The present status and potential future direction of the NASA space infrared astronomy program is reviewed. Projects and project concepts discussed include the Infrared Astronomy Satellite, Small Infrared Telescope on Spacelab 2, Cosmic Background Explorer, Shuttle Infrared Telescope Facility, Space Telescope, Large Deployable Reflector, Molecular Line Survey, and Infrared Interferometer in Space. Needs for continued engineering development in critical technology areas such as detectors, cryogenics, optics, and space structures are indicated.
The small helium cooled infrared telescope for Spacelab 2 will permit sensitive mapping of extended, low-surface-brightness celestial sources as well as highly sensitive investigations of the shuttle contamination environment. The development of the focal plane array for this instrument is described. A number of the Photoconductive detectors used in the focal plane were made at the University of Arizona, and we describe the fabrication procedure and some test results. A design for a thermally isolated, self-heated J-FET transimpedance amplifier is presented. This amplifier is Johnson noise limited for feedback resistances from less than 10 8 Ω to greater than 2 x 10 10 Ω at T = 4.2K. Work on the focal plane array is now complete. Performance testing for qualification of the flight hardware is discussed, and results of this work are presented. All infrared data channels are measured to be background limited by the expected level of zodiacal emission.
The COBE satellite, under study by NASA since 1976, will map the spectrum and the angular distribution of diffuse radiation from the universe over the entire wavelength range from 1 micron to 1.3 cm. It carries three instruments: a set of Differential Microwave Radiometers (DMR) at 23.5, 31.4, 53, and 90 GHz, a Far Infrared Absolute Spectrophotometer (FIRAS) covering 1 to 100 cm , and a Diffuse Infrared Background Experiment (DIRBE) covering 1 to 300 microns. They will use the ideal space environment, a one year lifetime, and standard instrument techniques to achieve orders of magnitude improvements in sensitivity and accuracy, providing a fundamental data base for cosmology. The instruments are united by common purpose as well as similar environmental and orbital requirements. The data from all three experiments will be analyzed together, to distinguish nearby sources of radiation from the cosmologically interesting diffuse background radiations. Construction is planned to begin in 1982 for a launch in 1987.
The use of infrared spectroscopy in astronomy has increased dramatically in the past ten years. The broad design considerations are discussed in terns of wavelength coverage and resolution. Three rough resolution ranges,VAA % 100, 1000 and 10000, are identified in which various types of astronomical problems can be studied. Numerous existing systems are briefly discussed and references are given to more complete descriptions.
AFGL will begin a series of rocket borne experiments in the summer of 1981 to survey the sky in the infrared. Two cryogenically cooled telescopes systems will be used, each with a 36 cm diameter primary aperture. One system is cooled with super-critical helium and surveys in three broad spectral bands centered at 11, 20.5 and 27 um. The other telescope system has a super-fluid helium reservoir and incorporates long wavelength arrays centered at 40 and 95 pm in addition to the three shorter wavelength arrays. These experiments will be flown on ARIES sounding rockets to an altitude of 380 km. Each experiment will cover 22 to 35 percent of the sky to a noise equivalent flux density (NEFD) of 1 to 3 x 10-17wcm-2 in each of the spectral bands. This is a factor of about 20 improvement over the previous instruments we used for the 11 and 20 pm observations. The survey instrumentation is described in this report as well as how the experiment will be conducted.
CIRRIS (Cryogenic Infrared Radiance Instrumentation for Shuttle) is a high resolution, cryogenic, Michelson interferometer-spectrometer coupled to a high straylight rejection telescope with coaligned photometer and cameras. CIRRIS is designed to explore the infrared phenomenology of the upper atmosphere from 30-300 km in the 2.5 - 25um LWIR region. In addition, CIRRIS will measure and characterize the gaseous and particle contamination in the Space Shuttle environment. This paper will describe the objectives, instrument fea-tures, and mission profile as they pertain to these measurements.
Self-heated JFets, suitable for use in low background focal planes cooled to cryogenic temperatures, have been developed and tested. For feedback resistances of 2 E 10 ohm at 4.2K the noise contributed by the amplifier is less than the Johnson noise of 2 μv/Hz1/2. Excellent dc stability has been demonstrated at a power level of 0.1 mw per JFet. These devices offer the possibility of much improved performance, greater reliability and less sensitivity to nuclear radiation when compared to MosFets.
