Cryogenic/thermal system for the SIRTF cryogenic telescope assembly

The Cryogenic Telescope Assembly (CTA) houses the SIRTF Science Instruments and provides them a 1.3 K temperature heat sink. It also provides the telescope, which is maintained at 5.5 K temperature in order to achieve the low photon background required for the 160 micron detector array. This unique cryogenic/thermal system provides the necessary cooling through passive means along with use of vapor cooling from the helium gas vented from the 360 liter superfluid helium cryostat. The cryostat vacuum shell temperature is low enough that the heat load to the helium reservoir is due almost entirely to instrument power dissipation, thus resulting in a predicted lifetime over 5 years. The corresponding helium loss rate is over 7 times lower than achieved by previously flown helium-cooled instrument systems, such as IRAS, COBE, and ISO. This extraordinary performance is made possible by the highly favorable thermal environment achieved in an Earth-trailing solar orbit at a distance of about 0.3 AU from the Earth. Attaining this outer orbit with the slight lift capacity of a Delta-II launch vehicle is made possible by the mass-saving approach of having the telescope outside the cryostat and warm at launch. The general end-to-end system architecture, verification approach, and predicted performance are discussed.