At the Advanced Technology Center of Lockheed Martin Missiles and Space, we have created an integrated optical, mechanical, thermal and structural design and analysis program called `Optics Plus'. This program is a rapid and accurate system for mechanical design and analysis of optical systems, including thermal, stray light and structural properties. Accurate cost and performance balancing trades are possible in very short time frames, thereby reducing overall costs previously much greater. There are three factors in its success: (1) An Integrated Team of analysists, domain experts and software experts that design, build and use the tool, (2) Integrated Tools that include OPTIMA, IDEAS, ASAP Plus, MSAT, and TMG, and (3) Outstanding Support from the laboratories broad expertise in optics, thermal and structural analysis. This paper will describe how the integrated system operates and a flow diagram will spell out the nodes of program interactions. The resultant will be demonstrated with several examples of cryogenic instruments.

Optical surface deformation of metal mirrors used at cryogenic temperatures is reduced through the use of a new process of plating amorphous aluminum on aluminum. The AlumiPlate^TM process (produced by AlumiPlate, Inc. in Minneapolis, MN) plates a layer of 99.9+% high purity aluminum about 125 micrometers thick atop the substrate. Very good surface finishes are produced by direct diamond turning of the plating, with some samples below 40 angstroms RMS. Optical testing of a 175-mm diameter, 550-mm optical radius of curvature 6061-T651/AlumiPlate^TM aluminum sphere was performed at 65 K to determine cryogenic optical surface figure stability. In five cycles from 300 to 65 K, an average optical surface change of 0.047 wave RMS (1 wave equals 633 nm) was observed. A total optical figure change of 0.03 wave RMS at 65 K was observed from the first to last cycle. The cause of this relatively small long-term change is not yet determined. The test mirror is bi-concave, with a semi- kinematic toroidal mount, and is machined from the axis of a billet. An `uphill quench' heat treatment consisting of five cycles from liquid nitrogen to boiling water temperatures is used to minimize residual stress in the test mirror. Initial diamond turning of the mirror by the Optical Filter Corp., Keene, NH, produced a 300 K unmounted optical surface figure of 0.380 wave peak-to-valley and 0.059 wave RMS. A second effort at diamond turning by II-VI, Inc., Saxonburg, PA produced a 300 K optical figure of 0.443 wave peak-to-valley and 0.066 wave RMS, with a surface roughness varying from 29 to 42 angstroms.

The Cassini Composite InfraRed Spectrometer half-mirror diameter beryllium flight telescope's optical performance was tested at the instrument operating temperature of 170 Kelvin. The telescope components were designed at Goddard Space Flight Center (GSFC) but fabricated out-of-house and then assembled, aligned, and tested upon receipt at GSFC. A 24-inch aperture cryogenic test facility utilizing a 1024 X 1024 CCD array was developed at GSFC specifically for this test. The telescope's image quality (measured as encircled energy), boresight stability and focus stability were measured. The gold coated beryllium design exceeded the cold image performance requirement of 80% encircled energy within a 460 micron diameter cycle.

The Composite InfraRed Spectrometer (CIRS) instrument flying on the Cassini spacecraft to Saturn is a cryogenic spectrometer with far-infrared (FIR) and mid-infrared channels. The CIRS FIR channel is a polarization interferometer that contains three polarizing grid components. These components are an input polarizer, a polarizing beamsplitter, and an output polarizer/analyzer. They consist of a 1.5 micron (micrometers ) thick mylar substrate with 2 micrometers wide copper wires, with 2 micrometers spacing (4 micrometers pitch) photolithographically deposited on the substrate. This paper details the polarization sensitivity studies performed on the output polarizer/analyzer, and the alignment sensitivity studies performed on the input polarizer and beamsplitter components in the FIR interferometer.

The Composite Infrared Spectrometer (CIRS) of the Cassini mission to Saturn has two interferometers covering the far infrared and mid infrared wavelength region. The mid infrared wavelength interferometer has a focal plane consisting of a germanium focus lens and HgCdTe array. System level calibration of the CIRS Flight Unit indicated a discrepancy between the expected and actual signal levels. Testing on the CIRS breadboard and Engineering Unit indicated that defocus of the germanium lens could significantly reduce the modulation efficiency of the interferometer in the presence of a moderate degree of wavefront shear. Defocus of the lens in the focal plane was of concern because of the temperature dependence of the index of refraction of germanium and the nominal operation temperature of 170 K. The shear/defocus interaction was extensively investigated and correlated to a newly developed analytical model. It was eventually determined that the CIRS instrument was in focus, had no appreciable wavefront shear and was operating near theoretical limits. The shear/defocus effect is however, of considerable interest, since it has not been described in previous literature on interferometers.