We describe an infrared imaging photometer employing a monolithic 32x32 pixel bismuth doped silicon charge injection device array. The device is primarily useful in the 8-13 pm atmospheric window. The detector is sufficiently sensitive to provide good performance on ground-based telescopes and promises to be very good for low background space flight operation.
Low background far infrared detector responsivity and low background spectral response calibrations can be performed simultaneously with a cooled, temperature-swept blackbody. This makes the use of a cooled, low background spectrometer unnecessary in many applications. Calibrations of a beryllium-doped germanium detector showing unwanted 100 pm response and of a gallium-doped germanium detector are described.
One of the most significant developments in radio astronomy has been the recent discovery of over 50 different molecules in the interstellar medium. These observations have changed our picture of the distribution of mass in the galaxy, altered our understanding of the process of star formation, and has also opened up a new and lively field of interstellar chemistry. This achievement was made possible not only by the development of sensitive heterodyne receivers (front-end) in the centimeter and millimeter range, but also by the construction of sensitive RF spectrometers (back-end) which enabled the spectral lines of molecules to be detected and identified. Traditionally, spectrometers have been constructed as banks of discrete adjacently tuned RF filters or as digital autocorrelators. However, a new technique combining acoustic bending of a collimated coherent light beam by a Bragg cell followed by detection by a sensitive array of photodetectors (thus forming a RF acousto-optic spcectrometer (AOS) promises to have distinct advantages over older spectrometer technology. An AOS has wide bandwidth, large number channels, high resolution, and is compact, light weight, and energy efficient. These factors become very important as heterodyne receivers are developed for ever higher frequencies.
Air Force development of various types of space-based surveillance systems is expected to become increasingly important during the 1980's. Sensors carried on orbiting spacecraft can detect and track targets both in space and in the atmosphere. Those sensors which detect infrared radiation from a target and have higher sensitivity than present operational systems will require very low temperature (cryogenic) cooling of focal planes and optical elements in the sensor. For long duration missions, this cooling should be provided by direct, passive thermal radiation or by several types of closed cycle refrigerators (cryo-coolers) now being developed. The particular cooling method chosen depends on the required temperature, the amount of cooling needed, and other factors such as the satellite orbit and weight or size restrictions. This paper will describe the development status, and projected capabilities of several cryo-coolers being developed by the Flight Dynamics Laboratory for space applications.
A series of 23 data flights was made using the 30 cm telescope on board the NASA Lear Jet Observatory to investigate the magnitude and spatial structure of IR atmospheric radiance in the spectral region 5 to 30 μm. Data were obtained in five wavebands with two multi-channel, Ge:Cu, detector systems having IFOV's of 200 and 500 μrad. Measurements were made in three geographical locations: 9°, 35° and 60° north latitude and at altitudes from 15 to 45 kft. A dual channel spectrum analyzer was used to process the recorded data which included a number of accelerometer channels. A high level of excess noise on the IR signals was observed during flight and was due to vibration effects and cabin gas leaks. The RMS value of the spatial variations in the atmospheric background was estimated to be less than 7 x 10-5 to 10-3 times the DC sky radiance. This result is discussed in terms of a theo-retical model for sky noise based on upper atmospheric turbulence. Values for the in-band sky radiance are compared to the AFGL atmospheric model calculations.
We have constructed a simple interferometer placed in front of a liquid helium cooled grating instrument, and have attained a spectral resolution of 0.1 cm-1 in the wavelength range around 64 cm-1 (157 microns) during observations from the NASA Kuiper Airborne Observatory.
We have built a liquid-helium-cooled far infrared spectrophotometer and used it to make low resolution observations of the continua of several kinds of astronomical objects using the Kuiper Airborne Observatory. This instrument fills a gap in both sensitivity to continuum sources and spectral resolution between the broadband photometers with λ/Δ/λ ≈ 1 and spectrometers with λ/Δ/λ > 50 . While designed primarily to study planetary nebulae, the instrument permits study of the shape of the continua of many weak sources which cannot easily be observed with high resolution systems.
InSb hot electron bolometer mixer receivers have been used for submillimeter line studies of the interstellar medium up to frequencies of about 500 GHz (600u). Detections of new interstellar lines have been made, such as the ground state fine structure transition of atomic carbon at 492 GHz, and various transitions of molecules such as carbon monoxide and water. The bulk of this work has been performed with the NASA Kuiper Airborne Observatory telescope which is transported to an altitude of aboutl2,000 km by a C141 aircraft, so avoiding most of the effects of the Earth's atmosphere. Some observations have also been made at ground observatories with the 5 m Hale telescope at Mount Palomar and the NASA In-frared Telescope Facility at Mauna Kea, Hawaii. The heterodyne bolometer receivers have achieved noise temperatures of less than 400 K at all frequencies up to 500 GHz. Develop-ment work continues to extend the frequency range further into the submillimeter band.
