The Space Infrared Telescope Facility (SIRTF) will be the last of NASA's great observatories and uses a 1 meter cryogenically cooled telescope to allow three focal plane instruments to make background limited measurements in the wavelength range 2- 700 microns over the entire celestial sphere for at least 5 years. The idea for the SIRTF mission was developed in the early 1970's and has been studied intensely since the success of the Infrared Astronomical Satellite (IRAS) mission in 1983. From 1980- 1987 SIRTF was constrained to a near earth orbit since the only available launch vehicle was the Shuttle. With the decision to go to a mixed fleet in 1987, the SIRTF Project Office at the Ames Research Center began an intensive investigation of mission options. A new mission design using a circular 100,000 km orbit was developed and culminated in a decision by NASA HQ to change the mission baseline to this new design in March of 1989. The new mission utilizes a Titan IY/Centaur and is launched from the ETh. The design meets all the Level I science performance requirements established by HQ and the SIRTF Science Working Group. The design is an elegant solution to a difficult problem and results in a significant reduction in observatory mass, elimination of the need for on-orbit servicing, a factor of two higher on target efficiency and better long wavelength performance.

The Space Infrared Telescope Facility (SIRTF) is a cryogenically cooled, space based, one meter class telescope for infrared astronomy. A recent mission option study has moved SIRTF from a previous low Earth orbit (900 km) shuttle launched design to a high Earth orbit (100,000 km) Titan IV/Centaur launched design. The mission option study requirements and trades relating to the structural configuration and the chosen SIRTF design are described. Also discussed is a dynamic stress analysis of the new SIRTF baseline structural design which has been performed using finite element modeling and simulated launch interface loads.

Three designs of the Space Infrared Telescope Facility (SIRTF) for a 100,000 high earth orbit are considered with particular attention given to the evaluation of the aperture stop position. The choice of aperture stop position will be based on stray light considerations which are being studied concurrently. It is noted that there are advantages in cost, mass, and astronomical aperture to placing the aperture stop at or near the primary mirror, if the stray light circumstances allow.

The primary mirror assembly (PMA) requirements and concepts for the Space Infrared Telescope Facility (SIRTF) program are discussed. The PMA studies at NASA/ARC resulted in the design of two engineering test articles, the development of a mirror mount cryogenic static load testing system, and the procurement and partial testing of a full scale spherical mirror mounting system. Preliminary analysis and testing of the single arch mirror with conical mount design and the structured mirror with the spherical mount design indicate that the designs will meet all figure and environmental requirements of the SIRTF program.

Scaling laws for light-weight optical systems are examined. A cubic relationship between mirror diameter and weight has been suggested and used by many designers of optical systems as the best description for all light-weight mirrors. A survey of existing light- weight systems in the open literature has been made to clarify this issue. Fifty existing optical systems were surveyed with all varieties of light-weight mirrors including glass and beryllium structured mirrors, contoured mirrors, and very thin solid mirrors. These mirrors were then categorized and weight to diameter ratio was plotted to find a best fit curve for each case. A best fitting curve program tests nineteen different equations and ranks a "goodness of fit" for each of these equations. The resulting relationship found for each light-weight mirror category helps to quantify light-weight optical systems and methods of fabrication and provides comparisons between mirror types.

Four lightweight solid contoured back mirror shapes (a double arch, a single arch, a modified single arch, and a double concave mirror) and a cellular sandwich lightweight meniscus mirror, have been considered for the primary mirror of the Space Infrared Telescope Facility (SIRTF). A parametric design study using these shapes for the SIRTF 40 inch primary mirror with a focal ratio f/2 is presented. Evaluations of the optical performance and fundamental frequency analyses are performed to compare relative merits of each mirror configuration. Included in these are structural, optical, and frequency analyses for (1) different back contour shapes, (2) different number and location of the support points, and (3) two gravity orientations (ZENITH and HORIZON positions). The finite element program NASTRAN is used to obtain the structural deflections of the optical surface. For wavefront error analysis, FRINGE and PCFRINGE programs are used to evaluate the optical performance. A scaling law relating the optical and structural performance for various mirror contoured back shapes is developed.

