Requirements for limb emission measurements have led to the design of a cryogenic Fourier transform spectrometer which combines a number of features for the first time. The critical design considerations were interferometric fringe visibility and interferogram sampling accuracy. The features include a parallelogram scan mechanism with flex pivots, a beryllium scan carriage with back-to-back cat's-eye mirrors, an optimized scan turnaround profile, a flexure-mounted KBr beamsplitter, and linear phase filtering. The instrument, which is currently under test at Perkin-Elmer Corporation, will serve as a breadboard for future spaceborne atmospheric sensors.

The Zodiacal Infrared Project uses advanced absolute radiometers to measure diffuse infrared emission from the interplanetary dust. The telescope is a conductively cooled off-axis doubly reimaging system with a total effective collecting area of 81 cm². A resonant tuning fork chopper is located at the second focus while a fifteen color detector array spanning the 2 to 30 μm spectral region is placed at the final focal plane. This paper describes the design and testing of the sensor.

This detector system was designed primarily for spectroscopic applications in astronomy. The main design goal was to provide a fast, high throughput optical system for adapting cryogenically cooled, self-scanned solid-state arrays to an existing KPNO spectrograph. The final system design combined the optics, mechanical housing, cryogenic cooling, detector, and the first stage of detector electronics into a single package. The system includes an F/7.6 collimator, grisms (prism and transmission grating combined), an F/1.0 Schmidt-type optical camera, a cryogenically cooled, high resolution, solid-state CCD array with a specially designed electronics package for the detector. The system was optically and mechanically interfaced to an existing KPNO spectrograph for added flexibility and cost savings. The system can, with minimum reconfiguration, be used for slit spectroscopy, two-dimensional slitless spectroscopy, or two-dimensional photometry. The spectroscopic applications can be configured for low resolution use with a transmission grism or for medium resolution use with a reflection grating.

A unique flexural pivot mirror translation system was used in the fabrication of a cryogenic Michelson interferometer spectrometer. Uniform cooling was obtained by placing the interferometer inside a convective atmosphere within a liquid nitrogen dewar. The interferometer was flown as part of the EXCEDE SPECTRAL rocket payload which also included electron accelerators to produce artificial auroras. The interferometer successfully measured infrared upper-atmospheric emissions within the spectral range 2.0 to 5.6 μm as induced by the pulsed electron accelerators. Observed emissions include the N2 Wu Benesch infrared system, the N₂ McFarlane infrared system and CO₂. In addition, H2O outgassing from the payload was observed. The 2 cm^-1 spectral resolution of the interferometer clearly assisted in the unique identification of the dominant emissions based on their characteristic molecular band profiles.

A liquid helium cooled grating spectrometer system designed and constructed for field operations was used to obtain atmospheric emission spectra from California to the South Pole. Details of the design of the instrument are discussed and samples of the spectra obtained are presented.

A cryogenically-cooled multi-detector Fourier transform spectrometer has been constructed to measure minor stratospheric constituents via high resolution earth limb emission spectroscopy from a balloon-borne platform . The instrument consists of a standard Bomem model DA2.02 dynamically aligned interferometer but with special vacuum and cryogenic motion and optical interfaces so that transducers remain at ambient temperature while the inter-ferometer operates at low temperature.

The Cryogenic Limb-Scanning Interferometer/Radiometer (CLIR) is being developed to observe infrared emissions of the earth's upper atmosphere from space. The earth's surface is an extended source of intense background radiation with a small angular separation from the desired scene. The CLIR employs an off-axis Gregorian Telescope whose primary mirror and baffles are cooled by an open-cycle cryogen system. A system of specular annular baffles has been developed to minimize both stray light problems and cryogen consumption by retro-mapping the aperture into itself. Each off-axis ray which enters the aperture and strikes the specular baffle surface is reflected so that it passes out of the aperture and is not absorbed on a cryogenic surface. The specular baffle which lies closest to the aperture is an ellipsoid whose foci trace out the circular aperture on revolution about the axis. Its theoretical "ray trace" efficiency is 100 percent. A subsequent baffle has an elliptical cross section whose near focus traces out the central hole in the ellip-soidal baffle and whose far focus traces out the aperture. Its theoretical efficiency is about 90 percent. These baffles reduce the earth radiation heat load on the cryogenic cooler by an order of magnitude,changing it from the dominant cause of cryogen consumption to a relatively small effect. An aperture shield is also desirable to reduce cryogen consumption, stray light, and contamination.

This paper describes the development of advanced Acousto-Optic Tunable spectral Filter (AOTF) for infrared imaging applications. The device is made of Te0₂ and operates at 80K and covers a range from 2 to 5.2 µm. The filter bandpass width is l% in this region.