A submillimeter heterodyne radiometer, developed for astronomical applications, uses an optically pumped laser local oscillator and a quasi-optical Schottky diode mixer. The resultant telescope-mounted system, which has a noise temperature less than 4000 K (double sideband) and high frequency and spatial resolution, has been used to detect the J = 6 -4- 5 rotational transition of carbon monoxide at 434 micrometers in the Orion molecular cloud. The measurements, when compared with previous millimeter-wave data, indicate that the broad carbon monoxide emission feature is produced by an optically thin gas whose temperature exceeds 180 K.
Infrared heterodyne spectroscopy is an extremely useful tool for doppler limited studies of atomic and molecular lines in diverse astrophysical regions. The current state-of-the art is reviewed and the analysis of CO2 lines in the atmosphere of Mars is outlined. Doppler limited observations have enabled the discovery of natural laser emission in the mesosphere of Mars and the discovery of failure of local thermodynamic equilibrium near the surface of Mars.
Modest modification could convert the Small Helium-Cooled Infrared Telescope from a broad passband to a spectroscopic instrument for use on future Spacelab reflights. The goal of a spectroscopic survey would be the study of the energy budget the Galaxy allocates to shocks in neutral gas. Observational and theoretical studies to date indicate that shocks are common phenomena in neutral gas regions surrounding supernova explosions and nova outbursts, in clouds that border exoanding shells of planetary nebulae and HII regions, and in gases near stars that suffer rapid mass loss. This investigation would be conducted by surveying the Galaxy in three of the most prominent emission lines known to originate from post-shocked gas, the 6.9 μ m line of H2, the 63.2 μm line of , and the 157.4 μm [CII] line. Changes to the Present instrument would include replacement of its current optics by spectrometer optics of a design used successfully in repeated airborne observations. Existing electronics, methods of scanning the sky, data collection, and data analysis could otherwise remain the same.
As the DOD makes the transition into the Shuttle era, experimenters are becoming more concerned about the environmental contamination of the Shuttle Orbiter. Their concern is that Shuttle contamination could prevent major planned experiments from obtaining required data, particularly sensitive infrared systems (e.g., Talon Gold, SIRE, STMP). The performance of optical experiments could be limited by the natural background, by light scattering and emissions from particulates and molecules, and by molecular absorption. Deposition and optical surface degradation may prove to be extensive problems, particularly for cryogenic optics. Other experiments such as communications and space environment tests may also be affected by deposition as well as electromagnetic interference. It has been known that the Shuttle's environment could cause contamination problems during water dumps, thruster firings, paint outgassing and other sources. Predictions have been made, but the contamination species and extent of these problems will not be known definitely until space measurements are made. This paper identifies the contamination types, sources, and their possible effect on particular types of space experiments. The paper also discusses NASA's plans for contamination measurements and the Space Test experiments which could contribute to early resolution of the contamination questions.
A computer data base of infrared astronomical observations has been established at NASA/Goddard Space Flight Center. It contains a summary of all infrared (1μm-1000 μm) observations of celestial sources outside the solar system, published in the major scientific journals since 1960, as well as the contents of infrared surveys and catalogs. Catalog of Infrared Observations (CIO) has been developed from the data base in printed an magnetic tape versions. A bibliographic Guide to the Infrared Astronomical Literature, and an Atlas of Infrared Source Names and Positions will be published in conjunction with the catalog. Future plans include development of an interactive data system at Goddard which will give a user direct access to the computerized data.
Autonomous stellar-reference attitude updates can be obtained by the use of an infrared sensor system's capability of generating dense and accurate infrared-star catalogs. Existing lists of stellar objects, such as the Two-Micron Sky Survey and the AFGL-Four-Color-Infrared Sky Survey, provide stellar densities of about 0.1 star/deg2 at galactic latitudes greater than 30 degrees with a star threshold spectral irradiance of approximately 7 x 10-16 W-cm-2-μm-1. This results in waiting times for new updates of up to 30 minutes in regions of the celestial sphere where the stellar density is low. IR.-CCD mosaic surveillance sensors can detect stellar sources with much lower thresholds and thus generate their own high-density star catalog. The accuracy of CCD-mosaics for a single star sighting is only 1/2 pixel in width, but this is improved by recurrent sightings of the same star and by blurred-image-centroiding algorithms. Precision centroiding is discussed in terms of primary functions and theoretical performance limits. Performance variations with algorithm complexity, blur circle size, and number of sightings are presented.
