2.2 Heating, Cooling and Focus
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 three cameras have significantly different foci, hence parallel observations are degraded. The difference between the NIC1 and NIC2 foci, however, is sufficiently small that an intermediate focus yields good quality images in both cameras.
- The NIC3 focus has moved outside of the range of the PAM. In an attempt to bring it to within the focus range, the secondary mirror was moved during two brief NIC3 campaigns in Cycle 7. During this time, HST performed exclusively NIC3 science, since no other HST instrument was in focus. Because of the extreme impact on all other instruments, no such campaigns are planned in the future. At the maximum PAM position, the degradation in terms of encircled energy at a 0.2" radius is only 5%. This is considered sufficiently small, and NIC3 will be offered "as is" in Cycle 11 and beyond.
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 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 77.15±0.10 K. Therefore, many NICMOS parameters are different from Cycle 7. Most notably the detector quantum efficiency (DQE) increased by ~30-50%.
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