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.
CRYOGENICS | |
---|---|
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 |
THERMAL CONTROL | |
Optics, Focal Plane | Cryogenic |
Aperture cover | Cryogenic |
Sunshade | Passive radiator, heater |
Electronics | Surface coatings, blankets |
Main Dewar | Multilayer insulation, shading, passive radiator |
OPTICS | |
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 |
FOCAL PLANE ASSEMBLY | |
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 |
ELECTRONICS | |
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 |
MASS | |
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.