After NICMOS was installed in HST, the dewar was planned to warm up to about 57 K, a temperature never reached during ground testing. The ice expansion caused by this temperature increase resulted in an additional dewar deformation, to the extent that one of the (cold) optical baffles made mechanical contact with the vapor-cooled shield (VCS). The resulting heat flow caused the ice to warm up beyond expectations, to about 60 K, which in turn deformed the dewar more. The motion history of NICMOS and the resulting image quality are discussed in Chapter 4 and a more detailed history of the dewar distortion can be found at
http://www.stsci.edu/hst/nicmos/design/history
This unexpectedly large deformation had several undesirable effects, the most important of which are:
The thermal short increased the heat flux into the inner shell (and therefore the solid nitrogen) by a factor of 2.5 and thereby reduced the lifetime of NICMOS from 4.5 to ~ 2 years. The cryogen depleted in January 1999, and NICMOS was unavailable for science operation between January 1999 and June 2002, when the NICMOS cooling system was activated and reached expected operating temperatures.
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The installation of the NICMOS Cooling System (NCS), a mechanical cryocooler has re-enabled NICMOS operation, and restored infrared capability to HST. The NCS is capable of cooling the NICMOS dewar to temperatures 75-86 K, significantly higher than during Cycle 7. So far, the temperature control is good enough to keep the detector temperature to 77.1+/-0.07 K. Therefore, many NICMOS parameters are different from Cycle 7. Most notably the detector quantum efficiency (DQE) has increased by ~30-50%. Users should pay close attention to the new NICMOS performance which is discussed in Chapter 4, Chapter 5, and Chapter 7. |
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