The Space Infrared Telescope Facility (SIRTF) Project will perform infrared astronomical studies including imaging, photometry, and spectroscopy over the wavelength range of 1.8 μm to 700 μm. A complement of three Science Instruments designed to meet those objectives has been selected for an Instrument Definition Phase. The Instruments are: A. A Multiband Imaging Photometer (MIPS), to be developed by the University of Arizona; Principal Investi-gator - Dr. George Rieke. The MIPS is baselined to image individual sources at the background limit and dif-fraction limit between 3 and 7 gm, to provide super resolution between 10 and 30 μm, to provide imaging, super resolution, and surveying between 30 and 200 gm, and to provide point source photometry, radiometry, and surveying at wavelengths longer than 200 μm. The MIPS will also provide polarimetry in selected bands. The MIPS will cover the wavelength interval from 3 to 700 μm. B. An Infrared Array Camera (IRAC), to be developed by the Smithsonian Astrophysical Observatory; Principal Investigator - Dr. Giovanni G. Fazio. The IRAC is baselined for two-dimensional photometry, imaging and imaging polarimetry in either a wide field mode (5 arcmin square field of view) over the wavelength interval from 1.8 to 30 μm or a diffraction-limited (1.25 arcmin square field of view) mode, over the wavelength interval from 2.5 to 30 μm (with image quality at 1.8 μm no worse than at 2.5 μm) in a 1.25 X 1.25 arcmin field of view.

A new design for the entrance shade for the Space Infrared Telescope Facility (SIRTF) is presented. The evolution of the entrance shade began with a simple frustum, symmetrical about the telescope axis, when SIRTF was expected to be shuttle-attached. With the change to a free-flying SIRTF this frustum was cut off at an angle. The telescope will be operated so that whenever not in the earth's shadow the high side is kept toward the sun. However, the entrance shade interior itself will be so warm that the optics, including the secondary mirror and its mechanisms and support structure, will be restricted to the rear part of the barrel, termed the aftbaffle, which is shaded from the interior of the entrance shade by the forebaffle. This is best accomplished by the most recent design in which the axis of the entrance shade is offset from the telescope axis. This results in a shorter entrance shade, shorter forebaffle, and a shaded region within the barrel which is symmetrical about the telescope axis. All of these are advantageous. The new design procedure can also be applied to entrance shades for other space telescopes.

The Space Infrared Telescope Facility (SIRTF) comprises a telescope system and a spacecraft. Both of these structures will experience temperature distribution changes due to variations in the sun/earth thermal radiation exposure while on-orbit. The more significant temperature changes (e.g., greater than 5K) and structural distortions occur principally in the outer shell of the telescope system. The telescope's internal optical and instrument components are structurally linked to the outer shell, and will experience some misalignment (decenter, defocus, and tilting) even though the telescope system structure is designed to minimize this effect. Misalignment magnitudes have been evaluated using a finite element structural model of the telescope system. The predicted misalignment results are presented for a telescope optical pointing sensor that may be either internally mounted to the instrument or externally mounted to the outer shell. It is shown that the primary and secondary mirror misalignments due to the thermally induced load of the chosen orbital scenario do not vary significantly. This paper is the first progress report on the topic for SIRTF. Additional orbital scenarios and other environmental assumptions are to be investigated later.

We describe our concept for a liquid helium temperature prototype secondary mirror assembly (PSMA) for the Space Infrared Telescope Facility. SIRTF, a NASA "Great Observatory" to be launched in the 1990's, is a superfluid heliumcooled 1-meter class telescope with much more stringent performance requirements than its precursor the Infrared Astronomical Satellite (IRAS). The SIRTF secondary mirror assembly must operate near 4 K and provide the functions of 2-axis dynamic tilting ("chopping") in addition to the conventional functions of focus and centering. The PSMA must be able to withstand random vibration testing and provide all of the functions needed by the SIRTF observatory. Our PSMA concept employs a fused quartz mirror kinematically attached at its center to an aluminum cruciform. The mirror/cruciform assembly is driven in tilt about its combined center of mass using a unique flexure pivot and a four-actuator control system with feed-back provided by pairs of eddy current position sensors. The actuators are mounted on a second flexure-pivoted mass providing angular momentum compensation and isolating the telescope from vibration-induced disturbances. The mirror/cruciform and the reaction mass are attached to opposite sides of an aluminum mounting plate whose AL/L characteristics are nominally identical to that of the aluminum flexure pivot material. The mounting plate is connected to the outer housing by a focus and centering mechanism based upon the six degree of freedom secondary mirror assembly developed for the Hubble Space Telescope.