The combined effects on performance of shear between the two arms, defocus of the detector, and difference in wavefront between the two arms of a Fourier transform spectrometer using cube corner retroreflectors were investigated. Performance was characterized by the amplitude of the fringe signals coming from a detector as the path-length difference was scanned. A closed-form expression was found for the combined effects of shear and defocus, and it was found that defocus had no effect in the absence of shear. The effect of wavefront error was modeled numerically and assumed to be independent of shear and defocus. Results were compared with measurements make on the breadboard and engineering model of the Composite Infrared Spectrometer for the Cassini mission to Saturn, and good agreement was found.

A combination of a single mode AlGaAs laser diode and a broadband LED was used in a Michelson interferometer to provide reference signals in a Fourier transform spectrometer, the Composite Infrared Spectrometer, on the Cassini mission to Saturn. The narrowband light from a laser produced continuous fringe throughout the travel of the interferometer, which were used to control the velocity of the scan mechanism and to trigger data sampling. The broadband light from the LED produced a burst of fringes at zero path difference, which was used as a fixed position reference. The system, including the sources, the interferometer, and the detectors, was designed to work both at room temperature and at the instrument operating temperature of 170 Kelvin. One major challenge that was overcome was preservation, from room temperature to 170 K, of alignment sufficient for high modulation of fringes from the broadband source. Another was the shift of the source spectra about 30 nm toward shorter wavelengths upon cooldown.

The development of an optical camera based on superconducting tunnel junctions has now reached a stage where practical applications in optical or UV astronomy can be considered. A prototype cryogenic camera (named S-Cam) has been developed, based on a high quantum efficiency 6 X 6 detector array of tantalum Josephson junctions, and operating at a temperature of about 0.4 K. This paper describes the general characteristics of the camera, sensitive in the waveband from 350 to 700 nm and designed to be installed in 1998 at the Nasmyth focus of the William Herschel Telescope in La Palma, Spain. In addition to the performance of the overall system, the preliminary detector unit test results will also be presented. The present S-Cam system performance is discussed in view of future versions of the camera. Provided the field coverage of these cameras can be extended through the development of larger format detector arrays and adequate read-out electronics, they have the potential to provide a significant additional tool for optical and UV astronomy in the next century.

This paper describes the thermal design and analysis of the HOSS (Hydrogen On-Orbit Storage and Supply) liquid hydrogen dewar. This task is being carried out by the Space Dynamics Laboratory at Utah State University under contract from NASA Lewis Research Center. The vacuum jacketed 80-liter dewar is designed for a mission life greater than 30 days. The design uses concentric G-10 fiberglass support tubes and multilayer insulation to thermally isolate the hydrogen tank. Heat load trade off studies were performed based on the support tube thickness, plumbing size, and vacuum shell temperature. The dewar employs a liquid nitrogen cooled shield to provide a non-venting ground hold capability of more than 96 hours for launch preparation. Analysis has shown that a greater than 30 day mission is feasible even with a mechanically robust design capable of withstanding most launch environments.

Solar Thermal Propulsion systems require long-term storage of liquid hydrogen. NASA's Hydrogen On-Orbit Storage System experiment was intended to demonstrate technologies critical to this concept, including a LAD and TVS. This paper describes the analysis and re-design of the TVS to function in the small (80 liter) dewar designed by the Space Dynamics Laboratory at Utah State University in conjunction with the NASA Lewis Research Center. The re-design uses the same approach as the NASA design for a larger system. The heat exchanger and Joule-Thomson device in particular are addressed. The design theory of a LAD is also presented.

