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Hubble Space Telescope

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.

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.

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.