A review of engineering philosophies used for the design of optical systems launched into space and operating a vacuum or cryovacuum environment is presented herein. Sources of energy dissipation which are usually lumped under a single modal parameter denoted as the equivalent viscous damping coefficient are reviewed. Optical systems operating in a cryovacuum environment are especially difficult to design for the launch inertia and acoustic loadings unless the components of the system are either caged or clamped since general viscoelastic effects, interface and/or joint relative motion are designed to be minimal. Moreover, stress levels and stress gradients are also designed to be low in precision instruments.

A coordinated technology program for the Space Infrared Telescope Facility (SIRTF) is described. The program encompasses detector technology, cryogenic mechanisms technology, and an adiabatic demagnetization refrigerator. Discrete detectors, detector arrays, detector readouts, and testing of engineering models under simulated flight environment conditions are considered. Several focal planes will be optimized at a particular wavelength range to make up over 247,000 detector pixels from about 1.8 to 1000 microns.

A 50 cm diameter, lightweight, Amersil TO8E, fused-natural-quartz mirror with a single arch cross section was tested at the NASA/Ames Research Center Cryogenic Optics Test Facility to measure cryogenic distortion and hysteresis. The mirror was cooled to 77 K in four serial tests and the mirror figure was measured with a phase-measuring interferometer. On the basis of the repeatability of room temperature and cryogenic optical measurements, it was determined that the Single Arch Mirror had no measurable hysteresis and displayed repeatable cryogenic distortion. The Cryogenic Optics Test Facility, optical and thermal test methods, test results, and measurement accuracy are described.

A thermal model of the dewar and optical system of the Cryogenic Optical Test Facility at NASA-Ames Research Center was developed using the computer codes SINDA and MONTE CARLO. The model was based on the geometry, boundary conditions, and physical properties of the test facility and was developed to investigate heat transfer mechanisms and temperatures in the facility and in test mirrors during cryogenic optical tests. A single-arch, fused-natural-quartz mirror was the first mirror whose thermal loads and temperature distributions were modeled. From the temperature distribution, the thermal gradients in the mirror were obtained. The model predicted that a small gradient should exist for the single arch mirror. This was later verified by the measurement of mirror temperatures. The temperatures, predicted by the model at various locations within the dewar, were in relatively good agreement with the measured temperatures. The model is applicable to both steady-state and transient cooldown operations.

The NASA Ames Research Center has developed a baseline design for a One Meter Cryogenic Optical Test Facility (OMF) incorporating a liquid helium Dewar for cryogenic optical testing of one-meter-class mirrors, principally the Space Infrared Telescope Facility (SIRTF) primary mirror. The primary requirements for the test facility are as follows: 1) The facility must be capable of interferometrically testing any mirror, positioned face-up or face-down, with a diameter less than 1.5 meters and a radius of curvature from 1.5 to 5.5 meters. 2) The facility must achieve and maintain, for 24 hours, a mirror temperature of 4 K with a maximum radiative heat load of less than 500 mW to the mirror, using stored liquid helium and liquid nitrogen. 3) The facility must include a vibration-isolated metrology structure between the test mirror and an external interferometer. 4) The facility must be operated by two persons and a test, starting from the mounting of a mirror to the removal of a mirror, must be conducted within one week.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) telescope concepts are briefly discussed, and a new air-bearing design philosophy is presented. The telescope mounting system inside the hull of a Boeing 747 SP aircraft encompasses a large spherical air-bearing which supports the telescope in the rear bulkhead of the aircraft cavity in order to make it independent of the rotary movements of the airplane and to isolate it from aircraft vibrations through an additional vibration isolation system.

The thin meniscus for SOFIA has reduced weight at the cost of mirror stiffness. As this stiffness is reduced, the support system becomes more complicated in order to hold the mirror in the required optical shape regardless of the deformation of the supporting cell structure. The coupling between mirror cell and meniscus is equivalent to a kind of whiffle tree, which reduces its large number of axial support points down to exactly three points, eliminating any mirror deformation. This reduction system is verified for SOFIA by a hydraulic axial support system for 64 support points in an optimized arrangement in four support rings. Contour line plots and the optical performance are given for this system. A dynamic analysis of the mirror and its support system has shown a remarkable insensitivity of the system against axial accelerations. This mechanism is studied and explained using a simplified beam model. The lateral support system does not carry the mirror at the outer rim as usually designed, but at the mirror's rear side in combination with the axial support system. All the lateral support forces are momentum compensated to transfer the actual force position into the flexural neutral line of the mirror. The self balancing hydraulic support system can be superposed by an active correction system, which adds forces generated by a lever system with a motor driven dead weight. This system can be used to correct long wave errors of the mirror or flight related shape errors of time independent behavior.

