The NICMOS dewar (view schematic) is used to cool the detectors and optical elements of the three NICMOS cameras. Initially following installation onboard HST during Servicing Mission 2 (SM2), February 11, 1997, the dewar contained solid Nitrogen encased in an Aluminum foam. However, due to a thermal short following SM2, the solid Nitrogen was depleted in January 1999. A cryocooler, cirulating a cold Neon gas, will be installed on NICMOS to cool the detectors during SM3B.
Prior to the SM2 launch, an extensive ground testing program was executed, during which the NICMOS dewar was filled with about 240 pounds of liquid nitrogen. The nitrogen was then solidified by passing cold helium gas through a coil located at the aft end of the dewar. This reduced the temperature of the nitrogen to about 40 K. During testing and storage, passive heat inputs caused the block of solid nitrogen to slowly warm up - an expected behavior. To avoid reaching the triple point at about 63 K, the block was recooled approximately every 6-8 weeks, again using cold Helium gas circulating through the aft end cooling coil. During this process, nitrogen gas froze onto the cooling coil. This reduced the vapor pressure at the aft end, effectively pumping gas from the warmer fore end to the aft.
As the dewar was allowed to warm up, the ice at the aft end expanded, pushing into the interior surfaces of the dewar and deforming it. By mid-1996 the three cameras in NICMOS were no longer parfocal although there were good reasons to expect that they would return to a nearly parfocal state after a fraction of the nitrogen had evaporated on orbit. At that time a total deformation of about 4 mm had been observed and steps were taken to assure that the dewar remained flight worthy and that subsequent recooling Cycles did not stretch the dewar further.
After NICMOS was installed in HST, the dewar was planned to warm up to about 57 K. This high a temperature was never allowed to be 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 warmer vapor-cooled shield (VCS). The resulting heat flow caused the ice to warm up even more, to about 60 K, which in turn deformed the dewar more. The thermal short increased the heat flux into the inner shell by a factor of 2.5 and thus reduced the lifetime of NICMOS from 4.5 to 2 years. The cryogen depleted in January, 1999, and NICMOS was unavailable for science until the installation of NCS in March 2002.
In March 2002, the NCS was successfully installed on the NICMOS, and the instrument is now performing even better than it did during Cycle 7. Now operating at a slightly warmer temperature, 77.1 K, the Instrument is more sensitive than it was during Cycle 7 and the temperature is also more constant. Furthermore, the focii for all three cameras have remained stable, and the NIC3 focus has moved in the positive direction relative to its Cycle 7 position, so as to be nearly in focus! Details about the operation of the NICMOS under NCS, as well as recommended strategies for observing proposals can be found in the Cycle 17 Instrument Handbook.