The Superfluid Helium On-Orbit Transfer (SHOOT) Flight Experiment is designed to demonstrate the components and techniques necessary to resupply superfluid helium to satellites or space station based facilities. A top level description as well as the development status of the critical components to be used in SHOOT are discussed. Some of these components include the thermomechanical pump, the fluid acquisition system, the normal helium and superfluid helium phase separators, venturi flow meter, cryogenic valves, burst disks, and astronaut compatible extra vehicular activity (EVA) coupler and transfer line. The requirements for the control electronics and software are given. A preliminary description of the requirements that must be met by a satellite requiring superfluid helium servicing are given. In particular, minimum and optimum plumbing arrangements are shown, transfer line flow impedance and heat input impacts are assessed, instrumentation described, and performance parameters considered.

The Superfluid Helium On-Orbit Transfer Flight Experiment (SHOOT) is designed to demonstrate the techniques and components required for orbital superfluid (He II) replenishment of observatories and satellites. One of the tasks planned in the experiment is to cool a warm cryogen tank and a warm transfer line to liquid helium temperature. A math model, based on single-phase vapor flow heat transfer, has been developed to predict the cooldown time, component temperature histories, and helium consumption rate, for various initial conditions of the components and for thermomechanical pump heater powers of 2 W and 0.5 W. This paper discusses the model and the analytical results, which can be used for planning the experiment operations and determining the pump heater power required for the cooldown operation.

A summary of the cryogenic testing of glass and metal mirrors performed at NASA Ames Research Center (ARC) and two other places is presented. Recent improvements to the ARC Cryogenic Optics Test Facility are described. The purposes of the tests were to determine: (1) how glass mirrors would perform at cryogenic temperatures compared with metal mirrors and (2) how various mirror mounts would affect the cryogenic performance of mirrors. Details of a cryogenic test of a 50 cm "double arch," fused-silica mirror with a three-point mount and with a radially-compliant, flexured mount are given. Within the accuracy of the measurements, it was determined that the flexured mount did not induce appreciable distortion in the double arch mirror. Results of the cryogenic tests of a number of glass mirrors and two beryllium mirrors are included. The cryogenic distortion of the glass mirrors was found to be less than that for the beryllium mirrors. Within the accuracy of the measurements, no hysteresis was found in the glass mirrors. It was possible to measure hysteresis in one of the beryllium mirrors.

Five interferometric tests were conducted at cryogenic temperatures on a lightweight, 50 cm diameter, hot isostatic pressed (HIP) beryllium mirror in the Ames Research Center (ARC) Cryogenic Optics Test Facility. The purpose of the tests was to determine the stability of the mirror's figure when cooled to cryogenic temperatures. Test temperatures ranged from room ambient to 8 K. One cycle to 8 K and five cycles to 80 K were performed. Optical and thermal test methods are described. Data is presented to show the amount of cryogenic distortion and hysteresis present in the mirror when measured with an earlier, Shack interferometer, and with a newly-acquired, phase-measuring interferometer.

A titanium flexure assembly for the Space Infrared Telescope Facility (SIRTF) 1-m primary mirror has been designed to accommodate (1) the cryogenic cool-down effect on the optical performance of the mirror, (2) the shuttle launch-load envi:onment, and (3) the support-baseplate manufacturing tolerances. Numerous iterations involving a multi-dimensional design space search led to an assembly design that provides the stiffness and strength in the vertical (optical axis) and tangential directions to accommodate launch loads, but is compliant radially to accommodate cryogenic cool down. A "folded back" titanium flexure system was required because of the differential thermal contraction of the aluminum telescope baseplate support and the fused-silica mirror. This unique and innovative flexure assembly represents a totally passive mechanism for accommodating the design launch loads, cryogenic cool down, and out-of-plane baseplate effects.

The dynamic analyses and design criteria that lead to the design of a titanium flexure mount of the primary mirror of the Space Infrared Telescope Facility (SIRTF) are presented. This flexure system passively accommodates the differential thermal contraction between the glass mirror and the aluminum structure of the telescope during cryogenic cooldown and supports the one-meter-diameter, 116-kg (258-1b) primary mirror during a severe space shuttle launch environment. Design criteria and procedures used to establish the required strength and radial compliance using computer programs NASTR AN and FRINGE are discussed. Several methods of combining modal responses resulting from a Displacement Response Spectrum analysis are given and the combination methods of Modified Root of Sum Squares (MRSS), Square Root Sum of the Squares (SRSS), and the Absolute Sum (ABS), are compared to the results of a Modal Frequency Response analysis.

A concept is presented for a national cryogenic optics test facility capable of optical characterization of 1.25-m-diameter optics having focal lengths up to 6.2 m at temperatures from 300 K to near 4 K. The facility will be comprised of a large Dewar with a phase-shift interferometer, a two-stage vacuum system employing a turbomolecular pump, and an integral vibration isolation system. The entire facility will be housed in a concrete site with a massive floor to assist in reducing vibration during optical tests. By providing interchangeable sections, the overall height of the Dewar can be adjusted to provide for testing of shorter focal length optics. This paper discusses the background for the facility, the facility location, and the requirements and the performance considerations which drive the Dewar design with respect to the vibration isolation system, vacuum system, and optical interferometry.

