The infrared space observatory

The Infrared Space Observatory (ISO), a programme of the European Space Agency, is an astronomical satellite operating at wavelength from 2.5 to 200 m. It will be launched in 1995.The ISO optical subsystem is a cryogenically cooled telescope with its baffling system (main baffle and sunshade). The telescope, a 60 cm Ritchey-Chrétien type, focuses the beam to the four scientific instruments located in its focal plane. The extremely low temperature, 1.8 K, is provided by the payload module (PLM) cryostat, filled with superfluid He.This paper presents the main choices done for the telescope design together with their rationale and the performances achieved on the flight model (FM) of the telescope. The FM telescope is presently installed inside the payload module, ready for the system final verifications.