The Shuttle Infrared Telescope Facility (SIRTF) is a cryogenically cooled telescope in the one-meter aperture class designed for sensing in the infrared from 2-200μm. This facility is designed to be flown many times on the space shuttle with varying instrument complements. All components of the SIRTF within the field of view of the optics are cryogenically cooled. The primary coolant is supercritical helium which is stored in an external tank and routed through the telescope-cooling the instruments first, then the optical components and finally the baffles. For detector cooling below 6K small reservoirs of superfluid helium (Heil) are provided. The SIRTF cryogenic system is designed to automatically control the tank pressure and telescope flow rates during prelaunch operations and flight as well as meet the space shuttle environmental and safety requirements. Temperatures maintained in the telescope are a function of instrument operation, design, energy dissipation, and telescope pointing angle. By control of the cryogen flow rate any selected instrument temperature can be maintained within fixed limits, and the critical secondary mirror can be maintained below 10K throughout a 14-day mission. Instrument thermal design is a critical factor in the maintenance of proper temperature differences in the experiment package. Design of a cryogenic telescope for space use presents many problems which do not exist in earth based systems. The limited opportunity for servicing, the restricted coolant supply, and the remote instrumentation and control all provide new considerations for the instrument and system designers.

A study was performed to determine the optimum focal plane configuration including optics, filters and detector-preamplifier selection. The configuration was optimized particularly with respect to minimizing the noise level, but fabrication considerations for a cryogenic environment were also taken into account. The noise terms from source, background, detector electronics and charged particle radiation were quantitatively evaluated. It appears that noise equivalent spectral radiance less than 10^-11 watts/cm²-sr-cm^-1 can be achieved between 2.5 and 20 microns.

A mathematical model is given for a frequency-compensated detector/preamplifier appropriate for cryogenically-cooled infrared sensors operating under low background conditions. By use of a digital computer, this model is used to rapidly select the optimal combination of design values. These parameters include load resistance, compensation resistance, compensation capacitance, chopping frequency and detector area to meet desired specifications of noise equivalent power, frequency response, dynamic range, and level of output noise. This computer-aided optimal design approach is demonstrated using a contemporary infrared sensor application.

Closed cycle cryogenic coolers are used extensively for cooling infrared detectors and other specialized electronic devices. Because of the special requirements of each electro-optical system it is generally necessary to custom design the cryocooler to fit the requirements. Early and close cooperation between the electro-optical systems designer and the cryocooler manufacturer is important to the successful marriage of the cryocooler with the total electro-optical system. Limitations of various cryocooling techniques are presented, and consideration for cryocooling integration are addressed.

Three cooling methods are now being used by the Army for cooling Night Vision devices. Closed cycle mechanical coolers are being used in applications that can tolerate the weight and power associated with this mode. Three closed cycle coolers are now being used on Army systems. The TE cooler is being used on handheld systems. This option has the quietest mode, since there are no moving parts. This method lends itself to the lowest weight, cost, and size option, but is limited to temperatures around 200K. The J.T. cooler option is being used for 80K operation. This method offers the lowest weight and quietest method for cooling to 80K, but has severe logistic problems. An analysis of these cooling methods will be presented. In addition to the problems of designing and maintaining cryogenic coolers, the interface between cooler and dewar is critical. Several interface methods will be discussed and the results of a new method that will greatly reduce vibrations into the focal plane by mechanical coolers, will also be presented.

This paper describes the features and gives the thermal performance capabilities of the high-capacity (Hi Cap), Vuilleumier (VM), cryogenic refrigerator. This refrigerator simultaneously cools at three different temperature levels, the lowest being below 15 K. It is designed to cool electro-optical sensors in a spaceborne environment. The basic design of the Hi Cap refrigerator is discussed, and the individual machine elements are identified. Data on the thermal performance of this refrigerator is presented for various thermal load conditions on the three cold stages. The capability of this refrigerator for off-design-speed operation is also discussed.

This paper discusses the present status of development of rotary reciprocating refrigerators, which are closed cycle, cryogenic refrigerators specifically designed for cooling space-borne sensors on extended missions. Three machines will be described: a two temperature level Brayton cycle refrigerator which has been built and tested, a three temperature level Brayton cycle refrigerator which is in the early stages of development, and a two temperature level Stirling cycle refrigerator which is in the conceptual design stage.