This paper describes an approach to mounting Potassium Bromide (KBr) optical elements that are expected to survive launch vibrations and a cryogenic environment. These KBr optics constitute the beamsplitter and compensator for a high-resolution, infrared Fourier transform spectrometer. This spectrometer is part of the Tropospheric Emissions Spectrometer (TES) instrument which will operate in the 3.2 to 15.4 micrometers spectral range. TES is part of NASA's Earth Observing System initiative to better understand our Earth's environment. TES is designed to obtain data on tropospheric ozone and other gas molecules that lead to ozone formation. These data will be used to create a 3D model describing the global distribution of these gases to better understand global warming and ozone depletion. TES uses a Connes interferometer where the clear aperture (CA) responsible for splitting the science beam is distinct and separated by 108 mm from the CA with recombines the split beams. KBr has a low elastic limit and a high coefficient of thermal expansion, is highly soluble in water and is susceptible to degradation from humidity. These characteristics make it a rather difficult optical material to mount and protect from environments typically resisted by glass optics. The design described here uses a diameter to thickness aspect ratio of 6:1 (based on a 190 mm diameter) resulting in a rather massive element. Due to instrument mass and volume constraints in the interferometer, a pseudo-rectangular shape for the optical elements was devised and a graphite/cyanate ester support structure was designed to minimize the mass of the entire beamsplitter assembly. Vibration isolation of the optical elements was provided by RTV silicone pads, which were also designed to meet thermal stress concerns for the 180 K operating environment. Both structural and thermal analyses were performed to verify the initial design. Further vibration and thermal testing of development units is expected to uncover any unforeseen problems and to verify compliance in areas of concern. This paper addresses RTV silicone material properties required to properly support the KBr optics and predicted KBr stresses and RTV preloads and deflections derived from an analytical model of the design configuration. Results from thermal and vibration testing of development units will also be presented (if available) and compared to preliminary thermal and structural models.

The Composite Infrared Spectrometer (CIRS) instrument on the Cassini Mission launched in October of 1997. The CIRS instrument contains a mid-infrared (MIR) and a far-infrared interferometer and operates at 170 Kelvin. The MIR is a Michelson Fourier transform spectrometer utilizing a 76 mm (3 inch) diameter potassium bromide beamsplitter and compensator pair. The potassium bromide elements were tested to verify effects of cooldown and vibration prior to integration into the instrument. The instrument was then aligned to ambient temperatures, tested cryogenically and re-verified after vibration. The stringent design optical figure requirements for the beamsplitter and compensator included fabrication errors, mounting stress and vibration load effects. This paper describes the challenges encountered in mounting the elements to minimize distortion and to survive vibration.

The Composite InfraRed Spectrometer (CIRS) instrument aboard the Cassini spacecraft en route to Saturn is a cryogenic spectrometer with far-infrared (FIR) and mid-infrared channels. The CIRS FIR focal plane, which covers the spectral range of 10 - 600 cm^-1, consists of focusing optics and an output polarizer/analyzer that splits the output radiation according to polarization. The reflected and transmitted components are focused by concentrating cones onto thermoelectric detectors. These thermoelectric detectors consist of a gold black absorber on top of a gold foil that is welded to a thermoelement consisting of two semiconductor pyramids. After the detectors were integrated into the focal plane assembly and the CIRS instrument, the detectors proved to be extremely susceptible to two environmental survivability conditions: acoustics and airflow. Several changes were investigated to improve the integrity of the detectors including detector airflow geometry, structural changes to the detectors, and more intensive screening methods. The geometry of the air paths near the sensing elements was modified. Two structural modifications were implemented to improve the stability of the sensing elements. These were changes in the geometry of the thermoelectric pyramids by ion milling, and a change in the gold foil thickness. New screening methods, centrifuge and modulated force testing, were developed to select the most rugged detectors. Although several methods gave significant improvements to the detector's stability, the modification that allowed the detectors to meet the environmental survivability requirements was the change in the geometry of the air paths near the sensing elements.

This paper describes the state-of-the-art precision bonding technique for cryogenic fused-silica optics. It was developed for assembling the fused-quartz Gravity Probe-B science instrument, which will be used to prove or disprove Einstein's Theory of General Relativity with unprecedented accuracy and precision. This room-temperature bonding process is based on hydroxide catalysis. The resulting bonding strength is comparable with that of fused silica or fused quartz. The interface is typically 200 nm essentially limited by surface figure mismatch. It is as precise as optical contacting, as reliable as high-temperature frit bonding, as transparent as optical epoxies. So far it is the only bonding approach that meets all the stringent requirements for GP-B's applications at 2.5 Kelvin.