The X-Ray Spectrometer (XRS) instrument on the Advanced X-Ray Astrophysics Facility (AXAF) will use X-ray detectors that operate at 0.1 K. The detectors will be maintained at 0.1 K by an Adiabatic Demagnetization Refrigerator (ADR) that operates inside a liquid helium dewar. The ADR rejects approximately 2 mW of heat to the stored liquid helium. With this low instrument heat load, the liquid helium dewar will have a long lifetime if the parasitic heat load on the helium from the surrounding, warm facility is minimized. Spaceborne helium dewars typically use up to 3 vapor cooled shields to intercept the parasitic heat load. The XRS will add mechanical coolers to provide additional cooling to the outer vapor cooled shield. The cryogenic system consists of an ADR, a liquid helium dewar, mechanical coolers, and a thermal strap to connect the coolers to the dewar. The lifetime of the stored cryogen is calculated to be up to 5 years. This cryogenic system is described, with particular attention given to the dewar, mechanical cooler, and ADR design, testing, and trade studies. A breadboard ADR is presently being fabricated and tested. The status of the construction and testing of this breadboard will be described.

NASA's Cosmic Background Explorer (COBE) is designed to investigate the Cosmic Background Radiation (CBR), that permeates the universe as a consequence of the Big Bang. This 3 degree Kelvin radiation is a fossil that contains much information about the early universe. The Far Infrared Absolute Spectrophotometer (FIRAS), will investigate the spectral isotropy of this ancient remnant and look for clues as to the subsequent evolution of the universe. The instrument is a cryogenically cooled, modified Michelson interferometer which operates in the 1 cm to 100 micron wavelength range. FIRAS is designed to provide absolute spectral information, therefore, all possible perturbations to the instrument response must be investigated to minimize distortions of the data. This paper discusses the methodology and resultant variations in the instrument performance noted during room temperature, and liquid nitrogen, (LN2) temperature vibration qualification. Reference alignment shifts in critical components such as the instrument wire-grid beamsplitter are correlated to changes in the instrument spectral response.

Optical alignment methods have been developed for thermal, vibration and assembly testing of the Diffuse Infrared Background Experiment ( DIRBE ) telescope. Much of the data was aquired Using videotape of CCTV images. Vibration and performance testing of the 32 hz tuning-fork chopper was carried out using strobe videography.

Characterization techniques used to verify the quality and spectral performance of the large free standing wire grid polarizing beamsplitters and input/output polarizers used in the Far Infrared Absolute Spectrophotometer (FIRAS) are presented. The clear aperture of these grids is lined with 20.8 micron diameter gold coated tungsten wire, spaced 33 microns apart. The grid characteristics measured throughout fabrication and space flight qualification are the center to center wire spacing and wire plane flatness. Ideally, the wire grids should produce coherent wavefronts with equal reflectance and transmittance properties. When the spacing is inconsistent, these wavefront intensities are unequal, thus decreasing the efficiency of the grids and reducing the output signal of the FIRAS. The magnitude of the output interferogram is also reduced by incoherence in the interfering wave fronts caused by uneven flatness.

This paper describes the system concept for the Stratospheric Observatory for Infrared Astronomy (SOFIA), as developed by in-house (Ames Research Center) Phase A level studies of the Telescope System and Ground Support/Operations System, and by contracted studies of the Aircraft System performed by the Boeing Military Airplane Company. The SOFIA facility will be a 3-meter class optical/infrared/submillimeter telescope mounted in an open cavity in the forebody of a Boeing 747 aircraft, to be operational in 1992. It represents the next generation of Ames' existing airborne IR facilities, including the Kuiper Airborne Observatory (KAO), which is a 0.91 meter telescope flown on a Lockheed C-141 aircraft. The SOFIA telescope will be about 10 times more sensitive than the KAO, will have 3 times better angular resolution, and will be able to detect all of the far-infrared point sources discovered by the IRAS (Infrared Astronomical Satellite) survey in 1983. We first present an overview of the SOFIA Phase A Telescope System concept, including its major requirements and design attributes. The Telescope System consists of the Telescope Assembly (optical train and support structures) and the Consoles and Electronics Subsystem, which provides the system's command, control, displays and communications. The major requirements and concept for the Aircraft System are next described, including the cavity modification and its supporting subsystems such as the cavity doors and shear layer control devices. Finally, a brief description of the Ground Support/Operations System is provided, including the ground-based facilities and equipment needed to support the airborne observatory, in addition to an overview of the operational scenarios and organization.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a proposed 3-meter class telescope in a Boeing 747 aircraft, anticipated as a joint development by NASA and West German Science Ministry. SOFIA would have the capability to make astronomical observations over a wavelength range from 0.3 microns to 1.6mm. The concept is based on the design and 13 years of experience in the Kuiper Airborne Observatory (KAO), a lockheed C-141 jet transport with a 0.9-meter diameter telescope, which SOFIA would replace. Relative to the KAO the larger telescope on SOFIA would provide a factor of 10 improvement in sensitivity for compact sources and a factor of 3 improvement in (diffraction limited) angular resolution at wavelengths beyond 30 microns. In addition, SOFIA will retain the major features of the KAO which have made the airborne astronomy program so successful. Among these are: continuous in-flight access to focal plane instruments while flying at or above 41,000ft altitude; pointing stability of 0.2 arc seconds; mobility and scheduling flexibility to accommodate targets of opportunity such as comets, eclipses, occultations, and novae. In this paper we present the scientific background, the scientific potential, a comparison with other astronomy missions, and the overall justification for an expanded airborne observatory.

