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 where the nitrogen temperature was kept around 40K. Passive heat inputs caused the block of solid nitrogen to slowly warm up and it was re-cooled every 6-8 weeks. During this process, nitrogen gas froze onto the cooling coil, reducing the vapor pressure at the aft end. As the dewar warmed up, the ice at the aft end expanded, causing a slight deformation of the dewar. It was expected that this small deformation would disappear after a fraction of the nitrogen had evaporated in orbit. The internal focus mechanism (the Pupil Alignment Mechanism PAM) was replaced with a version providing twice the focus range to accommodate any deviations from parfocality. 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 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:
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 (NCS) was activated and reached expected operating temperatures. The installation of the 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. Therefore, many NICMOS parameters are different from Cycle 7. Most notably, with the higher operating temperature, the detector quantum efficiency (DQE) increased by ~30–50%.
Each camera can be operated in a de-focused mode, where the PAM is
set to a non-optimal position relative to mechanical zero. The offset positions are -3mm, -5mm and -0.5mm for NIC1, NIC2 and NIC3, respectively. This mode can be used in grism and imaging modes and is useful for increasing the efficiency when observing bright objects. The photons are spread over a larger area (more pixels), leading to longer exposure times before saturation is reached. In order to select the de-focus mode, the observer will need to contact the Program Coordinator.