A He II-Cryostat for the Far Infrared and Submillimetre Space Telescope

The Far Infrared and Submillimetre Space Telescope is one of ESA's cornerstone missions in the “horizon 2000” programme. One option to cool the scientific instruments of this satellite is a superfluid helium (He II) cryostat. A trade-off analysis was performed about different sizes and configurations of a cryostat such as to meet the operational lifetime requirement of 2 - 4 years on either a highly elliptical 24 hours orbit or on a far-away from earth orbit. The possible size alternatives for a cryostat using to a maximum extent existing ISO-technology are presented. Upon ESA-decision a final cryostat size for the 24 hours orbit which provides at least 3 years operational lifetime was selected. This cryostat is described and presented.