The Space Infrared Telescope Facility (SIRTF) is a one meter class cryogenically cooled infrared observatory under study by NASA for launch and operation in the mid 1990's. It operates in the spectral range 2-700 µm and represents a factor of over 1000 increase in sensitivity over the first NASA IR space mission, the Infrared Astronomical Satellite (IRAS) . SIRTF will be the first true infrared observatory in space and is complementary in wavelength coverage and comparable in sensitivity to the Hubble Space Telescope (HST), Gamma Ray Observatory (GRO) and the Advanced X-Ray Astrophysics Facility (AXAF) missions. such, it is considered part of the great observatories program being developed by NASA .

The usable lifetime of the Space Infrared Telescope Facility (SIRTF) has been assumed to be limited to about two years by the lifetime of the superfluid helium carried in the telescope dewar. Concepts are presented for extending the system life by replenishing the cryogen on orbit, and for replacing the focal plane instruments. The operational aspects and the modifications to the baseline SIRTF are examined. It appears to be feasible to perform these servicing operations based on either the Space Shuttle or on the Space Station.

The design and development of systems for collecting and delivering liquid cryogens to a supply tank drain inlet presents unique problems in hydrodynamic and thermodynamic control. Solution of these problems requires perceptive interpretation of available technology and realistic appraisal of any need for new supporting technology. In general, the state of the art is sufficiently advanced at this time to warrant proceeding with designs to control liquid cryogens in low-g environments. There must be assurance that there are no failure modes that cannot be accommodated by design schemes. Analyses are necessary to determine the draining efficiency of these devices because low residuals in the supply tank are essential. Three types of fluid control have been demonstrated successfully in flight.

Gravity Probe-B (GP-B), also known as the Stanford Relativity Gyroscope Experiment, will test two fundamental predictions of Einstein's General Theory of Relativity by precise measurement of the precessions of nearly perfect gyroscopes in earth orbit. This endeavor will be the result of over 25 years of research and embodies state-of-the-art technologies in many fields including, among others, gyroscope fabrication and readout, cryogenics, super-conductivity, magnetic shielding, precision optics and alignment methods, and satellite control systems. These technologies are necessary to enable measurement of the predicted precession rates to the milli-arcsecond/year level and to reduce to "near zero" all non-General Relativistic torques on the gyroscopes. This paper, the first of six on GP-B at this conference, will provide a brief overview of the experiment followed by descriptions of several specific hardware items with highlights on progress to date and plans for future development and tests.

Current sensors and experiment systems take full advantage of the space environment to obtain extreme precision for scientific measurements. The systems therefore perform to design levels only in space, and can only be tested in the space environment. The sensors and support systems use specially developed technologies and also apply existing technologies in ways that push performance to the natural limits. These requirements place high emphasis on the task of the Systems Engineer to meet the challenges of integrating a broad range of technologies and verifying performance so that residual risk is tolerable at each stage of development and at launch. Gravity Probe-B (GP-B) is typical of this modern system challenge, as it represents the state of the art in sensors (gyroscopes and readout) and magnetic shielding, and incorporates state-of-the-art requirements for cryogenics, optics, satellite control, atmosphere-drag makeup, electronics, and supporting disciplines. Systems Engineering for GP-B will be called upon for innovative use of simulation and analytical techniques in conjunction with carefully selected development testing. For example, an existing error analysis is being used to develop the technology interactions and to support decisions (tradeoffs) on the configuration of the experiment system. This paper discusses the requirements that the GP-B system must meet, and describes our approach to integrating the technologies developed by Stanford University over the past 22 years with cryogenics and other disciplines developed for spaceflight by the aerospace community.

The Infrared Array Camera for the Space Infrared Telescope Facility (SIRTF/IRAC) is capable of two-dimensional photometry in either a wide field or diffraction-limited mode over the wavelength interval from 2 to 30 microns. Three different two-dimensional direct readout (DRO) array detectors are being considered: Band 1 - InSb or Si:In (2 - 5 microns) 128 x 128 pixels, Band 2 - Si:Ga (5 - 18 microns) 64 x 64 pixels, and Band 3 - Si:Sb (18 -30 microns) 64 x 64 pixels. The hybrid DRO readout architecture has the advantages of low read noise, random pixel access with individual readout rates, and non-destructive readout. The scientific goals of IRAC are discussed, which are the basis for several important requirements and capabilities of the array camera: 1) diffraction-limited resolution from 2 - 30 microns, 2) use of the maximum unvignetted field of view of SIRTF, 3) simultaneous observations within the three infrared spectral bands, 4) the capability for broad and narrow bandwith spectral resolution. A strategy has been developed to minimize the total electronic and environmental noise sources to satisfy the scientific requirements.