Fourier transform spectroscopy in the asymmetric mode is performed using a two mirror wavefront dividing interferometer. Synchrotron radiation is 100 to 1000 times brighter than black body radiation throughout the infrared region from 1 to 1000 microns. The spatial coherence of synchrotron radiation makes it possible to construct new types of Fourier transform spectrometers in the entire infrared region. Fourier transform spectroscopy in the asymmetric mode may simultaneously yield the optical constants n and k of a material.

The performance of a multielement Ge:Ga linear array under low-background conditions is investigated. On-focal plane switching is accomplished by MOSFET switches and the integrated charge is made available through MOSFET source followers. The tests were conducted at 106 microns and the radiation on the detectors was confined to a spectral window 1.25 microns wide using a stack of cold filters. At 4.2 K, the responsivity was measured to be nominally 584 A/W, and the NEP was 1.0 x 10 exp -16 W/sq rt Hz. A detailed description of the test setup and the procedure is presented.

The CLAES is calibrated with a full-aperture blackbody on the instrument-aperture door. In laboratory calibration, the blackbody is resistively heated. On orbit, the blackbody is intended to be heated by exposure to radiation from the earth while the door is open; calibration data are then taken at several temperatures after closing the door, as the blackbody cools to the temperature of the instrument's cryogenic telescope. An analysis of radiometric calibration-source accuracy is shown, indicating a nominal value of 2.7 percent at 12.63 microns. Preliminary analysis of calibration data indicates a measurement repeatability of about 1.25 percent. Details of the blackbody design, construction, and thermal instrumentation are given.

An Ebert-Fastie spectrometer and a filter radiometer for a cryogenically cooled IR-sensor of the Infrared Background Signature Survey experiment are described. The spectrometer covers the spectral range from 2.5 to 22.6 microns with a resolving power of greater than 300 and uses Si:IN, Si:Bi, and Si:As detectors. The radiometer is based on 29 detector elements of different size made from Si:In material. The light is modulated by two tuning fork choppers at 238 Hz, which can be switched on within 0.5 sec. The detector signals are digitized and a digital signal processor performs synchronous rectification and integration for up to 16 channels simultaneously providing amplitude and phase for each detector.

Methods used by the Space Dynamics Laboratory of Utah State University (SDL/USU) to calibrate infrared sensors are described, using the Infrared Background Signature Survey (IBSS) spatial radiometer and grating spectrometer as examples. A calibration equation and a radiometric model are given for each sensor to describe their responsivity in terms of individual radiometric parameters. The calibration equation terms include dark offset, linearity, absolute responsivity, and measurement uncertainty, and the radiometric model domains include spatial, spectral, and temporal domains. A portable calibration facility, designed and fabricated by SDL/USU, provided collimated, extended, diffuse scatter, and Jones sources in a single cryogenic dewar. This multi-function calibrator allowed calibration personnel to complete a full calibration of the IBSS infrared radiometer and spectrometer in two 15-day periods. A calibration data system was developed to control and monitor the calibration facility, and to record and analyze sensor data.

Prediction of the dynamic performance of the Infrared Background Signature Survey (IBSS) is considered in terms of the expected system response, system noise, photon noise, and signal-to-noise-ratio (SNR). The prediction is based on a preliminary radiometric analysis that facilitates the calibration test program. The IBSS sensor exhibited a design noise-equivalent sterance of 9.72 x 10 exp -9 W/sq cm/sr at 6.86 microns for a 6.13 x 10 exp -6 sr detector, a dynamic range of 1 x 10 exp 6 with three gain ranges, and an SNR between 1 and 2400. Theoretical predictions are found to be in an excellent agreement with measurements.