A preliminary first-order optical design for the Stratospheric Observatory for Infrared Astronomy (SOFIA) is presented. This is a Cassegrain design with a 3 meter diameter, approximately f/1 primary mirror. Phenomena limiting the image quality of the telescope are divided into "seeing", optics, and guidance. An error budget is presented for these categories and specific effects contributing to each. The seeing effects from the shear layer between the telescope cavity and the external air are expected to be dominant. Results are presented on the necessary thermal, optical, structural and guidance requirements to maintain contributions of these phenomena below that of the shear-layer seeing.

Studies have been conducted at the Ames Research Center on the feasibility of concepts for Stratospheric Observatory for Infrared Astronomy (SOFIA). This paper summarizes studies of the feasibility of the SOFIA telescope primary mirror and its mounting. The primary mirror is required to be very lightweight (areal density approximately 100 kg/m2), have an f/ratio near 1.0, and have surface quality that permits imaging in the visible as well as the infrared. Data on large mirror technology is presented to represent the space of areal density and size defined by current and projected technology. The desired subspace for the SOFIA primary mirror is identified. Also described in the paper are the results of the design study conducted to assess the feasibility of designing a suitable mounting system for the primary mirror. The requirements for the mount design are given both in terms of the environmental conditions and the expected optical performance. The mirror and mounting was modeled using Programs PATRAN and NASTRAN. The mirror studied was a sandwich-type made of Ultra Low Expansion (ULE) silica with square cells in the core, and with a continuous edge band. The mirror is modeled using equivalent solid elements for the core. Several mount designs were evaluated, having different numbers of axial supports. The design study produced primary mirror surface deflections in 1 g as a function of mirror elevation angles. The surface was analyzed using an optical analysis program, FRINGE, to give a prediction of the mirror optical performance. Results from this analysis are included.

The SOFIA telescope concept was discussed with respect to the ambitious NASA specification. Comparisons were drawn between SOFIA and other Zeiss optics like 3.5 m Telescope MPIA and ESO NTT. Recently developed technologies in producing ZERODUR blanks were presented. A distribution of tolerances for the optical system SOFIA was made. The axial mirror support in the telescope was optimized and the lateral support was calculated accordingly. Results for a radial backface support with correcting moment were given. For the SOFIA primary the deformation modes with lowest plate stiffness were calculated and load tolerances were derived. Fast flexible tools for figuring a F/1.0 asphere were discussed - developed in the ROFT program - that could be adapted to the SOFIA thin meniscus. Also a fast interferometric metrology was presented.

The behaviour of a thin meniscus in the thermal environment of SOFIA was calculated for different boundary conditions. Temperature fluctuations and time constants are given for significant nodes of the thermal Finite Element (FE) model. The worst case temperature distribution within the mirror was used, to calculate the mirror's deformation and its optical wavefront aberration. The figuring process of an equivalent thin meniscus, the ESO- Very Large Telescope (VLT) mirror, was calculated and found to be very sensitive against radial thermal gradients, caused by the meniscus shape of the mirror.

To increase production efficiency in the manufacture of infrared focal plane components, test techniques were refined to enhance testing throughput and accuracy. The result is an integrated package of high performance hardware and software tools which performs well in high throughput production environments. The test system is also very versatile. It has been used for readout (multiplexer) device characterization, room temperature automated wafer probing, and focal plane array (FPA) testing. Tests have been performed using electrical and radiometric optical stimulus. An integrated, convenient software package was developed and is used to acquire, reduce, analyze, display, and archive test data. The test software supports fully automated operation for the production environment, as well as menu-driven operation for R&D, characterization and setup purposes. Trade-offs between handling techniques in cryogenic production testing were investigated. " atch processing" is preferred over "continuous flow", primarily due to considerations of contamination of the cryogenic environment.

For many years the cryogenic hardware utilized in the development and quality control testing of infrared focal plane arrays and modules has been custom designed for each application. Conversion of a system to test devices in different packages usually required expensive and time consuming rework of the system or construction of new hardware. This paper describes a system with modular construction which can be quickly reconfigured to satisfy a wide variety of test requirements. Other problems, such as the large number of vacuum chamber penetrations, and the associated potential for leaks, rapid cool-down/warmup, and the need for cooled optics have been successfully addressed. Variations on the basic design allow for signal lead counts as high as 250 50 ohm (impedance) signal lines in a compact unit without sacrificing the low noise and crosstalk characteristics of systems with discrete feedthroughs.

