2.2 Heating, Cooling and Focus

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:

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 (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%.

 

In September 2008 the NICMOS Cooling System failed to restart after a planned temporary shut-down. The cause is believed to be the accumulation of water ice in the circulator pump housing. Several unsuccessful attempts to restart the NCS, using different procedures, were done in the following months. In December 2008 the circulator was successfully restarted but a problem with the compressor loop forced the NCS to be shut down once again. In January 2009 it was decided to defer further NCS restart attempts to the Servicing Mission 4. As of April 2009, the NCS remains shut-down. Please see the NICMOS web page for the latest updates on the NCS status.

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.