Abstract
This paper presents an overview of the current status of astrometry with the Fine Guidance Sensors (FGSs) on the Hubble Space Telescope. The FGSs have two astrometric modes of operation, one for positional (POS) and one for transfer function (TRANS) astrometry. The positional mode is intended for parallax and angular velocity work. Owing to optical aberrations in the Optical Telescope Assembly (OTA) (i.e.,the main telescope optics) beyond the spherical aberration, and instabilities within the combined OTA/FGS optical system, it is undergoing additional feasibility review. Preliminary engineering tests showed changes in stellar pair positions of ~ 100 milli-arc seconds which are far too large to allow for the expected ~ 1 mas calibration. Alternate calibration procedures and data reduction algorithms are currently being developed to maximize the scientific potential of this mode which can be fulfilled at the ~ 3 mas level.
Published Version
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