Many advanced electro-optical systems require cryogenic temperatures for optimum performance of their sensors. This cooling requirement makes the cryogenic refrigerator a critical component of the sophisticated system. Too frequently an expensive system is completely designed without adequate provision for the power, space and performance requirements of the refrigerator. There are more than 20 trade-offs an experienced cryogenic cooler design team can make for the system designers if the interface is discussed at an early stage. The important cooler design and interface parameters together with the associated weighting factors will be presented on a pragmatic basis.

Thermoelectric coolers (TECs) are solid-state devices with no moving parts, and therefore are inherently very reliable. The inherent reliability of the cooler, however, is determined by the electrical arrangement of the p- and n- type thermoelements (elements). Typically, TECs are manufactured with the elements in series electrically, resulting in the highest voltage and lowest current requirements for a given number of elements, but also the lowest possible inherent reliability. For maximum TEC reliability, all of the elements should be connected electrically in parallel, but this results in the need for an expensive high current power supply. By using a redundant element design, the inherent reliability of the TEC can be dramatically improved with very minimal impact on TEC cost and performance, and more acceptable increases in current requirements. A brief description of the exponential reliability model is presented followed by an analysis of various electrical arrangements of the TEC elements in order to optimize the TEC design as it relates to 1) reliability, 2) electrical requirements, and 3) impact on system design and cost. Comparisons are made between the configurations and the results are tabulated and graphed.

The status of a program which develops and characterizes integrated infrared (IR) detector array technology for space astronomical applications is described. The devices under development include intrinsic, extrinsic silicon, and extrinsic germanium detectors, coupled to silicon readout electronics. Low-background laboratory test results include measurements of responsivity, noise, dark current, temporal response, and the effects of gamma-radiation. In addition, successful astronomical imagery has been obtained on some arrays from this program. These two aspects of the development combine to demonstrate the strong potential for integrated array technology for IR space astronomy.

Aspects of the design and performance of an InSb Focal Plane/Dewar/Electronics System developed by Cincinnati Electronics for Grumman Aerospace are discussed. The system is housed within a liquid helium dewar. The InSb array and feedback resistors operate at liquid helium temperatures while part of the electronics operate at liquid nitrogen temperatures.

A unique focal plane array (FPA) assembly combining both electronic and optical components in a single hermetically sealed hybrid package has been designed by Cincinnati Electronics Corporation to meet the performance requirements imposed on the focal plane assembly in the Visual and Infrared Mapping Spectrometer (VIMS) for the Mars Observer (MO) mission. Inside the FPA package is a configuration of three multiplexed linear arrays containing 320 detector elements, a combination of silicon and indium antimonide (InSb), allowing continuous spectral coverage from 0.35 to 5.14 microns. An optical subassembly consisting of two spectral order-sorting filters with intrinsic field-of-view apertures requiring critical optical alignment is also internal to the hybrid. Several engineering issues arose during the MO/VIMS FPA development phase which had challenging design ramifications. FPA performance requirements, design approach, and critical issues are discussed.

For many applications, for example, in ground-based astronomy, detector systems using small arrays or small numbers of discrete IR. detectors are very useful. In particular, systems using indium antimonide (InSb) detectors are approaching background-limited performance throughout most of their useful wavelength band of 1 to 5 μm. The general requirements of preamplifiers for background-limited performance of InSb IR detector systems are presented. These amplifiers require a cryogenic Field-effect transistor (FET) first stage to achieve this level of performance. The electrical and thermal design tradeoffs of such amplifiers are presented. The noise performance of transimpedance amplifiers (TIAs) is estimated from a noise model. Measurements of noise voltage and noise current versus temperature are presented for a sample of commercially available junction FETs (JFETs). It is concluded that if JFETs with reasonably low values of noise voltage are selected, then the IR TIA system noise will be dominated by an input noise current from the JFET, typically 5 x 1047 A/.117z at 10-Hz frequency. Most significantly, this source appears to be correlated over temperature with the input noise voltage. This suggests that the voltage and current noise sources are caused by the same mechanism. A JFET selection procedure is proposed that includes a minimum of low-temperature testing.

A new infrared array camera system has been successfully applied to high background 5 - 14 μm astronomical imaging photometry observations, using a hybrid 58 x 62 pixel Si:Ga array detector manufactured by Hughes/Santa Barbara Research Center. The off-axis reflective optical design incorporating a parabolic camera mirror, circular variable filter wheel and cold aperture stop produces diffraction-limited images with negligible spatial distortion and minimum thermal background loading. The camera electronic system architecture is divided into three sub-systems: 1) high speed analog front end, including 2-channel preamp module, array address timing generator, bias power suppies, 2) two 16 bit, 3 microsec per conversion A/D converters interfaced to an arithmetic array processor, and 3) a LSI 11/73 camera control and data analysis computer. The background-limited observational noise performance of the camera at the NASA/IRTF telescope is NEFD (1a) = 0.05 jy pixe -1 min -1/2.

