This paper summarizes some of the basic system trade studies of the Space Infrared Telescope Facility (SIRTF) telescope. The basic requirements of the telescope are that it be background limited and have diffraction-limited imaging capability. This background limit is calculated as a function of wavelength in the 100- to 800- μm region and has been done versus primary mirror temperature. It is shown in the first section of this paper that the optics temperature must be less than 8 K. The contribution to the focal plane background by the other elements of the optical system have also been analyzed; this was done to enable determination of what effect the other parameters, e.g., temperatures and scattering properties of the sunshade, baffles, and other critical structures, have on the background noise. The noise generated by scanning the image motion compensation (IMC) systems is also discussed. This noise is generated by the effects of temperature and emissivity variations on the surfaces of the various optical elements. This paper also discusses the selection of materials usable for the primary mirror of cold optics systems. Fused silica is a particularly viable material for this type of system. Finally, there is a discussion on the fine guidance sensor (FGS) and its influence on the design of the telescope, including the optical system, the focal plane, and the potential heat load the focal plane may present to the system. The various locations of the FGS, both external and internal, are examined.
SIRTF is a planned cryogenically-cooled earth orbiting telescope mission for infrared astronomy. The selection of orbital inclination for the long-life SIRTF mission will have significant impacts for both telescope design and mission operations because the orientation of the inclined orbit plane slowly changes, affecting the relative positions of the earth, sun and SIRTF and influencing aperture sunshade design, cryogen consumption and telescope pointing constraints. These considerations provide useful insight for mission planning and orbit selection.
The primary goal of SIRTF is to conduct astronomical observations to a sensitivity limited by the photon noise produced by zodical dust emission. This fundamental natural background limit to sensitivity cannot be exceeded, regardless of telescope design. To approach this limit a cryogenic, baffled telescope is a necessity. For the SIRTF Technology Integrated System (TIS) Study for NASA, Perkin-Elmer studied the implications of obtaining natural background limited sensitivity with the baseline Cassegrain design. The major contributions to an infrared signal originating from the telescope that could exceed this level are as follows: 1) Stray Light from the sun and moon that results from scatter and diffraction; 2) Photon noise due to thermal emission from the telescope; 3) Scan noise due to a dc offset that results from telescope chopping in the presence of thermal gradients; 4) Thermal drift noise, which results from the offset caused by telescope temperature changes. Our major results are as follows: 1) The noise specification for the 200p - 300p band was the major system driver 2) Chopping removes stray light as a major noise contribution; 3) The black surfaces associated with the secondary mirror constitute the major source of photon, scan, and drift noise in the far infrared and should be masked with a cold stop from the view of the focal plane; 4) Heat dissipation flowing into the secondary mirror from the chopper can be a major source of noise in the far infrared and low dissipation and thermal isolation should be chopper requirements; 5) The telescope should be cooled to less than 5K; 6) Thermal control to hold the mirror surface thermal changes to the level of a mK/s is necessary; 7) Barrel baffle temperatures as high as 30K are within reauirements; the sunshade is a negligible contributor to the background.
The SIRTF Phase A baseline provided a 10 to 20 K cold telescope facility to allow for infrared astronomy to be conducted onboard the shuttle. SCHe cryogen tanks provided the necessary cooling for the duration of the 15-day sortie-type mission. Recently, LPARL completed a study to assess the feasibility for extending the lifetime of the baseline SIRTF concept to between 6 and 24 months. From this study, LPARL concluded that extending the lifetime by simply increasing the size of the SCHe tank size was not practical. An alternative approach was found, one in which solid hydrogen was used to cool the facility components. With solid hydrogen, 6 months of cooling could be provided by an 1870-liter supply having a combined tankage plus cryogen mass of less than 400 kg, both comparable figures to those required by the 15-day SCHe baseline. Detailed analyses have shown that facility temperatures can be maintained below 10 K and that the temporal temperature gradients of both the primary and secondary mirrors are less than one-tenth the maximum allowed excursion of 3.6 K/h. Study constraints imposed were that the telescope must allow for pointing to within 60 deg of the earth and sun, optics temperature must be less than 10 K, and that the total instrument power will be at 1.0 W.
