The Space Infrared Telescope Facility (SIRTF), planned for an early 21st century launch into a 100,000 km orbit, will be the fourth of NASA's great observatories. It contains a 1-meter class Ritchey-Chretien telescope, all the components of which will be cryogenically cooled to superfluid helium temperatures. Efficient use of SIRTF's limited cryogenic lifetime (5 years, with a sensitivity of approximately 0.5 month/mW) requires a very low power means for tilting the secondary mirror to relocate rapidly the telescope's line of sight and to map small regions of the sky. Furthermore, at the longest SIRTF wavelengths (200 - 1200 $mu@m) the emission from the cooled telescope will be significant, thereby requiring the use of periodic tilt oscillations of the secondary mirror to modulate the astronomical signal. In addition, achievement of diffraction-limited performance at wavelengths as short as 3 $mu@m will require realignment of the secondary mirror following cool-down and launch. The technology for achieving the secondary mirror assembly requirements at liquid helium temperatures with space-qualifiable hardware had not been developed at the inception of the prototype secondary mirror assembly (PSMA) program in January 1988. We describe in this paper the results of the first liquid helium temperature testing of the SIRTF PSMA. In addition to providing the conventional functions of telescope focus (0.8 $mu@m secondary mirror resolution) and collimation adjustment, the PSMA must achieve 0.063 $mu@m rms wavefront error and millikelvin/sec mirror thermal stability at about 2 K while providing the functions of stable two-axis pointing (0.15 arcsec rms secondary mirror jitter) and dc and dynamic tilting (`chopping,' up to $POM 14 arcmin at 5 Hz) with square, sawtooth, and triangle waveforms. The cryogenic power dissipation budget is 5 mW. We present liquid helium temperature measurements of the PSMA tilt jitter, drift, and range, the reaction mass dynamics, cryogenic power dissipation, and mirror thermal stability. The initial tests of the PSMA strongly indicate that the SIRTF requirements can indeed be met.
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