The Diffuse Infrared Background Experiment (DIRBE) is one of three experiments to be carried aboard the Cosmic Background Explorer (COBE) satellite scheduled to be launched by NASA on a Delta rocket in 1989. The DIRBE is a cryogenic absolute photometer operating in a liquid helium dewar at 1.5K. Photometric stability is a principal requirement for achieving the scientific objectives of this experiment. The Infrared Astronomy Satellite (IRAS), launched in 1983, which used detectors similar to those in DIRBE, revealed substantial changes in detector responsivity following exposure to ionizing radiation encountered on passage through the South Atlantic Anomaly (SAA). Since the COBE will use the same 900 Km sun-synchronous orbit as IRAS, ionizing radiation-induced performance changes in the detectors were a major concern. We report here on ionizing radiation tests carried out on all the DIRBE photodetectors. Responsivity changes following exposure to gamma rays, protons, and alpha particle are discussed. The detector performance was monitored following a simulated entire mission life dose. In addition, the response of the detectors to individual particle interactions was measured. The InSb photovoltaic detectors and the Blocked Impurity Band (BIB) detectors revealed no significant change in responsivity following radiation exposure. The Ge:Ga detectors show large effects which were greatly reduced by proper thermal annealing.

The Diffuse Infrared Background Experiment (DIRBE) is designed to measure the absolute brightness of the entire sky in the 1-300 μm spectral range. It is one of three instruments on NASA's Cosmic Background Explorer (COBE) satellite, to be launched in 1989 on a Delta rocket. The DIRBE will map the entire sky in 10 photometric bands with 1% relative accuracy at a spatial resolution of 0.6 °. Due to the wide dynamic range of signals expected in many bands, this requirement imposes stringent constraints on system linearity, which is particularly difficult to achieve in those spectral channels using photovoltaic or photoconductive detectors. For these detector types, we employ transimpedance amplifiers (TIA) requiring the use of very high impedance feedback elements for low noise operation. Typically, high value resistors used at cryogenic temperatures have quite nonlinear current/voltage relations. We have evaluated several resistor types and found that Victoreen MOX-400 resistors offer good linearity over the relevant signal range, and if procured without epoxy encapsulation, consistently survive repeated cooling to cryogenic temperatures. Approximately 50 flight and flight spare Victoreen MOX-400 resistors were tested for linearity and temperature coefficient of impedance over the 1.5-4.2K temperature range. The devices were found to have small voltage coefficients of impedance, and good uniformity from sample to sample. In this paper, we present a description of the measurement techniques used and show the measured electrical characteristics of the resistors.

The Far Infrared Absolute Spectrometer (FIRAS) and Diffuse Infrared Background Experiment (DIRBE) instruments on the Cosmic Background Explorer (COBE)1 mission require bolometers with a relatively short time constant, high throughput, and low NEP. At the same time these detectors must be flight worthy and have very small microphonic response. None of the bolometers available at the beginning of the program, either monolithic or composite, met all of the above requirements. Therefore, the Goddard Space Flight Center developed composite bolometers in-house that meet the stringent requirements of COBE. Small chips of doped and compensated silicon are used as sensing elements. The absorbing substrate consists of diamond wafers coated with thin layers of chromium and gold. Maximum IR absorptance is accomplished by controlling the total surface resistance of these layers. The throughput for the FIRAS bolometers is 1.28 sr-cm², while for DIRBE it is 0.21 sr-cm². At 1.6 K, the time constants range from 3 to 40 ms and their NEP range from 4x10-15 to 1x10-14 W/Hz1/2. The noise spectra of these bolometers are flat above 1 to 2 Hz.

The Cryogenic Limb Array Etalon Spectrometer (CLAES) is one of a complement of instruments on the NASA Upper Atmosphere Research Satellite (UARS) which will study atomospheric photochemistry, energy input, and dynamics following a 1991 launch. CLAES will measure stratospheric altitude profiles of temperature, pressure, 03, H₂O, CH4, N₂0, NO, NO₂, N₂05, HNO3, C1ONO2, HC1, CFC-11, and CFC-12. These data will be obtained typically between 10 and 60 km, with 2.5-km vertical resolution and 500-km horizontal grid size. Coverage will be obtained between latitudes 80° north and south, thereby providing substantial coverage of the Antarctic spring polar ozone-hole region. The experiment will have a minimum useful on-orbit operating lifetime of 15 months, as dictated by the lifetime of the stored solid-neon/solid-CO₂ cryogen. CLAES derives the listed geophysical parameters from measurement of earth-limb spectral emissions between 3.5 and 13 μm. Brief discussions of the measurement concept, instrument design, and performance are presented, followed by a more detailed discussion of scientific capabilities and measurement modes.

