A CRYOGENIC OPTICAL TELESCOPE FOR INERTIAL POINTING IN THE MILLI-ARC-SEC RANGE

We report the test results of the prototype telescope which is part of a system that provides the inertial reference frame for Gravity Probe B. The fused silica star-tracking telescope, which has an effective f ocal length of 3.8 m at room temperature, experienced a 5 cm focal pos ition shift upon cooling to 4.2 K. A method to compensate this focal s hift is described. The photo sensor for the telescope uses Si PIN phot odiodes and Si JFET preamplifiers, all mounted on a miniature thermal platform to accommodate the approximately 80 K operating temperature o f the JFETs. An electronics noise level of 17 aA/(Hz)(1/2) at 10 Hz ha s been achieved At this noise level, photon noise dominates for 5th ma gnitude candidate guide stars selected for the mission.