Radiometric calibration of Michelson interferometric spectrophometers for the use in atmospheric sounders has been extensively studied, but its practical demonstration is not well reported. This paper describes one such demonstration for the InSb spectral region. The spectral resolution was 2 cm-1, and specific attention is given to the radiometric correctness of the fine scale spectral information. This work is paticularly applicable to the High-Resolution Interferometer Sounder (HIS) experiment, where the interferogram is used directly as the data element.

Rapid scanning of Fourier Transform Spectrometers (FTS) has become the norm for most commercial and military FTS systems. Optimization of scan (retardation) rates is normally predicated on detector response time, recording limitations and mechanical factors. This approach will not necessarily lead to optimum performance when additional sources of noise are included in the total error budget. Examples of such sources of noise are amplitude and phase modulation errors as well as dynamic range limitations in the recording process and truncation errors in the transform process. All these sources of noise are subject to what is sometimes called, ironically, "Fellgett's disadvantage" and that their magnitude depends on the integrated intensity of the source. The effect of these noise sources and techniques to minimize them will be discussed with relevance to some practical and real situations.

The utilization of detector arrays in the focal planes of FTS sensor systems allows simultaneous spectral and spatial measurements. However, spectral lineshapes and wavenumber locations depend upon the size and location of the detector elements with respect to the Haidinger fringe pattern of the FTS sensor. These spectral distortions can be generalized as a shift and shape change of the FTS sensor lineshape. Depending on the distortions that can be tolerated, a degree of field-widening can be obtained for a given Haidinger fringe pattern. An exact model for predicting the FTS lineshape distortions is presented. The model is applied to several contemporary applications in order to quantify the magnitude of distortions to be expected.

FTS sensor systems have significant potential for a wide range of applications compared to other spectrometric systems. Absolute wavelength calibration, high spectral resolution, high throughput, two-dimensional imaging, and wide spectral range are readily achieved advantages. However, spectral fidelity and radiometric precision have been more difficult to achieve, and even to specify, than with other spectro-radiometers. The difficulty originates partly in the operation mode of the FTS sensor which requires instrument errors to undergo a Fourier Transform prior to presentation in the spectral domain. We have developed and described here an FTS sensor performance model to allow evaluation of the effects of a wide range of instrument errors on the spectrum. Likewise, spectro-radiometric performance requirements can be quantitatively related to specifications for subsystems in the instrument (e.g. the scan mirror drive). We illustrate the use of our performance model through application to a contemporary measurement scenario with particular emphasis on classifying and quantifying FTS unique error sources in spectro-radiometric applications.

Utah State University has developed a technique to automatically align the optical surfaces in cryogenic interferometers. Although the technique depends on microprocessor sensing of the highest obtainable interferogram peak, it is not necessary that an interferogram signal be present for alignment to begin. The technique is thus particularly useful when cooling interferometers from room temperature (273° K) to liquid nitrogen temperature (77° K) or colder causes large displacement of optical elements, or when mechanical shock results in similar displacement.

A cryogenically cooled Fourier interferometer spectrometer capable of producing a 0.1 cm-1 resolution was constructed for the primary objective of observing the infrared atmospheric emission spectrum at various balloon altitudes. The instrument was a Michelson interferometer equipped with a Ge beamsplitter coated on a KC1 substrate and two cat's-eye retroreflectors, one stationary and another movable. The entire interferometer was kept at temperature of 77°K in a cryogenic chamber. The optical path difference of the interferometer was monitored by a HeNe laser line at 6329.9A which was provided to the interferometer circuit from a single-frequency cw HeNe laser mounted outside the cryogenic chamber. The spectrometer with a GeCu detector and its auxiliaries were flown on 7 October 1981 at Holloman AFB, producing the analyzable data at altitude of 27000 ~ 28000 m along both the horizontal and the vertical line of sight for more than 2 hours. The interferogram data with other various data were transmitted to the ground station from the balloon package via telemetry radio link. In the post-flight process, the interferogram data were extracted from the telemetry signal recorded on a magnetic tape and then they were converted to the spectral data using the CDC Cyber computer.

This paper presents a first-order performance analysis of the Cryogenic Limb Array Etalon Spectrometer (CLAES), one of several experiments intended for flight on the Upper Atmospheric Research Satellite (UARS). The experiment is based around a solid Fabry-Perot spectrometer which provides spectral resolution of 0.25 cm-1 for atmospheric emission spectroscopy over the 3.5- to 13-μm infrared wavelength range. A solid hydrogen cryostat sized for a 2-year in-orbit lifetime provides cooling for the detector array, spectrometer, and telescope optics.