We compare two methods of spatial chopping, namely, (a) symmetrical chopping and (b), asymmetrical chopping. In method (a) the IR object and the sky background are alternatingly observed at equal and opposite chopping angles. In method (b) the IR object is observed on axis. In both methods chopping is implemented by articulation of the secondary mirror. The basis of comparison is the achievable size of an imaging field in which the angular resolu-tion is diffraction-limited at 2 pm. The comparison is made for telescopes with fixed diam-eters of 850 mm, but with f numbers ranging from f/15 to f/30. In method (a) we find usable fields between 2.2 arc min at f/15 and 1.7 arc min at f/30, in case the secondary mirror pivots near its vertex. The field diameters can be increased to 6.0 arc min at f/15 and 4.1 arc min at f/30, if the pivot center of the secondary mirror is placed at the so called "neutral point". However, this has some severe technical disadvantages. In method (b) the size of the imaging field is not limited by aberrations, but by the restriction that it can not be larger than one third of the total field diameter. The latter is defined by the ac-quisition field of the Fine-Guidance System (FGS) and is equal to 15 arc min. With regard to figure imperfections in the cooled mirrors, method (b) offers a much larger margin than method (a). It also offers superior image quality in the FGS field. For further develop-ment of SIRTF we recommend method (b), but consider method (a) acceptable, if the secondary mirror pivots near its vertex.
It has long been baselined that the SIRTF facility must provide spatial chopping at all observing wavelengths. Requiring tnat the image be stabilized to a small fraction of the Airy disk then implies that the spatial chopping must be combined with high-accuracy image motion compensation. An earlier system study found that this requirement is a major driver of the SIRTF design. We have reviewed the underlying requirements for modulating the infrared signal and examined the various kinds of modulation tnat could be used, and find that spatial chopping is not needed simultaneously with the highest-accuracy image stability. Decision trees and calculations are presented to assist in tne development of the final SIRTF facility chopping requirements.
Substantial improvements in infrared sensor technology, the ability to design optics suitable for use at cryogenic temperatures, and the advent of Space Shuttle operations have spurred the development of the Space Infrared Telescope Facility (SIRTF). SIRTF, operating at cryogenic temperatures, will be nearly three orders of magnitude more sensitive than the current generation of infrared telescopes. The unique ability of a cooled infrared telescope operating above the earth's atmosphere to detect and measure extremely faint objects is further enhanced by a technique called spatial chopping. Spatial chopping requires the telescope secondary mirror to execute a rapid back-and-forth motion in a pattern closely approximating a square wave. This motion, performed at frequencies ranging from 1 to 40 Hz with amplitudes between two and 30 arcminutes, permits a continuous comparison of an object-field containing background radiation only with an adjacent field containing background radiation plus source radiation. By collecting data from both object fields and subtracting the resultant outputs, the signal-to-noise ratio of very weak sources is substantially improved.
The advent of the Space Transportation System (STS) and the forthcoming Space Station makes on-orbit servicing of satellites not only technically feasible but highly desirable from an economic point of view. For satellites such as SIRTF with cryogenically cooled optical systems and instruments buried inside a high performance SFHe dewar, free access for servicing becomes a key design consideration. Alternative design approaches are discussed which provide this access and a comparison of the two approaches is given.
A number of applications require the precise tracking or position estimation of an object unresolved in the system optics. This paper evaluates several (NxN) centroid-like interpolation algorithms (N=2,3,4,5) designed to make these estimates to subpixel accuracy. Analytic and Monte Carlo results are presented. The tracking sensor examined was a staring mosaic array (100% coverage assumed) of detectors assumed to be device-noise (e.g., CCD noise) limited. The detector size was varied parametrically to determine the relative performance and to obtain the optimum configuration. The optics blur spot was assumed Gaussian. The sources of error considered to affect the algorithm performance were the systematic algorithm bias (or positional error), the random noise (or jitter error), and the postcalibration residual detector responsivity nonuniformities. The results were applied to the design of the SIRTF Fine Guidance Sensor. Track accuracy improves with signal-to-noise ratio (SNR), until limited by algorithm inaccuracies or focal-plane nonuniformity. But blur spot distortion has significant impact on algorithm performance. Among the algorithms tested, the relative SNR performance improved as N decreased. However, extreme sensitivity to algorithm bias error limited the use of the (2x2) algorithm to cases with positional requirements z L/25 (even with correction). The (3x3) algorithm is then optimum for positional requirements z, L/100 (with correction). Higher (NxN) algo-rithms are required for greater positional accuracy.