A conceptual design for an infrared spectrometer capable of both low resolution (λ/Δ-λ = 50; 2.5-200 microns) and moderate resolution (1000; 4-200 microns) and moderate resolution (1000; 4-200 microns) has been developed. This facility instrument will permit the spectroscopic study in the infrared of objects ranging from within the solar system to distant galaxies. The spectroscopic capability provided by this instrument for SIRTF will give astronomers orders of magnitude greater sensitivity for the study of faint objects than had been previously available. The low resolution mode will enable detailed studies of the continuum radiation. The moderate resolution mode of the instrument will permit studies of a wide range of problems, from the infrared spectral signatures of small outer solar system bodies such as Pluto and the satellites of the giant planets, to investigations of more luminous active galaxies and QS0s at substantially greater distances. A simple design concept has been developed for the spectrometer which supports the science investigation with practical cryogenic engineering. Operational flexibility is preserved with a minimum number of mechanisms. The five modules share a common aperture, and all gratings share a single scan mechanism. High reliability is achieved through use of flight-proven hardware concepts and redundancy. The design controls the heat load into the SIRTF cryogen, with all heat sources other than the detectors operating at 7K and isolated from the 4K cold station. Two-dimensional area detector arrays are used in the 2.5-120μm bands to simultaneously monitor adjacent regions in extended objects and to measure the background near point sources.

The Multiband Imaging Photometer for SIRTF (MIPS) is to be designed to reach as closely as possible the fundamental sensitivity and angular resolution limits for SIRTF over the 3 to 700μm spectral region. It will use high performance photoconductive detectors from 3 to 200μm with integrating JFET amplifiers. From 200 to 700μm, the MIPS will use a bolometer cooled by an adiabatic demagnetization refrigerator. Over much of its operating range, the MIPS will make possible observations at and beyond the conventional Rayleigh diffraction limit of angular resolution.

A liquid-nitrogen 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. Cryogenic cooling combined with the use of extrinsic silicon photoconductor detectors cooled to liquid-helium temperature allows the detection of weak emission features of gaseous species. The spectrometer has two basic scan modes: the first mode records the continuous spectrum from 650-2100 cm with 0.2 cm resolution; the second records simyltaneously four, preselected, narrow intervals (-175 cm bandpass, each) with 0.02 cm resolution, unapodized. Filtering of the interferogram signal is done by real-time, digital signal processing. The most important feature of this flat mirror Michelson, with respect to remote balloon-borne operation, is the dynamic alignment system which maintains the relative parallelism of the two flat reflectors of the interferometer. Species identified to date in data obtained during a November 6, 1984 flight include: CO₂, 0₃, H₂0, CH₄, HNO₃, N₂0, NO₂, NO, CC1₃F (Freon-11) and CF₂C1₂ (Freon-12).

This paper describes the design, development and preliminary testing of the cryogenic star-tracking telescope used as an optical reference for the gyroscopes in the Gravity Probe B Relativity Gyroscope experiment. The telescope is operated at 1.8 K; it is fabricated entirely from fused quartz components held together by optical contacting; it has a physical length of 14 in, a focal length of 150 in and an aperture of 5.6 in. Readout is by two photomultiplier chopper-detector assemblies at ambient satellite temperature. When fully operational the telescope may be expected to have a precision approaching 0.1 marc-s over a linear range of ±70 marc-s. Its projected noise performance corresponds to an angular resolution of 1 marc-s in 1 Hz bandwidth. The paper includes a theoretical analysis, a description of the design and fabrication of a laboratory version of the telescope, a discussion of techniques of optical contacting, an account of vibration tests on a separate mass model of the telescope, a description of the artificial star developed for optical tests, and an account of preliminary experimental results.

We examine the interrelationship of the issues of sensitivity, image centroiding precision, and field of view and how they interact with the SIRTF pointing requirement to determine a FGS focal plane configuration. While the examples are specifically oriented toward SIRTF, our approach is general and can be applied to other CCD-based star sensor systems.

Preliminary analyses and development tests are beginning to define the Gravity Probe B (GP-B) superfluid helium dewar and probe that contain the scientific instrument. The status of the current dewar and probe concept is reported, and supporting analyses used to define the neck tube region are described.

