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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.

The ISO-satellite consists of a Service and a Payload Module (PLM), the latter being essentially made up of the Cryostat. The paper describes essentially the hardware status of the PLM-flight model as it is delivered for final satellite integration and acceptance testing before flight. The various test activities and essential results obtained during development, qualification and PLM-flight model acceptance up to delivery to ESTEC are summarized.