Contamination measurements during development and testing of the SPIRIT III cryogenic infrared telescope

The 10.6 (mu) bi-directional reflectance distribution function (BRDF) of the SPIRIT III primary mirror was measured after each major phase of sensor development and testing. The compiled BRDF history provides useful insights with respect to the cleanliness levels that may be reasonably expected for a cryogenic infrared sensor of this size and complexity. The use of nitrogen blow-off was demonstrated as an effective, low-risk option for `touching-up' critical infrared optics. The BRDF measured before integration with the MSX spacecraft was consistent with the scatter that would be produced by a surface cleanliness of near Level 100. Follow-on BRDF measurements will not be possible, since the SPIRIT III sensor will remain cold and under vacuum throughout spacecraft processing. An internal cryogenic quartz crystal microbalance has been used to monitor molecular redistribution processes that may occur when internal temperatures change during cryogen refills of other cryo-vacuum operations. The CQCM data is easily understood, and will provide a valuable diagnostic during pre-launch processing of the SPIRIT III cryostat.