NICMOS was installed onboard the HST during the second Servicing Mission (SM2) in February 1997. 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. Also, the internal optical alignment and focus mechanism (the Pupil Alignment Mechanism PAM) was replaced with a version providing twice the focus range and a demonstrated capability for frequent movement. The PAM, originally intended to align the input beam onto the corrective optic and to bring NICMOS into parfocality with the WFPC2 (the only HST instrument without an internal focus mechanism), would be used to support a unique focus setting for each NICMOS camera and to switch between them routinely. 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.
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 order to enable execution of Cycle 7 programs that required the use of camera 3, two NIC3 campaigns were performed in January and June of 1998. These were periods of 2-3 weeks during which the HST secondary mirror was adjusted to bring the focus back into the NIC3 PAM range. During this time, HST performed exclusively NIC3 science, since no other HST instrument was in focus. While it was initially hoped that the NIC3 focus would eventually return to within the PAM range, we no longer expect this to happen. However, at the maximum PAM position, the degradation in terms of encircled energy at a 0.2" radius is only 10-15%. This is considered sufficiently small, and NIC3 will be offered as is in Cycle 11 and future Cycles.
- 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 has since been unavailable for science operation. To enable completion of the NICMOS science program despite the shortened lifetime, NASA and STScI adjusted the HST scheduling in such a way that NICMOS observations were assigned 40-50% of the total observing time in Cycle 7. Moreover, a second Call for Proposals (CP) for additional NICMOS science was issued in summer 1997. The proposals were put through the peer review process, and the full science program was executed before the cryogen depletion.
Following the recomemndations of the Independent Science Review committee, NASA has developed the concept of the NICMOS Cooling System (NCS) in order to restore and conserve an infrared capability on HST. The NCS is a mechanical cryocooler that will reenable NICMOS operation by cooling the instrument to temperatures around 75-86 K. This range is cool enough to allow operation of the NICMOS detectors, but is significantly higher than during Cycle 7 science operations. Therefore, many NICMOS parameters will be different from Cycle 7. In order to estimate the impact of the higher operating temperature on NICMOS sensitivity, a large monitoring program was executed during the instrument warmup following the cryogen exhaustion. The execution, data products, and results of the warmup analysis are described in detail in NICMOS ISR 99-001. A thorough discussion of a dark current anomaly observed during the warmup can also be found in this paper.
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.