The Gravity Probe-B star-tracking telescope Original Research Article

We describe the design, development, and testing of a cryogenic star-tracking telescope that provides the inertial pointing reference for the Gravity Probe-B gyroscopes, as established by a distant guide star. The telescope has a modified Cassegrainian configuration with an aperture of 14.4 cm and an effective focal length of 3.8 m, and operates at 2.5 K. It consists of high-grade fused-quartz components silicated together to maximize strength dimensional stability. The star image is split at the focal plane to provide quadrant pointing information for subtraction from the gyro readout signal and for spacecraft attitude control. We also briefly describe the status of the telescope readout electronics and the guide star selection.