The Infrared Astronomical Satellite (IRAS) has successfully completed its mission of providing an unbiased all-sky survey of astronomical objects in the 8 to 120 micrometer wavelength region. The design and performance of the Focal Plane Optics Assembly (FPOA) for the IRAS instrument is described in this paper. The FPOA consists of 62 survey field stops, 62 individual small field lenses, 124 small spectral filters, and a precision multi-part aluminum housing. The FPOA is capable of repeated thermal cycling from ambient temperature to 2 Kelvin. The spectral filters, along with the detector spectral responses, provide infrared bandpasses of 8-15, 18-30, 46-78, and 85-117 micrometers. The combination of very long wavelengths, liquid helium temperatures, and small size provided a significant design challenge. Spectral filter and field lens designs for the four spectral bands are described. Also, discussed are techniques which were developed for mechanical mounting of the small lenses and spectral filters, and to assure their optical alignment.
The Infrared Astronomical Satellite (IRAS) was designed to operate at 2K, and over the spectral range of 8 to 120 micrometers. The focal plane is approximately 2 by 3 inches in size, and contains 62 individual field stop apertures, each with its own field lens, one or more filters and a detector. The design of the lenses involved a number of difficulties and challenges that are not usually encountered in optical design. Operating temperature is as-sumed during the design phase, which requires reliable information on dN/dT (Index Coeffi-cient) for the materials. The optics and all supporting structures are then expanded to room temperature, which requires expansion coefficient data on the various materials, and meticulous attention to detail. The small size and dense packaging, as well as the high precision required, further contributed to the magnitude of the task.
In conducting thermal/vacuum testing on a test article that is to operate at cryogenic temperatures (below 77 K, liquid nitrogen temperature), the designer is faced with unique problems peculiar to these temperatures. Care must be taken to account for these problems in the design of the test and its associated test equipment if a successful test program is to be achieved. These problems are often aggravated when the test article has special test requirements of its own that must be met. Frequently, trade-offs must be made. When the test article is an optical system, as it is in this case, the designer has especially difficult trade-offs to make. The design that evolves may involve compromise, but it must still insure that the desired cryogenic test conditions will be satisfactorily provided.
A 32-cm diameter corrector plate with a 0.866 neutral zone is easier to fabricate than one with a 0.707 neutral zone. The making of a Wright corrector is chosen because it has twice the strength of a Schmidt plate. The Wright optical system permits three possible conversions: a f/2.7 camera and a f/2.7 Newtonian comet seeker both with a 5.50 flat fields and a f/8.3 Wright-Cassegrainian system. The specifications are listed in Table 1.
A liquid-helium-cooled, 24-detector grating spectrometer has been developed and used for low-resolving-power astronomical observations in the 5-14-pm spectral range. The instrument has operated on the 91-cm Kuiper Airborne Observatory (KAO), the 3-m IRTF (Mauna Kea), the 3-m Shane telescope (Lick Observatory), and the 152-cm NASA and University of Arizona telescope (Mt. Lemmon, Ariz.). The detectors are discrete Si:Bi photoconductors with indi-vidual metal oxide semiconductor field-effect transistor (MOSFET) preamplifiers operating at 4 K. The system uses a liquid-helium-cooled slit, order-sorter filter, collimator mirror, grating, and camera mirror arranged in a Czerny-Turner configuration, with a cold stop added between the collimator mirror and the grating. The distances between components are chosen so that the collimator mirror images the telescope's secondary mirror onto the cold stop, thus providing a very effective baffle. Scattered radiation is effectively reduced by using liquid-helium-cooled, black baffles to divide the spectrometer into three separate compart-ments. The system noise-equivalent flux density, when used on the 152-cm telescope from 8 to 13 μm with a resolving power of 50, is 4.4 x 10- 17 WCM-2 pm The main appli-cations are for measuring continuum radiation levels and solid-state emission and absorption features in regions of star and planet formation.