A simple optical method for studying vibration has been developed and applied to the investigation of a commercial closed cycle refrigeration (CCR) system. This method utilizes an amplitude modulation of a laser beam by the knife-edge attached to the cold finger of the CCR system. The sensitivity of the proposed optical technique is determined by the diameter of the focused laser beam and a displacement of 1 micrometers is readily detectable. For the system CRYO Model 396-022 based on CTI CRYODYNE Model 22 refrigerator, experimental studies were conducted for different cold finger temperatures, cold head orientations, and mechanical holders. The total amplitude of the displacement was on the order of 50 micrometers for a cold head fixed into rigid mechanical holder placed on the optical table and 30 micrometers for the same holder placed on a special stand decoupled from the optical table. Three main frequency components at 3 Hz, 60 Hz, and 120 Hz have been observed.

An estimate of the transparency of aluminized mylar `superinsulation' was obtained by measuring the far- infrared/submillimeter wave transmittances of 3 pieces randomly selected from a 25.4 micrometers -thick (nom. 001 in.) sheet of mylar that was aluminized on one side. Measured transmittance values were less than 1 X 10^-4 in the 100 micrometers - 1000 micrometers wavelength region. The emissivities of mylar and aluminum were computed from published optical constants to be, respectively, about 5 X 10^-2 and 2 X 10^-4 for temperatures near 20 K and an effective wavelength of 150 micrometers . Due to the strong attenuation of the aluminum layer, the radiant power from an elemental area on the outer surface of the superinsulation is about 10⁴ times more significant than radiance originating within the insulating mylar layer, for temperatures near 20 K. Radiant power passing through doubly aluminized mylar (the usual configuration) would be attenuated by a factor of about 10^-10.

This review paper focuses on measurement techniques and facilities for the study of the contamination and space environment effects on optical and thermal radiative surfaces. Laboratory measurements are reviewed and illustrate how cryogenic and relatively warm surfaces can be affected by contaminants, vacuum, and UV. The laboratory data are used to illustrate the important parameters that require consideration when trying to determine these types of effects on future satellite missions. Optical properties of thin contaminants films, BRDF measurements on cryogenic films, quartz crystal microbalance (QCM) measurements, and UV effects on silicone/hydrocarbon films are presented and discussed relative to their applications to satellite systems. The laboratory data are complemented with flight data from the Midcourse Space Experiment (MSX) satellite. Laboratory results were used to interpret MSX spacecraft flight data. The MSX demonstration and validation satellite program was funded by the Ballistic Missile Defense Organization. MSX had UV, visible, and infrared instruments including the Spirit 3 cryogenic telescope and had several contamination instruments for measuring pressure, gas species, water and particulate concentrations, and condensable gas species. Some of the data collected from the flight QCMs are presented.

NASA's Space Infrared Telescope Facility (SIRTF) is a 1- meter class cryogenically-cooled space observatory currently in the design and development phase. It is planned for launch in December 2001 by a Delta rocket into a heliocentric orbit. The SIRTF Observatory is comprised of the Cryogenic Telescope Assembly (CTA), the Spacecraft, and the three Science Instruments. The CTA has an 85 cm diameter aperture telescope which is cooled to its lowest operating temperature of 5.5 K by effluent vapor from the 360-liter superfluid helium cryostat. The three Science Instruments, which span an operating wavelength range from 3 to 180 micrometers , will be maintained at a temperature of 1.4 K inside the cryostat. The required SIRTF mission lifetime is >= 2.5 years. The CTA system and subsystem design as well as their technical challenges are described.

The Composite Infrared Spectrometer of the Cassini mission to Saturn has two interferometers covering the far infrared and mid infrared wavelength region. The instrument was aligned at ambient temperature, but operates at 170 Kelvin and has challenging interferometric alignment tolerances. Cryogenic alignment tests of the instrument indicated that it should suffer minimal degradation due to the cooldown from ambient to operational temperature. System level tests performed by the calibration team indicated a lower than expected signal level on the mid infrared channel, while providing ambiguous optical throughput data. Therefore it became imperative to develop a metric that could be used to determine the instrument performance at both the instrument and system levels, at ambient and cryogenic temperature. Modulation efficiency and throughput measurements were performed and new analytical models developed to evaluate the status of the instrument. Methodologies are detailed, empirical and analytical data are reconciled and deviations from design values explained.

