Cryogenic implications of orbit selection of the Space Infrared Telescope Facility (SIRTF)

An investigation has been conducted to determine how the choice of orbit for NASA's prospective Space IR Telescope Facility (SIRTF), between polar (99-deg) and low inclination (28.5-deg) alternatives, will affect the performance of the all-superfluid He-cooled IR optics employed. While the dewar design met both the service life and 200-micron background-limited performance criteria in the case of the polar orbit mission, the alternative orbit allowed the background-limited criteria to be met only 50 percent of the time. It is accordingly recommended that the 200-micron background-limited observations be made only for a limited portion of the mission, while meeting the 100-micron limit at all times.