The evolution of design approaches for high-performance superfluid helium dewars containing large-aperture telescopes are discussed. Particular attention is given to thermal-math modeling for the IRAS and the Cosmic Background Explorer (COBE) dewars. Correlation of the recent COBE flight data with the dewar thermal-math model is presented, and apparent predictive deficiencies of the model are discussed.

NASA's Cosmic Background Explorer (COBE) was launched into a polar orbit from the Vandenberg Air Force Base, California on November 18, 1989. The COBE conthin three scientific instruments. Two of these are infrared instruments housed within a 660 liter toroidal superfluid helium cryogen tank. The tank is designed to maintain the base of the instruments below 1.6 K for the duration of the planned one year mission. Boil-off helium is vented from the cryogen tank through a porous plug liquid vapor phase separator, and then overboard from the spacecraft. We discuss here the initial thermal set-up and operation of the dewar in general, and the helium vent system in particular. During the initial cooldown of the dewar from 1.72 K to 1.41 K, short term (1 mm ≤ t ≤ 3 mm) temperature and pressure oscillations were observed in the porous plug and in the vent line. These oscillations have continued throughout the mission life. A detailed flow model was developed to describe this phenomenon and is described below. We further detail the slow establishment of a steady state, 'mission mode' operation of the dewar. The various factors leading to a two week time to mission mode equilibrium for the dewar and cryogenic instruments are discussed. Finally we summarize the performance of the dewar and instruments through the first six months, and we project the expectations for the remainder of the mission through the final depletion of the liquid helium.

A superfluid helium dewar design for operation under supercritical conditions is described. The dewar consists of a 300 1 volume cylinder toroidal tank suspended from the vacuum shell by preloaded GFC chains. Three ventgas cooled radiation shields surround the tank. The optics is mounted to a flange at the He tank being conductively cooled. The optics temperature is allowed to float with the tank temperature going from 4.8 to 10 K while holding the tank pressure constant at 2.8 bar. The focal planes of the IR sensor are actively heated and held at a constant temperature of 10.5 and 14.2 K.

Cryogenic components and techniques for the superfluid helium on-orbit transfer (SHOOT) flight demonstration are described. Instrumentation for measuring liquid quantity, position, flow rate, temperature, and pressure has been developed using the data obtained from the IRAS, Cosmic Background Explorer, and Spacelab 2 helium dewars. Topics discussed include valves and burst disks, fluid management devices, structural/thermal components, instrumentation, and ground support equipment and performance test apparatus.

Recent developments concerning the performance and reliability of a spaceworthy adiabatic demagnetization refrigerator (ADR) for the AXAF X-ray spectrometer are considered. They include a procedure for growing the salt pill around a harness made up of 6080 gold-plated copper wires, a totally modular gas gap heat switch, and a suspension system utilizing Kevlar fibers.

Cryogenic coolers have historically been the weaklink in the overalireliability ofoptical systems and instruments incorporating these devices. The reliability limitation of these coolers incorporating rotary reciprocating compressor mechanisms (commonly referred to as rotary drive coolers) has been predominantly associated with contamination that can freeze out and foul critical operating surfaces within the cooler. This paper reports on the results of significant breakthroughs that have been made in controlling this life limiting feature leading to fivefold increases in coolers reliability over the last two years. Extensive testing ofcoolers with manufacturing improvements has been conducted since mid-1988. Twenty-five coolers (four cooler models) have been subjected to rigorous reliability cycle testing per military usage requirements, accumulating 50,000 hours of operation with only 9 failures. Mean-time-to-failure (MTTF) has been demonstrated to have increased from 300 to 1000 hours to 1500 to 3000 hours for this family ofcoolers. This breakthrough in cooler reliability has resulted in a family of miniature cryogenic coolers that can satisfy system reliability requirements heretofore considered unattainable with rotary driven machines.

A solid argon cooler (SAC) for attached Shuttle payloads has been developed and qualified to meet the need for low cost cooling of flight instruments to the temperature range of 60-120 K. The SACs have been designed and tested with the intent of flying them up to five times. Two coolers, as part of the Broad Band X-ray Telescope (BBXRT) instrument on the ASTRO-1 payload, are awaiting launch on Space Shuttle mission STS-35. This paper describes the design, testing and performance of the SAC and its vacuum maintenance system (VMS), used to maintain the argon as a solid during launch delays of up to 5 days. BBXRT cryogen system design features used to satisfy Shuttle safety requirements are discussed, along with SAC ground servicing equipment (GSE) and procedures used to fill, freeze and subcool the argon.

