Satellite Status

Cutaway diagram of ISO showing main components

Over the past few years, there have been a number of technical problems with the satellite development which have caused a delay in the launch date to September 1995. These have now been overcome and testing of the flight model is well underway.

The main problem areas have been:

• Cryogenic Valves: The liquid-helium sub-system contains a number of valves which need to operate over a temperature range from 350K down to 2K and to be "tight" to superfluid liquid helium. The valves are almost exclusively used during ground testing but there are some mission-critical actuations during the powered part of the Ariane flight. During the development phase, there were problems with leakages through the valves. By a long process of re-design and extensive test, these difficulties were overcome and flight-worthy valves were integrated into the flight-model cryostat in autumn 1993.
• Contamination: During assembly and test, the optics of the telescope became contaminated leading to a complete rebuild of the flight model telescope with a new primary mirror.
• Straylight: It is required that the thermal self-emission straylight from ISO reaching the IR detectors must be less than 10% of the expected minimum astronomical background at all wavelengths. Following concerns raised by the PI teams, there was an intensive engineering effort to close all potential straylight paths to the instrument enclosure. In February 1994, an extra test - the Dark Background Test - was carried out to check the self-emission straylight properties of ISO. For this test, the scientific instruments were used and the ISO telescope was closed with a special blinding plate at a temperature of 5K. From the results of these tests ISO is expected to meet its self-emission straylight specifications at all wavelengths.

The testing of the flight model service module has been successfully completed and it is in storage awaiting the cryostat. The flight-model scientific instruments and telescope have been integrated with the flight model cryostat and tested. Following these tests, the focal plane unit of ISOPHOT will be replaced. System-level integration will start in ESTEC in June 1994 with the satellite acceptance review foreseen for early 1995.

The industrial prime contractor for ISO is Aerospatiale, F. DASA. D, are responsible for the payload module.