ASTROMETRY WITH HUBBLE-SPACE-TELESCOPE FINE GUIDANCE SENSOR NUMBER-3 - POSITION-MODE STABILITY AND PRECISION

We report results from a test exploring the long- and short-term astro metric stability of Hubble Space Telescope Fine Guidance Sensor #3. A test field was observed 40 times over 522 days to determine the precis ion and accuracy of FGS astrometry and to measure the character and ma gnitude of possible secular scale changes. We examine the astrometric data and the associated guide-star data to determine random errors. Th ese data are also explored to find sources of systematic error. After correcting for some systematic effects we obtain a precision of 0.002 arcsec (2 mas) per observation (RSS of x and y). This is relative astr ometry within a central 2.5 arcmin FGS field of view for any orientati on. We find that the scale varies over time and confirm the sense of t he trend with independent data. From the 40 observation sets we produc e a catalog of an astrometry test field containing eight stars whose r elative positions are known to an average 0.7 and 0.9 mas in x and y. One reference star has a relative parallax of 3.1 +/- 0.5 mas. Finally , we report that eleven observation sets acquired over 387 days produc e parallaxes and relative positions with 1-mas precision.