Due to the thermal short between a cold baffle and the warm filter wheel housing, the solid nitrogen coolant in NICMOS evaporated at a higher than expected rate, resulting in a reduced lifetime. NICMOS reached cryogen exhaustion on 3 January 1999. On 6 January 1999, the temperatures for all 3 cameras exceeded the limit of 78.1 K and the filter wheel was commanded to the blank position.

During the third Servicing Mission (SM3b) in February 2002, a closed-loop cooling system (NICMOS Cooling System, NCS) will be installed to fully restore NICMOS science capabilities. The detectors under NCS operation will be at around 77 Kelvin, significantly warmer than the range of temperatures experienced during nominal operations. The different operating temperature will cause changes in quantities like dark current, quantum efficiency, and possibly focus position. In order to estimate NICMOS performance during the NCS era, a monitoring program was executed to follow the detector characteristics during the EOC warmup of the instrument.

• Proposal 7961 (pure parallel) took lamp flats four times a day in a number of filters in all three cameras. The goal was to follow the QE variations as a function of temperature and wavelength to provide sensitivity estimates under NCS. ASCII data tables of the relative DQE vs temperature are available for the following cameras and filters:
  • NIC1, F110W
  • NIC1, F160W
  • NIC2, F110W
  • NIC2, F160W
  • NIC3, F110W
  • NIC3, F160W
  • NIC3, F222M
• Prop. 7962 (pointed) observed a star cluster twice a week in order to monitor possible focus variations due to varying mechanical stresses in the dewar.
• Proposals 7963 and 8093 (internal) took darks in all three cameras once every orbit which was not affected by the South Atlantic Anomaly (SAA) or used for one of the other two monitoring proposals. This not only allowed us to monitor the dark current of the detectors, but also to check for any temperature-induced electronic effects.

All three programs were activated Nov. 15th and were continued until 11 January 1999 (however, the BLANK filter was in place after 6 January 1999).

The results of these programs are summarized in ISR-99-001 (PDF format). The information has also been used to refine the NICMOS instrument handbook for Cycle 11, so that NICMOS users have up-to-date information regarding the expected performance of NICMOS after the NCS installation.