galaxies. The Jet Propulsion Laboratory provides project management and science leadership. The Space Dynamics Laboratory at Utah State University will build and integrate the instrument. The cryostat will be built at the Lockheed Palo Alto Research Laboratory.

This paper describes the issues associated with thermal management of the Sounding of the Atmosphere using Broadband Emission Radiometer instrument, proposed by NASA LaRC and the Space Dynamics Laboratory at Utah State University. With the instrument subjected to severe mass and power constraints, the TRW miniature pulse tube cooler has been baselined to maintain the focal plane at less than 75 K over a required lifetime of two years. Cooler and electronics heat is to be rejected through the spacecraft bulkhead at 300 K to radiators at approximately 290 K. The optical cavity is to be maintained at 210 K by a separate radiator. Approaches to ensure that heat loads do not exceed cooler capacity, radiator studies, and interfacing the cooler to the sensor and spacecraft are discussed.

SPIRIT III, the primary sensor to be flown aboard the BMDO midcourse space experiment, will be cooled using a 9 K solid-hydrogen cryostat. Laboratory cold tests of SPIRIT III have relied on helium as the cryogen because of the inherent risks associated with hydrogen.

The Superfluid Helium On-Orbit Transfer (SHOOT) Flight Demonstration was designed to demonstrate the technology for resupplying space helium dewars on orbit. Secondarily SHOOT developed a number of components useful to other satellites using superfluid helium. SHOOT was launched on the Space Shuttle Endeavour as part of mission STS-57 on June 21, 1993. On orbit operations were conducted in three stages: pump down of the superfluid and electronics checkout, initial ground controlled helium transfers, and an astronaut controlled transfer. An overview of the SHOOT mission is given. A summary of experiences with a number of flight components developed specifically for SHOOT is presented. Questions that were answered by the flight are discussed.

The goal of this work is to design a space experiment that will evaluate the thermal savings on a cryogenic subsystem when manganin leads are replaced by the high temperature superconductor (HTS) leads printed onto a substrate with a low thermal conductivity. A conductive mathematical model has been developed and used as a key tool in the design process and subsequent analysis. The results indicate at least a 50% reduction in total heat load on the cryogen by replacing the currently used manganin wires with HTS leads.

A topical workshop on Cryogenic Optical Systems and Instruments was held on Monday 4 April, from 8:00 to 10:00 pm, moderated by James B. Heaney, NASA Goddard Space Flight Center.