FPA annealing studies for the WISE mission cryostat Original Research Article

The wide-field infrared survey explorer (WISE) is a MIDEX mission that is being developed by the Jet Propulsion Laboratory (JPL) to address several of NASA’s Astronomical Search of Origins (ASO) objectives. The WISE instrument, developed by the Space Dynamics Laboratory (SDL), includes a cryogenically-cooled telescope (at <13 K) and four focal plane assemblies (2 at 7.6 K, 2 at 32 K). Cooling of the instrument is accomplished by a dual-stage solid hydrogen cryostat that is developed by the Lockheed Martin Advanced Technology Center (LM-ATC). Experience from Spitzer suggests that it will be required to warm the two Si:As LWIR arrays used on WISE to above 20 K approximately 4-times a day—i.e. anneal the arrays—to re-establish detector sensitivity. For SFHe systems, detectors are typically vapor cooled. As a result, the warming associated with the annealing process is only indirectly coupled to the cryogen. However, since the WISE focal plane assemblies are conduction coupled directly to the solid hydrogen tank, the potential exists for system impacts on lifetime, temperature stability, and mission duty cycle. This paper summarizes the results of some top-level sensitivity studies that characterize the effects of FPA annealing on a solid hydrogen cryostat—using the WISE cryostat and focal plane assemblies as a model. These trades provide a vector for candidate system level changes to the WISE instrument to help mitigate the impact of the annealing process on the cryostat’s performance.