The Space Infrared (SIRE) sensor is being developed by the U.S. Air Force to obtain a comprehensive data base of long-wave infrared space target and background measurements. As originally designed by Hughes Aircraft Company, SIRE was to be placed in orbit aboard a modified Agena free-flying space vehicle (USAF P80-2). In 1979, the feasibility of flying the SIRE sensor as an attached (sortie) payload on the space shuttle was studied and the Air Force decided to implement this approach. This paper focuses on the interface requirements imposed by the shuttle, and the design efforts necessary to ensure successful integration of the SIRE payload with the necessary flight support equipment, the shuttle orbiter, and other elements of the Space Transportation System. Shuttle flight operations are constrained by contamination effects, data collection needs, and the comparatively high power and thermal dissipation requirements imposed by the SIRE cryogenic refrigerator. Modifications to the SIRE sensor as designed for the P80-2 space vehicle were necessitated by the new mission pro-file and by shuttle-imposed constraints.
This paper presents the results of a contamination analysis and computer modeling study performed for the Space Infrared Experiment (SIRE) using the Space Transport System (STS) Shuttle Orbiter as the launch vehicle for the proposed seven-day sortie mission. These results will provide an accurate description of the deposition levels on the telescope primary mirror and of the molecular number column density (NCD) along the telescope line-of-sight. The planned Helium Purge System was assumed not to be operating. The contri-bution to the contamination environment of any cargo element, other than SIRE and its pallet, was not considered in this study. The study considers five potential contamination sources, including the flash evaporator vent effluents and the vernier reaction control system (VCS) engines plume constituents.
The objective of the Space Infrared (SIRE) Sensor program is to measure LWIR radiation of natural and man made sources in space. Measurements will be used to support development and operation of space based space surveillance systems. This paper describes the planned concept for operating SIRE as a non-deployed payload within the payload bay of the Orbiter. The operations concept is prefaced with an overview of the SIRE system addressing the sensor, space segment, ground segment and supporting elements of the Space Trans-portation System. This is followed by a description of operational concepts and data processing that will be used within the ground segment during flights to plan, command and evaluate SIRE operations. This responsive system provides for inflight evaluation of data and replanning of measurements as necessary to accommodate operational perturbations from the Orbiter and react to unexpected measurement results.
The ARPA Mai Optical Station (AMOS) is an advanced electro-optical research and development facility located atop Mt. Haleakala at an altitude of 10,000 feet on the Hawaiian island of Maui. Co-located with AMOS and sharing common support facilities is the Maui Optical Tracking and Identification Facility (MOTIF), a Spacetrack unit of the Strategic Air Command (SAC). One of the prime sensors at the site is the AMTA, a long-wave infrared (LWIR) radiometer, which is routinely used for making infrared radiance measurements on exoand endoatmospheric targets and for making phenomenological measurements on the intervening atmosphere. The AMTA used one of the two MOTIF 1.2 meter cassegrain telescopes as its collecting aperture to obtain LWIR measurements in any of seven discrete spectral bands from 3 urn to 21 μm wavelength. This paper discusses the AMTA sensor instrumentation, data collection and recording schemes, and calibration techniques employed in preparation for IR data collection activity. Ongoing developments providing for an on-site infrared data reduction enhancement capability will also be discussed, as will the development of a new IR spectral radiometer for the AMOS 1.6 meter telescope.
One of the questions that must be investigated in the design of spaceborne IR telescope systems is that of the level of the background flux due to discrete celestial IR sources (stars, galaxies, etc.). This flux is a potential source of false targets and could decrease the efficiency of a system's data handling capability. This paper describes a technique for predicting 2.2 μm source counts based on an extrapolation of the Cal Tech Two Micron Sky Survey (IRC) using a model of galactic structure. These source counts can be used to estimate the celestial IR flux incident on a space-viewing sensor. For improved accuracy source count predictions should be based on a galactic model which includes, e.g., local spiral arm structure. The IRC is not complete to distances at which spiral arm structure becomes obvious. However, for some space systems now under consideration such galactic features should be considered. For this task as well as for an improved knowledge of stellar distributions in the galaxy deeper IR surveys would be very desirable. Appropriate surveys are possible using existing technology.