In early 1988 the Cosmic Background Explorer will be placed into a 900 km orbit from which the spectrum of diffuse radiation will be measured from 1 micron to 1 cm. A critical component of the observatory is the 650 liter superfluid helium dewar, which will house the two cryogenic instruments: a far infrared absolute spectrophotometer and a diffuse infra-red background experiment. Fabrication and testing of the dewar is complete, and measured performance exceeds requirements. The four-month test program included four major phases: (1) filling the dewar with superfluid helium and verifying basic functions, primarily fluid management and instrumentation, (2) vibration testing to verify the structural math model and prove Shuttle launch compatibility, (3) thermal performance testing to verify the thermal math model and prove the orbital lifetime requirement will be met, and (4) testing to prove that in a simulated orbital environment the aperture cover will eject properly. No significant flaws in the dewar performance were encountered during testing. In laboratory conditions the "hands-off" helium loss rate was measured at 0.75 percent per day, and the bath temperature (with pumping) was 1.65K. The boil-off rate during ground testing is about four times greater than during orbital operation because of differences in configuration and boundary conditions. To verify orbital performance through ground testing, it is therefore necessary to use a math model which is rigorously validated by test data. Testing was designed specifically to accomplish this. Orbital cryogen lifetime is predicted at 14 months, compared to 10 months required with a design goal of 12 months. Major design features, test results, thermal math model correlation, and orbital performance predictions are discussed.

The cryo-optical methods used to measure the spectral transmittances of filters and beamsplitters for the Cosmic Background Explorer's instruments are described. Measured results demonstrate the temperature sensitivity, or insensitivity, of various infrared filter designs within the wavelength range from 1μm to 1000μm.

We describe the superconducting readout system to be used for resolving 0.001 arc second changes in the gyroscope spin direction in the Relativity Gyroscope (GP-B) experiment. This system couples the London magnetic moment flux of the spinning gyro to a low noise superconducting quantum interference device (SQUID) detector. Resolution limits and noise performance of the detection system will be discussed, and improvements obtained and expected with advanced SQUIDs will be presented. We also describe the novel use of superconducting magnetic shielding techniques to obtain a 250 dB attenuation of the Earth's magnetic field at the location of the gyroscopes. In this approach, expanded superconducting foil shields (as developed by Cabrera') are coupled with fixed cylindrical superconducting shields and special geometric considerations to obtain the extremely high attenuation factor required. With these shielding techniques, it appears that the 0.5 Gauss Earth field (which appears to the gyroscopes as an AC field at the satellite roll rate) can be reduced to the 10^-13 G level required by the experiment. We present recent results concerning improvements in the performance of the superconducting foil technique obtained with the use of a new computer-controlled cooling system.

In 1960, Leonard Schiff predicted, using Einstein's General Theory of Relativity, that a gyroscope in orbit about the Earth would experience a precession of its spin axis relative to the "fixed stars". Two relativistic precessions are predicted: a "geodetic" precession associated with the orbital motion of the gyro about the Earth, and a "motional" precession due to the Earth's rotation. For a gyro in a 650 km altitude polar orbit with its spin axis initially pointed towards an inertial reference, in this case the star Rigel, and lying in the orbital plane, the geodetic precession is 6.6 arcsec/yr north , and the motional precession is 0.042 arcsec/yr east. This scenario is illustrated in Figure 1. To detect these relativistic drifts, a gyro is currently being developed whose absolute Newtonian drift rates are less than 10^-3 arcsec/yr. Even with such a "perfect" gyro, however, the question arises: Can the relativistic drifts be detected in the presence of random measurement noise, and other error sources such as satellite attitude control system errors, Rigel proper motion uncertainty, drifts due to gyro suspension forces, drift of electronic parameters such as instrument scale factors due to thermal effects, etc.? This paper describes an all-digital data flow simulation that demonstrates the Kalman Filter data reduction process for detection of the relativistic drifts. The simulation demonstrates successful optimal estimation of the relativity effects in the presence of the expected measurement noise and consistent with the experiment lifetime, and the other above-mentioned effects.

An analytical model for an infrared telescope is described. The model gives not only the prescription and the third-order aberrations but also the influence of tilts and decenters of the secondary on the first- and third-order properties of the system as a function of the input parameters. Predictions obtained with this model are compared with the results of real ray tracing.

The finite element method was used for the structural design of the Space Infrared Telescope Facility (SIRTF) primary mirror and its support system in a cryogenic environment similar to space-shuttle cargo-bay launch conditions. Shuttle loads were specified by power spectral density functions (PSDF) obtained from previous shuttle launches. The primary goal in the development of a design to withstand this random loading was to ensure the structural integrity of the support system, which comprised an aluminum baseplate and three titanium flexures. This design was an extension of a support system previously developed for cryogenic static effects only. The displacements and stresses of the support system are greatly affected by the damping characteristics of the flexures, which are very difficult to quantify. A parametric study illustrates the behavior of the system over the range of the estimated damping values. Recommendations and techniques for modeling this type of structure are presented. The methods and approaches used in the analysis and the effect of model refinement upon solution accuracy are discussed.