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Inside of NICMOS
The light detectors of NICMOS are contained in a cryostat
which kept them cool with a large block of Nitrogen Ice. |
A High-Tech Thermos
A schematic of the cyrogenic cryostat. The heat leak which
caused the nitrogen ice to deplete prematurely is shown
on the enlarged diagram. |
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Summary
Just
as a camera for recording visible light must be
dark inside to avoid exposure to unwanted light,
a camera for recording infrared (IR) light must
be cold inside because, in the infrared, warm objects
glow. In addition, the detectors must also be cold
- photoconductive infrared detectors, when warm,
effectively generate their own glow (called dark
current).
To make sure that NICMOS records infrared light
properly, the sensitive infrared detectors in NICMOS
must operate at very cold temperatures - below -321
degrees Fahrenheit, or 77 Kelvin.
The instrument's detectors (less than a square inch
in size) were cooled inside a cryostat (a thermally
insulated container much like a thermos bottle).
When NICMOS was installed, the cryostat contained
a 230-pound block of nitrogen ice.
Soon after NICMOS was operational, an unexpected
heat source was detected and it was apparent the
nitrogen ice would be prematurely depleted in about
2 years rather than the expected 5 years.
To compensate, NASA and the scientific community
agreed to have Hubble perform more NICMOS observations
than originally scheduled in its two years of operation.
The setback to infrared science still remained however,
and a solution was needed.
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