NICMOS Cooling System
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.
During Servicing Mission 3B, currently scheduled for February 2002, a closed- loop cooling system (NICMOS Cooling System, NCS) will be installed to fully restore NICMOS science capabilities. Currently, the NCS circulator is being refurbished by Creare, Inc. In June, it is expected to be returned to Goddard Space Flight Center and integrated into the NCS.
- NICMOS Cryocooler Independent Science Review (ISR) Report March 1999
- NICMOS Cutaway
- Filter Wheel Mechanism
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 suite of programs were executed to monitor the detector characteristics during the End Of Cryogen (EOC) warmup of the instrument; the programs were activated Nov. 15th 1998 and continued throug EOC.
- Program 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.
- Program 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) will took DARKS in all three cameras once during each 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.
The results of these programs were summarized in ISR-99-001-REVISION and used to refine the NICMOS Instrument Handbook V 4.1 for use in Cycle 11, so that proposers have the best possible information regarding expected NICMOS performance after the NCS installation.