FGS Astrometry in Two-Gyro Mode

The Fine Guidance Sensor 1R on board HST is used by observers as a science instrument for state of the art relative astrometry and for the high angular resolution of close binary systems. The post observation analysis of FGS science data requires the removal, among other things, of spacecraft jitter and drift that occurred during the observations. Under three-gyro mode this technique had been perfected and implemented as part of the FGS astrometry calibration pipeline. Special tests were conducted in February 2005 to assess FGS astrometric performance in twogyro mode. No degradation was noted, and no modifications to the calibration pipeline were found to be necessary. FGS science and calibration data acquired since the transition to two-gyro operations in August 2005 confirm the results of the February test. However, in two-gyro mode the scheduling of observations is more restrictive. This especially impacts parallax programs of distant targets at low to moderate declinations since such fields can be observed at only one of the two epochs of maximum parallax factor.