Initial Evaluation of the GP-B Cryogenic Telescope Performance in Orbit

The Gravity Probe B spacecraft was launched on April 20, 2004 to measure the geodetic and frame dragging effects predicted by the theory of general relativity. A cryogenic optical telescope is used to establish the inertial reference frame for the measurements by star tracking. With a year of flight data collected, we describe the initial evaluation of the optical performance of the telescope. We find that the pointing noise is sufficiently low to support the mission and in reasonable agreement with extrapolations from ground tests. Our results show that appropriately designed precision optics can be operated over the temperature range from 300 to 3.5 K with little degradation.