Cryogenic Performance of the Space Infrared Telescope Facility Prototype Secondary Mirror Assembly

The Space Infrared Telescope Facility (SIRTF), planned for an early 21st century launch, is the fourth of NASA's Great Observatories. It contains a 1-meter class Ritchey-Chretien telescope, all of whose components will be cryogenically cooled to superfluid helium temperatures. Achievement of diffraction-limited performance at wavelengths as short as 3 μm will likely require realignment of the secondary mirror following launch. Efficient use of SIRTF's limited cryogenic lifetime (5 years, with a sensitivity of 1 month/mW) requires a means for tilting the secondary mirror to rapidly relocate the telescope's line of sight and to map small regions of the sky. Furthermore, at the longest SIRTF wavelengths (200-1200 μm) the emission from the cooled telescope will be significant, thereby requiring the use of periodic tilt oscillations of the secondary mirror to modulate the signal.