Pre- and Post-Launch Performance of the NICMOS Dewar

Abstract

Approximately 14 months prior to launch, the NICMOS solid nitrogen dewar was filled with its flight load of cryogen and successfully completed subsystem verification testing. The dewar remained filled with solid nitrogen throughout instrument integration and testing, with periodic recooling of the cryogen to avoid venting. Thermal expansion and possible migration of the solid nitrogen during the recool-warmup cycles resulted in an unexpected shift in detector focus positions, revealed during an instrument-level optical system test. The results of an investigation of this anomaly are discussed. The instrument successfully completed system calibration and verification testing, and the cryogen sublimation rate measured during calibration was within 10% of the thermal model estimate. Subsequent recool-warmup cycles indicated that the expansion had stabilized at an acceptable level. However, shortly after launch and installation into HST, the solid nitrogen expanded further than before, moving one of the three detectors out of focus adjustment range. This expansion also caused physical contact of optical baffle segments, and a resulting large increase in the cryogen heat load and sublimation rate. Recent trends in the detector motion and heat load are discussed.