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.
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.
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 temperature2.6 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