II.C.1 Cryogenics

IRAS Explanatory Supplement
II. Satellite Description
C. Telescope System Overview
C.1 Cryogenics

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Figure II.C.2 Cross-sectional view of main cryogen dewar emphasizing components of insulation system.
larger largest

The telescope cryogenic system provided a 1.8 K thermal sink for controlling the temperatures of the optics and detectors. As shown in Fig. II.C.2, the main cryogen tank was toroidal in shape and surrounded the optics and focal plane. Because maximum mission lifetime required isolating the cryogen from external heat loads, the tank was suspended from nine fiberglass straps to isolate it thermally from the exterior main shell. Three shields cooled by venting gaseous helium and 57 layers of multilayer insulation provided additional isolation between the cryogen tank and the main shell. The helium gas left the main cryogen tank through a porous plug made from densely packed sintered stainless steel. The plug allowed vapor to vent while retaining the superfluid liquid. The telescope and focal plane instruments were cooled through the attachment of the optics subsystem to the main cryogen tank near the primary mirror. Heat loads from the aperture were coupled to the venting helium gas by a strap connecting the baffle assembly to a heat exchanger. The gas finally exited to space through two vent nozzles located symmetrically on the dewar exterior.

Table II.C.1 Telescope System Physical Characteristics
Outer shell temperature 195 K
Main dewar capacity 78 kg superfluid helium
Cryogen temperature 1.8 K
Aperture cover dewar capacity 6 kg supercritical helium
Optics, Focal Plane Cryogenic
Aperture cover Cryogenic
Sunshade Passive radiator, heater
Electronics Surface coatings, blankets
Main Dewar Multilayer insulation,
shading, passive radiator
Type Two mirror, Ritchey-Chretien
Mirror material Beryllium
Baffle material Aluminum
Entrance pupil diameter 57 cm
Obscuration diameter 24 cm
Operating temperature 2 to 5 K
Detector, feedback resistor
operating temperature
2.6 K
JFET operating temperature 70 to 80 K
MOSFET operating temperature 2.6 K
Number of detectors 62 infrared 8 visible
power dissipation 14 mW
Construction Modular: 8 infrared subarrays
2 visible subarrays
Preamplifier type trans-impedance amplifier,
one per detector
Number of subassemblies 15
Power consumption 48.3 W
Operating temperature 0 to 15 C
A/D sensitivity 125 µV/data number
Data rates
Engineering 128 bits per second (bps)
Infrared data 5888 bps
Visible data 128 bps
External thermal control 73 kg
Main Liquid helium dewar 432 kg
Liquid helium at launch 73 kg
Optics 72 kg
Focal plane instruments 11 kg
Electronics and cables 90 kg
Structure and Miscellaneous 58 kg
Total 809 kg

Typical operating temperatures of the cryogen tank and exterior shell during flight were 1.8 K and 195 K, respectively. The 73 kg of superfluid helium in the tank at launch gave approximately a 300-day lifetime.

The aperture cover was an independent cryogenic system which included a cryogen tank, multilayer insulation blankets, a vapor-cooled shield, and a balanced vent system. The covert contained supercritical helium and operated between 6 and 15 K. A back-pressure regulator maintains the tank pressure to 37 ± 2 psi. After one week in orbit, the entire cover assembly was ejected from the telescope in preparation for survey observations.

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