The purpose of the Cryogenic Limb Array Etalon Spectrometer is to measure the global concentrations of stratospheric species, and temperature, as a function of altitude. Of particular interest are ozone and ozone-destructive species. CLAES will detect N20, NO, NO2, N205 and HNO3 in the nitrogen family, and CFC1₃ (FC-11), CF2C12 (FC-12), HC1, and C1ONO₂ in the chlorine family, in addition to 03, H₂0, CH, and CO₂. This measurement set includes some of the more important source, ozone-destructive, and reservoir species in the ozone layer chemical system. CLAES is one of nine instruments on NASA's Upper Atmosphere Research Satellite (LIARS), scheduled for a space shuttle launch in 1991. UARS will monitor upper atmospheric chemistry, dynamics, and energy input; and its coverage will include the antarctic spring ozone depletion region. The limb-viewing instruments, combined with the 57-degree inclination, 600 km circular orbit, will allow UARS to take measurements to 80 degrees latitude, covering H,ore than 98% of the earth's surface. CLAES will function autonomously in its science mode, collecting IR measurement data. The measurement data will be passed to the UARS observatory computers for packaging into transmittable blocks with the data from the other eight instruments. Pointing, platform stability, propulsion, telemetry, and other such spacecraft related functions are provided by UARS. Figure 1 is a configurational diagram of the CLAES instrument.

The Cryogenic Limb Array Etalon Spectrometer (CLAES) will fly on the NASA Upper Atmosphere Research Satellite (UARS) in 1991. CLAES uses solid-state focal plane arrays to detect emission from the earth's atmosphere over the infrared wavelength range 3.5 to 13 μm. This paper discusses the design of the focal plane detector assembly and compares calculated performance with measurements. Measurements were made of focal plane noise and responsivity as functions of frequency (2 to 500 Hz) and temperature (12 to 19 K), pixel-to-pixel and across-array crosstalk, and linearity over a dynamic range of 105. The measurements demonstrate that the arrays satisfy the science requirements, and that, in general, there is reasonable agreement between the measurements and the analytical model.

The Cryogenic Limb Array Etalon Spectrometer (CLAES) to be flown on the Upper Atmosphere Research Satellite (UARS) is a limb viewing radiometer operating in nine wavelength channels between 3.5 and 13 μm. Each wavelength channel is selected by a combination of a solid Fabry-Perot etalon and suitable blocking filter. Spectral scanning within a given channel is achieved by tilting the etalon. The spectral transmission for a perfect etalon in a collimated beam is given by the Airy function. Deviations from perfect parallelism, absorption in the substrate and coatings, and the instrumental field of view produce changes in the spectral transmission. These changes affect the peak transmission (T), the full width at half maximum intensity (FWHM), and the overall shape of the transmission peaks. High-precision retrieval of geophysical parameters requires that the slit function of the instrument be accurately characterized. We have developed models to accurately characterize the slit function. These models involve a convolution of the Airy function for a given thickness with the distribution of surface thicknesses; the effect of absorption in the substrate, which must be taken into account for the CLAES etalon operating in the 10- to 13-1.tm region; and the field of view broadening as a function of etalon tilt angle. We compare our model calculations with experimental transmission data for CLAES etalons centered at 3.52, 5.72, 8.0, and 11.86 μm. The model is also useful in setting tolerance limits on surface quality and permissible substrate absorption for other potential etalon applications.

The S/N001 Cryostat2 tested consists of the originally designed solid hydrogen configuration coupled with an instrument thermal mass simulator (ITMS) for simulating the thermal behavior of the instrument package. The test program was designed to verify the operational and thermal performance of the Cryostat/ITMS system as monitored and serviced by the Data Acquiisition System (DAS) and Ground Support Equipment (GSE). After the DAS and GSE passed functional performance tests, the Cryostat/ITMS was slowly pumped from ambient to 10 torr to minimize any possible damage to the structures and Multilayer Insulation (MLI). The system was then pumped to a hard vacuum and tested for vacuum integrity of all 0-ring seals. During this initial pumpdown phase of the test, it was found that better communication was needed between the various compartments of the vacuum space for efficient evacuation of the cryostat. These improvements have been implemented on this unit as well as on the subsequent Ne/CO₂ Cryostat. Liquid Nitrogen flowing through the coolant lines was used to precool the Cryostat to slightly above 77 degress Kelvin, at which point the tank was filled with liquid Helium. The test data includes the rate of temperature decrease of the various parts of the Cryostat/ITMS and the final equilibrium temperatures of the shields, thermal links and tank. These temperatures as well as the boiloff rate of the liquid Helium were compared with the expected values as predicted by the thermal models developed in house. The liquid Helium was allowed to boil off and liquid Neon was introduced into the Cryostat. The Neon was solidified by first circulating liquid Helium through the coolant coils and maintained by pumping on the cooled Neon through the fill line. End of life temperature profiles across the tank were obtained and compared with predicted values. Additional temperature and heat rate values were obtained and compared with predictions. System warmup procedures were verified without problems. The overall test resulted in significant design improvements for the Ne/CO₂ Cryostat.