The Lockheed Sensor Test Facility, located in Sunnyvale, California, is a state-of-the-art LWIR sensor calibration resource designed to calibrate strategic seekers against a simulated exoatmospheric optical background. Increasingly accurate and sophisticated seeker technology has created a demand for improved performance in test equipment, particularly in the area of cryogenic optical systems. Diffraction-limited optics and sub-arcsecond pointing have become the norm rather than the exception in these systems. This paper chronicles the two-year development of several precision mechanisms for use in cryogenic environments to 20 K at pressures below 1 microtorr. The Lockheed mechanism development is highlighted by the successful adaptation of traditional mechanism design principles to the cryogenic environment through the judicious selection of materials, lubricants and electromechanical devices, and the appropriate use of both open- and closed-loop controls. All of the mechanisms developed are associated with the 500-inch effective focal length, eccentric pupil Ritchey-Chretien collimator which forms the basis of the Lockheed seeker calibration approach. Although fundamentally athermal in design, this collimator has traditionally exhibited unacceptable warm-to-cold alignment variations. This phenomenon has been precluded through the use of a precision, six-degree-of-freedom refocusing mechanism which allows the in situ positioning of the collimator's secondary mirror. Together with two precision scan mirrors and their associated positioning mechanisms, the optical performance of the system at operating temperatures and pressures is assured. A source select mirror and its associated drive mechanism has been completely redesigned to provide the accurate positioning of several LWIR radiometric sources at the collimator prime focus.

Systems for scan mirror positioning and filter wheel grating and indexing used in the Infrared Background Signature Survey sensor are described. A control loop which incorporates a cryogenic brushless torquer, an ironless inductive position sensor, and associated control electronics is used to achieve precise angular positioning with the angular range of +/- 7.5 deg. The motion programs include step, sawtooth, and staircase operations. A positioning accuracy of greater than 0.03 deg and a position resolution of greater than 0.001 deg have been achieved. Fixation of grating and mirror mechanism during launch is accomplished using short circuiting of motor windings for providing high braking torques. For a filter wheel indexing, the inductive position sensor is replaced by Hall probes, and the torque motor commutation uses Hall sensor signals. The same signals are applied to control the required 12 positions. A Hall sensor located at the filter wheel marks a reference position.

This paper describes the design and testing of an indexing system for optical-beam steering. The cryogenic beam-steering mechanism is a 360-degree rotation device capable of discrete, high-precision alignment positions. It uses low-precision components for its rough alignment and kinematic design to meet its stringent repeatability and stability requirements (of about 5 arcsec). The principal advantages of this design include a decoupling of the low-precision, large angular motion from the high-precision alignment, and a power-off alignment position that potentially extends the life or hold time of cryogenic systems. An alternate design, which takes advantage of these attributes while reducing overall motion, is also presented. Preliminary test results show the kinematic mount capable of sub-arc second repeatability.

A proposed baseline design for the Space Infrared Telescope Facility includes a Tertiary Mirror Assembly (TMA) which selectively redirects the telescope's converging science beam to each of several instruments. The TMA's mirror rotates on an axis coincident with the beam's axis,'and is held steady during observation by a kinematic mount. A bearing has been designed whose compliance causes minimal interference with the precision of the kinematic mount, and which is well suited to the particular requirements of a cryogenic sateffite such as SIRTF. The bearing suspends its rotor by taking advantage of the repulsion between a superconductor and a magnet. It potentially eliminates problems associated with mechanical bearings that arise in similar applications, such as lubricant loss or failure, bearing wear, and sensitivity to particulates, and does so without imposing the thermal load of a bearing heater or active magnetic bearing. The bearing shows promise of offering an alternative to ball bearings in cryogenic applications where some compliance is acceptable or advantageous.