Pioneering observations of far infrared lines from astronomical sources gave been made possible in the past few years by NASA's Kuiper Airborne Observatory (KAO). This is primarily because spectroscopy from ground based sites in the wavelength region from about 20 to 350 microns is precluded by telluric water vapor absorption. Lines in this wavelength range characterize all the phases - molecular, neutral atomic, and ionized - of the inter-stellar gas. They provide both unique informatio2 and information complementary to that available from measurements at other wavelengths.
Long-wave infrared (LWIR) sensors are evaluated against the exoatmospheric background and radiometric sources of the Lockheed Sensor Test Facility (STF) cryovacuum chamber. In order to provide realistic simulations of exoatmospheric scenes, the chamber is operated at a temperature of 20 K with a pressure of 10-6 to 10-8 Corr. The chamber contains mechanically adjustable source attenuators, source positioners, and precision source scanning mirrors as well as a cold scanning spectroradiometer for in situ source calibration. These mechanisms must function in the hostile environment of the chamber without generating unwanted IR radiometric background.
A test facility for interferometrically evaluating the performance of various optical surfaces at cryogenic temperatures is described. The complete facility utilizes standard commercially available equipment and provides rapid and convenient component assessment in cryovacuum environments. Surface figure distortions in reflection and transmission at 632.8nm have been determined for various optical elements mounted and unmounted at temperatures to 84 K. Data are presented for several elements including metal and fused silica mirrors with convex, concave, and piano surfaces. Also presented are data for several windows, lenses, and prisms of Ge, MgO, and ZnSe.
Since silica mirrors have been shown to have significantly less distortion due to cryogenic cooldown, a silica primary mirror of approximately 1 meter diameter is being considered for the Space Infrared Telescope Facility (SIRTF). This is a departure from the technology of the Infrared Astronomical Satellite (IRAS) Optical Subsystem, which was built mostly of beryllium and had a primary mirror of 0.62 meter diameter. Using a thermal and cryogenic transient analysis program developed previously, we have modeled the SIRTF Optical Subsystem, including the primary mirror, using lumped-parameter techniques. The mirror thermal response to variations in aperture heat load and in heat sink temperature have been explored. Calculations have been made of system noise components from primary mirror spatial and temporal temperature variations. We demonstrate that in spite of the relatively low thermal conductivity and diffusivity of glass as compared to beryllium, that noise sources arising from the thermal response of the primary mirror are sufficiently small to maintain background-limited infrared observations at wavelengths out to 200 micrometers. Mirror thermal performance with and without copper wires for cooling is also considered.
In response to technology needs for infrared (IR) telescopes operating at cryogenic temperatures, Eastman Kodak Company has developed a 0.5-meter (m), ultra lightweight, frit bonded, fused silica mirror capable of being scaled to a larger size that would provide a fast aspheric, smooth, low scatter optical surface.
This mirror has been evaluated by Kodak at a temperature of 100 degrees Kelvin (°K). This paper reports on a continued evaluation of the mirror jointly by Kodak and Ames Research Center (ARC) to a temperature of 8°K. Analysis of common interferograms by independent processing hardware and software has been carried out by Kodak and ARC. The results of both processes are compared and reported.
