Gravity Probe-B (GP-B) is a space mission that was launched in April 2004 that is intended to measure the prediction by General Relativity Theory that a rotating gravitational field, namely the Earth's, "drags" the space-time continuum by a definite amount. GP-B utilizes a telescope with silicon photodiode detectors. Light from a distant reference frame, namely, a star designated as IM Peg, is used to reference the orbital motion of the spacecraft about the Earth and Sun to within 200 milliarcseconds at a frequency of 10 Hz. Fine angular control of the spacecraft orientation uses the signals from the telescope detectors during the 55 minute portion of the orbit during which the star is visible. The performance of the detectors and the control system's resultant pointing are discussed.
The Gravity Probe B Relativity Mission uses a fused-quartz optical star tracking telescope as the sensor for the control system which points the spacecraft towards its guide star. The telescope is cooled to <5 K while the readout which uses photodiodes and JFET preamps operates at 72 K. It is mounted on the front end of the telescope with a thermal standoff. Analysis indicates that the telescope is capable of providing sub-milli-arc- second (marcs)pointing stability information with an angular pointing noise of (formula available in paper) for the guide star IM Pegasi. We describe the design of the telescope and test results under nominal operating conditions. Analysis of the expected performance of the telescope in flight, based on the test results, is also presented.