ASTROMETRIC PERFORMANCE-CHARACTERISTICS OF THE HUBBLE SPACE TELESCOPE FINE GUIDANCE SENSORS

Each of the three Fine Guidance Sensors for the Hubble Space Telescope constitutes a sixth science instrument to be used for astrometry. We detail the tests and results used in choosing one of the three sensors to be the prime astrometer. The Astrometry Science Team has chosen Fine Guidance Sensor (FGS) 3. FGS 3 produces position measurements on a star with V = 17 with a per-axis precision of 0.003 arcsec. The interferometer response function should permit double-star astrometry at least down to V = 16 for the central region of FGS 3. In a 10-h test of the stability of POS-mode astrometric measurements made in FGS 3, we found no scale or orientation variations greater than two parts in 10(5). During this same time period, we found no statistically significant systematic guide-star radial separation changes. Spacecraft jitter is found to be the prime determinant of astrometry success or failure.