Two unique aspects of gravity probe-B star-tracking space telescope: (1) focal-plane roof-edge diffraction and (2) fused-quartz bonding for 2.5-Kelvin applications

摘要

Abstract: The Gravity Probe-B cryogenic star-tracking telescopeprovides the inertial pointing reference, asestablished by a distant 'fixed' guide star, withmilli-arc-second resolution per year for theNASA/Stanford general-relativity gyroscope experiment.The design of the f/27 modified-Cassegrain telescopeis briefly reviewed. Then discussed are two of itsunique aspects: (1) focal-plane roof-edge diffractionand (2) fused-quartz bonding for 2.5 Kelvinapplications. The star image in the telescope is splitby roof prisms to generate quadrant pointinginformation within few arc-seconds about the guide-stardirection. The corresponding roof-edge diffractioneffects due to the roof- prisms were compared withtheoretical calculations, which successfullyinterpreted the test result and indicate the need forenlarging post-focal-plane detection apertures, ascompared with ray-trace situation. On the other hand, anovel fused-quartz bonding technique was developed andfound superior in many aspects to index-matchingepoxies, optical contacting, and Corning's proprietaryfrit bonding for both room-temperature and cryogenicapplications. The bonding was simple, economical, yetextremely reliable and optically precise. It resultedin strengths near that of fused quartz, with less than10 nano-meter interface thickness demonstrated. !9