Practical application of many low-temperature phenomena will demand capability for uninterrupted cryogenic cooling for periods of several years. The reversed Brayton turbo-refrigerator offers unmatched potential for satisfying this need. Life limitation due to wear is virtually eliminated through use of gas bearing turbo-machines for all dynamic components of the system. The turbo-refrigerator is suitable for cooling at temperatures down to liquid helium levels. The refrigerator concept is defined and the physical and operational characteristics are discussed. The input power, weight, and volume of turbo-refrigerator systems providing from 10 to 500 watts of cooling at temperatures from 10 to 100 K are presented, showing that performance of the turbo-refrigerator improves dramatically as the capacity increases.

Four Stirling Cycle mechanical refrigerators are presently providing orbital cooling of two gamma-ray spectrometer instruments.' Each instrument uses two refrigerators that can be operated together or separately. The units have been operating since February 1979 without failure. Performance has degraded due to the gradual leakage of the helium working gas, necessitating operation of two refrigerators simultaneously on one instrument to attain the desired temperatures. The two-stage refrigerators provide 0.3 W at 80 K and 1.5 W at 135 K and have demonstrated the longest duration orbital operation of any mechanical refrigerators to date. A summary of the design, ground tests, and orbital operation is presented for the cryogenic system. Apassive thermal switch was developed during the program, and its operation and performance is summarized. Orbital data that includes working gas pressure, temperatures, and heat rates are presented and compared with ground test data.

Refrigeration of the optics of a spectroradiometer equipped with a low background detector can greatly improve the radiometric sensitivity of the radiometer when observing optically thin or other weak sources. The well known advantages of multiplex spectroscopy, for example by use of a Fourier Transform Spectrometer (FTS) of the Michelson type, in general still apply. Because of the nature of the sianals from the FTS, the existing data base for low background detectors has however only limited applicability in predicting the performance in an FTS for low background conditions. A discussion of the various noise and error sources that affect performance and some practical solutions minimizing their effect (applicable to a Michelson Interferometer operating at a useful resolution and scan rates) will be presented. Results obtained with a number of cryogenic Michelson interferometers in a variety of applications will also be presented.

A phenomenon known as thermal acoustical oscillation, which can be very troublesome and difficult to eliminate, occurs in cryogenic systems when a tube closed on one end is inserted into a cryogenic container. This oscillation can cause an increase in the conduction heat transfer in the tube by several orders of magnitude. A design approach is presented for the elimination of thermal acoustical oscillation based upon the Roth theory. Techniques that have been found useful in improving stability include: Helmholtz resonator, perforated fill line, heat sinking of vent tube, connecting vent and fill lines, and low pressure check valve.

The operational restrictions on several current interferometer/radiometer instruments are such that focal planes, optical components, and telescope baffles must be maintained at temperatures near 8°K, 20°K, and 80°K respectively. One reliable method of achieving this is to expel supercritical helium from a supply dewar at constant pressure by applying energy to a dewar heater; the resulting flow then passes serially through three (or more) heat exchangers to achieve the desired cooling before being vented through a pressure regulator valve. A convenient set of approximate, time-dependent equations that can be programmed on a hand-held calculator for the preliminary design of a supercritical helium cooling system are presented in this paper. Predicted temperature and mass flow rates based on the simplified equations agree within 5 percent of those resulting from analyses by other researchers. The equations are applied to a particular cooler system for the CIRRIS 82 instrument scheduled to be flown on the Space Shuttle in 1982.

Results of an experimental study to ascertain how well the focal-plane location of cryogenically-cooled optical systems can be predicted are reported. These results indicate that if the required low-temperature thermal expansion and index-of-refraction data are available, the focal shift caused by cooling to cryogenic temperatures can be accurately predicted by simply computing the shift in the paraxial focus. In this study, the differences between the measured focal shifts and the computed shift in the paraxial focus were less than the diffraction-limited depth-of-focus tolerance. The results of this study also indicate that for off-the-shelf optical systems ray-tracing analysis may not adequately predict the absolute location of the focal plane. Thus, the following method of predicting the focal-plane location of a cryogenically-cooled optical system is suggested: first measure the focal-plane location with the optics at room temperature, and then add the computed paraxial focal shift to the measured location.

This review paper summarizes and identifies the sources of cryogenic refractive index measurements in the infrared. Representative refractive index versus temperature and wave-length data are presented for the following materials: germanium, silicon, hot-pressed cadmium telluride (Irtran 6), hot-pressed zinc sulfide (Irtran 2), polycrystalline cadmium telluride, cesium iodide, cesium bromide, zinc selenide, and birefringent thallium arsenic selenide. The measurement techniques and the errors associated with the measurements are discussed, and the need for additional measurements is delineated.