The Diffuse Infrared Background Experiment (DIRBE), a part of the Cosmic Background Explorer (COBE), has a requirement for internal calibration of its detectors. This requirement stems from COBE's orbital motion through the Van Allen Belts and South Atlantic Anomaly. Radiation hits require the annealing of the detectors and their calibration. The calibration required stimulation at six levels covering a dynamic range of 105 to a reproducibility of 1% in any three separate calibration runs. The IRS consists of four thermal greybody sources coupled to an integrating sphere. The goal of the IRS is to provide stable, linear, uniform radiant outputs for detector calibration. The design was heavily impacted by the 2K environment of DIRBE. Important considerations included thermal cycling, power dissipation, and cool down times of less than three seconds. The major design problem for the IRS was the thermal sources. They were required to have very low heat capacities in order to have rapid rise and fall times and low power dissipation but the stability more characteristic of sources with large heat capacities. A trade-off also existed with regard to stability and dynamic range. Ideally one would desire to be in the Rayleigh-Jeans limit of the thermal sources, since the linear dependence on temperature aids in stability. However, the high temperatures required for the near infrared bands were difficult to obtain and tended to swamp the detectors with signal for feasible source sizes. The relative insensitivity of the DIRBE bolometers and the linear temperature dependence of the Rayleigh-Jeans law made bolometer stimulation over a dynamic range of five orders of magnitude difficult, These problems were solved in part by having four different sources with emitting areas varying by a factor of 108. A low heat capacity design with the necessary stability appears to be thin nichrome films on a sapphire substrate. Extreme voltage stability (to 0.01%) is required for those near infrared bands that required stimulation on the Wien portion of the blackbody curve.
The Infrared Astronomical Satellite (IRAS) was the first far infrared space telescope to perform observations from orbit. Its excellent overall performance demonstrated life and temperature profiles, that should provide the confidence to proceed with the next major cryogenically cooled telescope, SIRTF. An overview of the cryogenic system design is presented along with the hardware flight performance. Specific flight performance parameters such as optics temperatures, initial stabilization times, and optics and cryogen system temperature profiles after depletion of the superflid helium are highlighted.
The COBE satellite includes two cryogenically cooled instruments, which are housed inside a 664 liter superfluid helium dewar to achieve an operating temperature of 1.5K. The dewar will provide a 14 month operating lifetime, during which the far infrared absolute spectrophotometer (FIRAS) and diffuse infrared background experiment (DIHBE) will conduct full sky surveys from a 900 km, sun-synchronous polar orbit. The spectrum of diffuse radiation will be characterized by these instruments over the wavelength region from 1 micron to 1 cm. Launch is planned for autumn 1987. The dewar is very similar in design and function to that used successfully as part of the Infrared Astronomy Satellite (IRAS). Some design changes, as compared to the IRAS dewar, have been made to meet STS safety and operations requirements, to accommodate the CUBE instrument package, to increase lifetime, and to improve functional reliability in several areas. Most noteable design changes are a 23 percent enlargening of the cryogen tank, de-letion of the aperture cover helium tank, and improved thermal radiation baffling to mini-mize the heat input through the area at the interface between the main dewar and the aperture cover. Dewar system testing is scheduled to start in summer 1985. The dewar design and design drivers, especially as related to changes from the IRAS design, are discussed along with thermal performance predictions resulting from computer modelling and observed performance of the IRAS dewar.
The Spacelab 2 Infrared Telescope Experiment has recently completed a series of extensive tests both in the laboratory and as part of the integrated Spacelab 2 payload. We report on the results of the cryogenic performance and servicing tests conducted to date.
Presented in this report is the transient thermal analysis of the cooldown of a solid cryogen when the pressure in the system is decreased. The method of analysis is discussed, and results consisting of time-temperature histories and the amount of solid cryogens vented from a two stage methane/ammonia cooler during the cooldown are presented.
Production of superfluid (4HeII) helium from a reservoir of supercritical helium offers an option in the cooling of instruments and the transfer of helium in a low-g environment. The problem with small Joule-Thomson Expanders (JTX) has been a tendency toward the plugging of capillary expanders. A JTX has been designed using porous plugs as expander elements to reduce the potential for plugging. Superfluid helium has been produced continuously for an hour and a half using unfiltered liquid helium.
Many space instruments will require very low cryogenic temperatures to perform satisfactorily. The performance of bolometers greatly increases at very low temperatures and instruments for telescopes such as SIRTF will require a cooler to provide -50 μW total cooling power at temperatures of 0.1 to 0.3 K. With advances in cryogenic technology and satellite lifetime, the cryogen system design lifetimes are becoming much longer. This paper discusses a long life open-cycle 3He cooler that provides low cooling (50 μW) to a temperature as low as 0.18 K by continuous evaporation of liquid 3He and loss of vapor through a vent tube.