The Cryogenic Limb Array Etalon Spectrometer (CLAES)1 is one of nine instruments that will fly aboard the Upper Atmospheric Research Satellite (UARS)2 in the fall of 1991. CLAES is an earth-limb viewing instrument that requires cryogenic cooling of its focal plane (<15.0K), spectrometer (<30K), telescope (<150K), and baffles (<180K) in order to achieve the required performance sensitivity. Initially, the CLAES baseline design incorporated a single-stage solid hydrogen cryostat to perform the necessary cooling, however, after the Challenger shuttle disaster, the UARS and CLAES Project Offices investigated the feasibility of incorporating a completely inert cryogen system for CLAES. The result of this study showed that a dual stage Ne/CO₂ cryostat would meet all sensor cooling requirements, provided that a significant increase in weight could be accommodated. In December '86, the Ne/CO2 design was adopted as the new cryostat baseline for CLAES. The focal plane and spectrometer are conductively cooled to the solid neon (13.2 K) while the telescope and internal baffles are cooled by the CO₂ (121.8K). This paper describes the design and performance of the Ne/CO2 cryostat.

This paper documents the engineering design, fabrication, assembly, and test activities at SSG, Inc. that produced the CLAES Telescope Assembly. It includes a brief outline of the optical design as given by Lockheed Palo Alto Research Laboratory. Several major design and assembly areas are reviewed to highlight the driving design and performance constraints of the telescope. These include the dip-brazing process utilized on the structural sub-assemblies, and the fabrication process for the three flight mirrors. The telescope system alignment techniques and processes are reviewed, and include an outline of the verification test plan which covers the optical, structural, and cryogenic test procedures for the telescope. Test data is given and compared to the performance specifications. The last topic is a brief discussion of the lessons-learned from this telescope, and the follow-on diagnostic tests that are currently in process at SSG, Inc.

A cooled, infrared spectrometer is required to operate in earth orbit. We discuss the spectrometer performance requirements to show how package volume constraints and the stressing enviornment of cryogenic operation affect the detailed design of spectrometer components such as relay imaging optics and fold mirrors. Design steps are described that were taken to insure that the spectrometer would perform well in orbit, with components that are easily manufactured, readily aligned and stable in operation. This design procedure has produced two components of special interest, fast doublets suitable for use at 20 °K over the wavelength band from 3.5 to 12 μm, and fold mirrors that can be adjusted to arc second tolerances.

The CLAES Telescope and Spectrometer were aligned as separate units. The optical interface between the two units is at the intermediate Lyot stop, where close angular and centering tolerances are required, with control by the use of matched machined tooling. In the alignme-L. of the Spectrometer, all optical components were centered to the chief ray using centering targets to align the optical components. The initial assembly was made at room temperature, and tested at 20K. One key reason for this testing is that the refractive indices for ZnS and ZnSe are not known below 90K, and therefore the exact location of the image plane is not known. The tests at 20K established the location of the image plane. A beam of collimated carbon-dioxide laser power illuminates the cryogenically cooled Spectrometer or the CLAES Instrument along the optical axis. The collimation of the beam is adjustable in small increments; the beam is scanned over the edges of the individual detectors creating edge scans that were used to determine where the image plane is located. Given the offset from exact collimation of the input beam, the corrections required to locate the image at the detector plane are computed. To determine "best focus", the inverse of the slopes of the edge-traces are plotted. Data obtained on both sides of best focus is plotted; the curves look like parabolas with upward arms. The minimum of this curve is defined as the location of the image plane. Shims that compensate for the focus errors are cut to the correct thickness, and installed. In addition to setting focus, the cryogenic tests were used to determine stability of the optics over the specified environment, and blur size measurements were performed at operational temperatures.

A rotary indexing mechanism for use in a cryogenic and vacuum environment has been designed and deployed in several spaceborne instruments. In the course of space qualification, this mechanism was subjected to severe environmental testing, including thermal cycling (40-800K), vibration (10.6 g's RMS), and acoustics (147 dB RMS). A typical mechanism positions sets of spectral and neutral-density filters in the optical path of a cryogenic radiometer. The filter sets operate independently, with stepper motors providing the indexing feature to each set. Gear-driven resolvers provide feedback to the motors to ensure accurate angular positioning. The sequence and timing of filter combinations is controlled by an on-board microprocessor which commands filter selection routines. This paper describes the overall mechanical architecture, specific design details, and command and control sequences for a typical mechanism. In addition, design tips are given to show how several operational problems were solved.

A brushless DC motor is ideally suited as a filter positioner in a cryogenic infrared spectrometer. Unlike stepper motors, a DC torque motor has infinite resolution for setting an etalon filter to any desired angle. However, the very precise angular positioning and rapid step response needed place rather constricting requirements on the electronic controller. Accuracy, stability, fast step response, and mechanism mechanical characteristics combine to pose a unique electronic design challenge. This paper describes one controller developed for the Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmospheric Re-search Satellite (UARS). An 18-bit control loop provides high accuracy, resolution, and stability. Details on loop stability and signal/noise considerations are discussed. Dynamic computer interaction allows the user to optimize step response.