The Cryogenic Limb Array Etalon Spectrometer (CLAES) is 1 of 13 instruments selected to fly aboard the Upper Atmospheric Research Satellite (UARS) in late 1988. CLAES is being developed by the Lockheed Palo Alto Research Laboratory for the NASA/Goddard Space Flight Center to obtain global measurements of those stratospheric trace specie concentrations affecting the ozone layer balance. It is an earth-limb viewing instrument that requires cryogenic cooling as shown in Table 1 to achieve the required performance sensitivity. Lifetime of the instrument is to be 1.5 years when operating on a 50 percent duty cycle; however, the cryogenic cooling subsystem will be designed to provide 2.0 years of cooling. This provides a 33 percent lifetime design margin for growth in instrument heat rates.

The hybrid cryogenic cooler is an intermittent Joule-Thomson refrigerator with a pre-cooler in the form of a passive radiant cooler. We will review the basic design of the cooler in terms of its evolution, temperature range, mass characteristics, and operation in a micro-gravity environment. We will then describe the special features and advantages that make it attractive for spaceflight applications. Finally, we will discuss application areas, including an example of a specific design.

This paper reports on the development of a two-stage radiative cooler for infrared detectors designed to operate in polar, sun-synchronous, low-earth orbit. Novelty is evident in three areas of the design; radiator/shield geometry, contamination protection, and provision for detector cooling in ambient air. An overview of the design is presented alone with the results of thermal, structural and contamination control tests.

In-situ infrared detector cooling in an ambient air environment is a desirable feature for space-borne instruments. Many system-level tests can be conducted at reduced cost (as compared to vacuum chamber tests) if reliable, controllable spot-cooling of the detectors can be achieved using a "bench cooler". This paper describes a bench cooler designed for spot-cooling the infrared detectors of the Thematic Mapper (TM) instrument. The mechanical and thermodynamic design of the bench cooling system is presented along with system test results.

Unfortunately, for user and manufacturer both, the closed-cycle cryogenic cooler to date has deserved its reputation as the "weak-link" in IR systems. When the cooler requires service at intervals of a few hundred hours at best, the quality of the system it serves is unfairly diminished. This paper addresses technological advances in the art of Stirling-cycle coolers which will increasingly cause that image of military cryocoolers to change for the better. A family of split-cycle coolers designed for long MTBF and in the final stages of development is the focus of the discussion. Their technological evolution, from multi-year-MTBF satellite system Stirling coolers developed in the U.S., and the UA 7011 cooler (tne first all-linear, military, production cooler) developed in Holland, is explained. Three new machines are discussed. Both 1/4 watt and 1 watt (nominal capacity) at 80°K linear-resonant, free-dispLacer Stirling coolers designed for thousands of hours of service-free operation are examined. The third machine is an advanced 1/4 watt at 80°K Stirling cooler incorporating the same component improvements in its free-displacer while utilizing a crankshaft-driven compressor. All three are designed to be compatible with standard U.S. 60 element and 120/180 element detector/dewars. The technologies of linear-resonant compressor and free-displacer expanders as embodied in these machines is discussed in sufficient detail that the reasons for their superior performance will he clear.

In February 1979, four Stirling cycle cryogenic refrigerators, designed and built by Philips Laboratories for the Johns Hopkins University/Applied Physics Laboratory, were launched into orbit aboard a P78-1 spacecraft. These refrigerators, designed to cool two identical gamma-ray spectrometers for a Lockheed experiment, have been highly successful; they hold the record for in-orbit operation of any closed-cycle cryogenic refrigerator. The refrigerators are still in orbit and have individually accumulated from 4700 to over 20,000 hours of operation. The in-orbit performance of the units demonstrated both high reliability and the capability of exceeding their one year maintenance-free life. However, telemetry data indicated degradation in the refrigerator's operating temperature. Specifically, the Lockheed Palo Alto Research Laboratory experiments indicated a 0.4°K/day short-term increase in temperature and a 16°K increase per year. Although these increases were within acceptable limits, Philips Laboratories initiated efforts to eliminate or minimize the reported degradation. A refrigerator identical to those in orbit was built, with one significant modification: flexible metal bellows were introduced between the crankcase and the working volume to prevent possible contaminents from migrating into the latter. During the life test of the modified refrigerator, the temperature increase for the first three-month run was 0.022°K/day, a negligible level. The unit has accumulated 11,000 hours of operation as of June 1982.

The SIRTF optics are intended for operation at 20°K (or less); it will be extremely inconvenient, expensive, and time consuming if it becomes necessary to accomplish all of the optical element testing, assembly, and alignment at comparable temperatures. The thermal strain behavior, including potential anisotropies and inhomogeneities, of a mirror substrate between room temperature and 20°K thus becomes a major factor in the selection of the substrate material, structural configuration, and joining methods for lightweight structures. With support from Space Projects, NASA Ames Research Center, Itek Optical Systems is executing an optical figure evaluation of a 0.65-meter, lightweight, fused silica mirror at a low-temperature goal of 20°K. The design details of a thermal shroud, provisions for extracting heat from the low-conductivity mirror, and wavefront error sources other than the mirror surface are discussed and preliminary test results presented.