It is desirable to expose and observe the implications of design decisions early in conceptual stages of design when the cost of design changes are relatively inexpensive. However, few quantitative design details are available at the conceptual design stages rendering traditional design analysis techniques ineffective. Using qualitative information, failure mode and effect recognition (FMER) analysis can expose many possible failure modes. In addition, it lays the foundation for the quantitative failure modes and effects analysis (FMEA) employed in the later stages of design. In performing FMER for a given design, system functionality and interactions between all subcomponents become better understood. The objectives, goals, and assumptions of the design become explicitly documented in the analysis. FMER design verification is based on first principles, geometric relationships, and general information about the components. This qualitative analysis can reveal critical failures due to overconstrained objects, under-constrained motions, conflicting information, and unrecognized assumptions. The designer quickly recognizes short comings of a design and is thus better able to make revisions. The designer specifies more details, making the transition from conceptual design to detailed design. Moving from qualitative to more quantitative analysis, more thorough design validations can be performed as detailed information becomes available. The effectiveness of FMER as an early design analysis tool was demonstrated. Failure modes and effects recognition (FMER) analysis was performed on the base-line (kinematic mount concept) design of the tertiary mirror assembly (TMA) of the space infra-red telescope facility (SIRTF). The findings presented here are not intended to be critiques of the design. But they did serve to identify areas of concern to the designer.

The mechanical testing performed on the Cryogenic Limb Array Etalon Spectrometer (CLAES) instrument installed on the Upper Atmosphere Research Satellite is discussed. The CLAES determines temperatures and concentrations of stratospheric minor species as a function of altitude by measuring the atmospheric infrared emission spectra. CLAES is based on a telescope optical system and infrared spectrometer which are cooled with cryogens.

A method for determining the detector electrooptical transfer function (DEOTF) at different discrete frequencies simultaneously is presented. It involves simulation of the detector with a waveform of unknown frequency composition, such as a square wave or impulse function. The DEOTF is calculated as the ratio of the discrete Fourier transform of the detector output to the transform of the input waveform. This technique was successfully applied to Golay cell and bolometer detectors and can be used for other linear detector systems.

The design of a novel, low profile indium seal for cryogenic optical windows is presented. Such a seal may be suitable to applications where restrictions on available space surrounding the window prevent the use of conventional approaches. The seal is applicable to asymmetric window geometries which can exhibit nonuniformity in mechanical stress that is sufficient to prevent the use of other sealing techniques. A seal between a non-circular germanium window and a metal frame has been fabricated. Modelling and experimental results for selected aspects of the seal design are given. In order to establish the effective life of the new seal, an apparatus has been developed which performs repetitive cycling between room and liquid nitrogen temperatures. Details of the apparatus are provided. The germanium window to metal frame seal has successfully withstood more than 200 such cycles with no detectable leak.

Optical problems which arose during final integration and testing of the DIRBE instrument in the closed flight dewar before launch are discussed. Simulation based on the optical breadboard and engineering unit components showed that these problems (stray light signals) originated outside the instrument in the dewar dome lid. Excellent performance of the instrument confirmed this fact.

Various and unique alignment tooling techniques were used to determine the angular alignments and stabilities of multiple components in a single coordinate system. The methods discussed were developed for alignment of the COBE Observatory1. Acost effective and unique precision alignment facility was devised for the COBE high fidelity Enneering Test Unit (ETU) Observatory. The ETU spacecraft and Cryogenic Optical Assembly (COA) with dummy components attached were measured to verify their structural integrity and mechanical stability. Optical-mechanical alignment techniques were also used to integrate the flight Observatory's attitude control system module (consisting of gyros, reaction wheels, and one of the Observatory's momentum wheels). The techniques for alignments and stabilities of the earth scanners, sun sensors, Far JR Absolute Spectrophotometer (FIRAS), Diffuse Infrared Background Experiment (DIRBE), and Differential Microwave Radiometers (DMR) antenna horn boresights are also discussed.

Metallurgical and physical property changes in Kovar, an Fe-Ni-Co alloy, associated with the phase transformation phenomenon and factors which affect the stability of austenitic structures are discussed. Martensitic phase transformation is considered to be a critical concern to designers of cryogenic instruments for avoiding transformation-induced failures. Problems under consideration include properties of iron-nickel alloys and Kovar alloy, austenite stability in Kovar alloy, and phase transformation testing.