Abstract
ABSTRACT We report results from a test exploring the long- and short-term astrometric stability of Hubble Space Telescope Fine Guidance Sensor #3. A test field was observed 40 times over 522 days to determine the precision and accuracy of FGS astrometry and to measure the character and magnitude of possible secular scale changes. We examine the astrometric data and the associated guide star data to determine random errors. These 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 astrometry within a central 2.5 arc minute FGS field of view of any orientation. We find that the scale varies over time and confirm the sense of the trend with independent data. From the 40 observation sets we produce a catalog of an astrometry test field containing 8 stars whose relative positions are well 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 11 observation sets acquired over 387 days produce parallaxes and relative positions with 1 mas precision.
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