Indium antimonide photovoltaic detector parameters which are critical to the design of high-performance focal planes have been measured and are reported. Detector capacitances as a function of voltage, area and temperature are presented and the data is shown to agree well with the abrupt junction model. Resistance-area products as a function of temperature have also been measured; values as high as 1011Ω -cm2 at 50 K. are reported. In addition, fabrication techniques are discussed which can minimize additional capacitances to the diode, as well as maximize the area definition.

We introduce a new P-channel, enhancement mode, Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) which is optimized for use at liquid helium temperatures. The new device, now labelled the ZK-111, operates very well to at least 1.8K. It can operate at power levels of a few picowatts, is zener diode protected, and has higher gain and transconductance, lower channel "ON" resistance, higher channel "OFF" resistance, and better thermal cycling stability than any MOSFET commonly used in infrared focal planes. It has a grounded gate 1/f spot noise that is less than 1μV/√Hz at 1 Hz, ,and is 1/f to beyond 50 KHz. It is now commercially available through Cryoelectronic Inc.

Two TIXM12 germanium junction field effect transistors (JFETs) were briefly characterized at 77, 4, and 1.8K. Both FETs could be operated independently as grounded gate source followers, or connected together in a dual-FET dc-coupled Transimpedance mode Am-plifier (TIA). In the source follower mode a cooled, 1.2E10-ohm load resistor was connected between the gate and ground for one JFET, while the other had its gate grounded directly. In the TIA mode the same load resistor was used as the feedback resistor. Source follower gain, input capacitance, characteristic I-V curves, input leakage current, and transconductance were measured over a small range of conditions. Noise spectral density curves for each device, when operated in the independent source follower mode and when combined as the dual-FET TIA, were measured from dc to 50KHz over a small range of parameters. For experimental devices that were made at least 10 years ago by dated techniques and crudely packaged, these Ge JFET's worked surprisingly well. At 4K and 1.8K, with proper biasing and a few hundred microwatts of power dissipation, the source follower gain is greater than 0.9; the input capacitance is close to 4 pF; the transconductance is about 3.000 micromhos: the I-V curves are quite orderly: the spot noise is about 80 nanovolts per root hertz at 1 Hz: the input leakage current can be forced to zero: and the TIA yields the Johnson noise of the 1.2E10-ohm feedback resistor. There is, however, a noise term at 4K and 1.8K that is sensitive to the gate bias. We believe this bias-sensitive noise source is process dependent and may be removed by modern manufacturing methods. We believe the test results imply, in principle, that an integrated-array focal plane using germanium detectors, germanium resistors, and germanium JFET preamplifiers in the same monolithic structure is feasible. Another implied use could be in an all germanium Charge Coupled Device [CCD] utilizing bump bonding to the germanium JFET's. The devices also have immediate application in discrete detector arrays where system constraints preclude the use of a thermally isolated and bulky silicon JFET housing.

A technique is presented for measuring many of the non-ideal characteristics of the large value [1E8 to 1E13 ohm] resistors used in the feedback loops of liquid helium cooled infrared (IR) detector circuits. These resistors are sensitive to both temperature and voltage and have a far from ideal response to transient events. Since these resistors are used to determine the transfer characteristics of a photodetector circuit, their non-ideal behavior can have a large effect on a system's photometric accuracy and linearity. They can also seriously degrade a system's signal to noise ratio in the presence of ionizing radiation. The method described in this paper has the sensitivity and versatility to accurately measure the non-linear characteristics of these resistors in the actual circuit configuration in which they will be used. This method relys upon the use of a small, stable, ceramic chip capacitor that is installed at the input summing node of a preamp, and an extremely narrow band, tuneable, RMS voltmeter. The capacitor is used to differentiate a number of controlled and well known input voltage waveforms into correspondingly well known input currents. These input currents flow through the resistor thereby creating the output voltage waveforms. These output waveforms contain information about the load resistor's electrical characteristics and allow the calibration of the resistor's response to a wide variety of signals. This method is useful in performing a dynamic system level calibration of a detector circuit since it simulates the many varied types of signal currents generated in either a photovoltaic or photoconductive IR detector circuit.

Stepper motors operating at liquid helium temperature have multiple applications in cryogenically-cooled telescopes such as the Shuttle Infrared Telescope Facility (SIRTF). These SIRTF applications include driving cryogen flow valves, operating the Multiple Instrument Chamber (MIC) beam splitter mechanism, and operating filters and grating wheel mechanisms in the scientific instruments. The positional repeatability of the beam splitter drive mechanism is especially critical since it feeds the optical beam to the scien-tific instruments. Despite these important applications, no significant data on the positional repeatability of stepper motors at cryogenic temperatures has been available. Therefore, we conducted a series of measurements to determine the positional repeatability of a modified, off-the-shelf Berger/Lahr stepper motor (model RDM 253/25, step angle 3.6°) which had demonstrated excellent performance in previous endurance testing at LHe temperature. These test results indicated that the positional repeatability of the motor was excellent at all temperatures, with somewhat better performance at cryogenic temperatures. Another important result was that the motor could be repeatedly turned off and on while still accurately retaining its rotor position.