ISO's Payload Module (see Figure 3.1) consisted of the helium tank, the telescope and the four scientific instruments -- a camera, ISOCAM, a photo-polarimeter, ISOPHOT, and the two spectrometers, LWS and SWS. Inside the vacuum vessel was a toroidal tank, which at launch was filled with 2286 litres of superfluid helium.
Some of the infrared detectors were directly coupled to the helium tank and were held at a temperature of around 1.8K. All other units were cooled by means of the cold boil-off gas from the liquid helium. This was first routed through the optical support structure, where it cooled the telescope and the scientific instruments to temperatures of around 3K. It was then passed along the baffles and radiation shields, before being vented to space. The cryogenic system enabled ISO observations of nearly 29 months (the design requirement was 18 months). Above the main helium tank was a small auxiliary tank (of volume about 60 litres); this contained normal liquid helium and met ISO's cooling needs on the launch pad for up to the last 100 hours before launch. Mounted on the outside of the vacuum vessel at the entrance of the telescope was a sunshade, which prevented direct sunlight from entering the cryostat. Cooling of the telescope and the instruments to close to absolute zero practically eliminated their thermal emission -- an undesirable `foreground' radiation source -- and enabled observations to be made at high sensitivities.
The telescope itself, located at the
centre of the cryostat, was a Ritchey-Chrétien Cassegrain telescope.
This configuration was deemed to be the most appropriate for
provision of a wide spectral range through a limited field of view,
while remaining free from any coma or spherical aberration effects.
The Ritchey-Chrétien telescope had an effective aperture of
60cm and an f/ratio of 15. The
optical quality of its mirrors was designed to be adequate for
diffraction-limited performance at a wavelength of 5
m.
The optical system consisted of a weight-relieved fused silica primary
mirror and a solid, fused silica secondary mirror with
straylight control via baffles and imposition of viewing
constraints. Stringent
control over straylight, particularly that from bright infrared sources
outside the telescope's field of view, was necessary to ensure that the
system's sensitivity was not degraded. This was accomplished by means of
the sunshade, the Cassegrain and main baffles, and a light-tight shield
around the instruments. Additional straylight control was provided by
constraining ISO from observing too close to the Sun, Earth and Moon.
The scientific instruments were mounted on an optical support structure
(which carried the primary mirror on its opposite side). Each
occupied an 80
segment of the cylindrical volume available. The
20
total unvignetted field of view of the telescope was
distributed radially to the four instruments by a pyramid mirror. Each
experiment received a 3 unvignetted field, centred on an axis at
an angle of 8.5 to the main optical axis, i.e. the instruments
viewed separate areas of the sky.
Data was gathered at the detectors and transferred to the `warm' components in the Service Module for processing before downlink to the ground station.