Instrument Host Overview
  ========================
 
  The IRAS satellite consisted of two main parts, the spacecraft and
  the telescope system. The overall dimensions of the satellite with
  deployed solar panels were 3.60 meters in height, 3.24 meters in width,
  and 2.05 meters in depth. The mass of the satellite at launch was
  809 kilograms.
 
 The satellite attitude was controlled by three orthogonal reaction wheels.
 Excess momentum was dumped by magnetic coils to the Earth's magnetic field
 as necessary. The attitude, and changes in attitude, were sensed by a
 combination of a horizon sensor, a sun-sensor, and three orthogonal gyros.
 The z-axis gyro was used in all modes of control and was duplicated to
 provide a redundant backup.
 
  The telescope system comprised the upper part of the satellite. The
  system was mounted within a toroidal superfluid helium tank, which in
  turn was mounted within the evacuated main shell. The telescope was a
  f/9.6 Ritchey-Chretien design with a 5.5 meter focal length and a
  0.57 meter aperture. The mirrors were made of beryllium and cooled to
  approximately 4 K.
 
  The optical system was protected from contamination before launch and
  during the first week of the mission by an aperture cover cooled with
  supercritical helium.  After the cover was ejected, the sunshade limited
  heat flow to the aperture by blocking direct solar radiation and
  reflecting away terrestrial infrared radiation. The telescope was
  cooled by contact with the superfluid helium tank to temperatures
  ranging from 2 to 5 K.  The surfaces of the sunshade which could be
  viewed by the telescope aperture were cooled by a three-stage radiator
  to about 95 K.
 
  The focal plane assembly contained the survey detectors, visible
  star sensors for position reconstruction, a Low Resolution
  Spectrometer (LRS) and a Chopped Photometric Channel (CPC). The
  focal plane assembly was located at the Cassegrain focus of the
  telescope and was cooled to about 3 K.
 
  The survey array consisted of 62 rectangular infrared detectors
  arranged in staggered rows such that any real point source crossing
  the focal plane as the satellite scanned would be seen by at least
  two detectors in each wavelength band. Most of the detectors in
  each band had standard size apertures, with one or two being half-sized.
 
  The LRS was a slitless spectrometer sensitive from 7.5 to 23
  microns with a resolving power of about 20.
 
  The CPC operated during some pointed observations. It mapped
  sources simultaneously at 50 and 100 microns, and used a cold
  internal chopper for flux reference. However, the focal plane
  temperature was lower than expected, which resulted in CPC detector
  anomalies that rendered CPC data very difficult to use.
 
  For more information about the instrument, see Beichman et al. 1988
  [BEICHMANETAL1988] and Neugebauer et al. 1984 [NEUGEBAUERETAL1984].