The Composite Infrared Spectrometer of the Cassini mission to Saturn has two interferometers covering the far infrared FIR and mid infrared, MIR wavelength region. The FIR is a polarizing Michelson interferometer, which presents a collimated output beam to the FIR focal plane. The focal plane consists of a parabolic focus mirror and an analyzer grid, which splits the output beams into transmitted and components. The two orthogonal polarizations are focussed onto two thermopile detectors, each consisting of a gold black absorber on top of a 100-nanometer thick gold foil welded to the top of two bismuth pyramids. The gold black is 30 microns thick, and the weld area is approximately 5 microns in diameter. The detectors are extremely fragile and the weld can be broken with a minuscule amount of airflow across the surface of the foil. The detectors consistently passed acoustic testing (at the detector level), to qualification levels that simulated the launch environment of the Titan IV launch vehicle. However, they experienced a 50% failure rate when installed in the focal plane assembly during instrument level acoustic tests. A test focal plane was developed with small pressure transducers in the nominal detector locations. These tests indicated over 10 dB of acoustic amplification in the focal plane in the instrument due to the geometry of the focal plane. New techniques were developed to allow testing of the focal plane without over testing the instrument, and modifications were made to the focal plane assembly to successfully attenuate the amplification.

For the past several years, cryogenically cooled sensors have become a popular method of observation and study for both space-based and ground-based operations. Accordingly, various cooling techniques have been developed to accommodate this group of sensors. Because of rising performance standards and escalating cost limitations, cryocoolers have become an impressive cooling technique to consider. This report focuses on the use of a mechanical cryocooler in conjunction with the Russian American Observational Satellites (RAMOS). RAMOS consists of two co- orbital satellites which will map using infrared radiometers. The telescope focal plane assembly will be cooled using a multiple cryocooler configuration to approximately 60 K. The use of multiple coolers introduces redundancy into the cooling system. The cooling system will also incorporate various other new technologies, such as thermal disconnects, a thermal storage unit, and low- resistance flexible thermal links to meet the overall system requirements. Incorporating thermal switches and thermal storage units into a cooling system design can alleviate the concerns of cryocooler vibration and parasitic heat loads from the redundant cooler. An understanding of these concepts and configurations will assist in the design of similar optical instruments for both space-based and ground- based exploration campaigns.

Utah State University/Space Dynamics Laboratory, teaming with NASA Langley Research Center, is currently building the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument. Stringent mass and power constraints, together with a greater than two year mission life, led to the selection of a TRW miniature pulse tube refrigerator to cool SABER's infrared detectors to the required temperature of 75 K. This paper provides an overview of the SABER thermal management plan and the challenges encountered in matching the refrigerator characteristics with instrument performance requirements under the broadly variant space environments expected for this mission. Innovative technologies were developed to keep heat loads within the limited cooling capacity of the miniature refrigerator, as well as mechanically isolating but thermally connecting the refrigerator cold block to the focal plane assembly (FPA). A passive radiator will maintain the SABER telescope at an average temperature of 230 K while a separate radiator will reject heat from the refrigerator and electronics at approximately 260 K. Significant breadboard tests of various components of the SABER instrument have taken place and the details of one of these will be discussed. The test included attaching a miniature mechanical refrigerator, borrowed from the Air Force, to the SABER FPA. This opportunity gave the SABER team a significant head start in learning about integrating and testing issues related with the TRW miniature pulse tube refrigerator. SABER is scheduled to be launched in January 2000 as the primary instrument of NASA's TIMED (Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics) spacecraft. The TIMED program is being managed by the Applied Physics Laboratory at Johns Hopkins University.

The focal plane assembly of the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument is supported using Fiber Support Technology (FiST) which utilizes high performance fibers in tension to mechanically support and thermally isolate a cooled component from a warm environment. Details of this approach were presented in detail at SPIE meeting in Denver in 1996. The SABER team deemed it necessary to perform optical stability testing on this never-before-flown technology for supporting focal plane assemblies to determine if precise positioning could be maintained through vibration and thermal cycling. After subjecting the support system to vibration and thermal cycling, the angular orientation between the warm outer support structure and the inner cold block was measured. Since the outer support structure serves as the reference location for positioning the focal plane assembly and the cold block is where the detectors reside, it was possible to determine if FiST meets the optical stability requirements for the SABER instrument. The results from this testing are presented, discussed, and compared to the optical requirements of the SABER instrument. A brief summary of current thermal and mechanical enhancements to the system will also be discussed.