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.
The other operational mode-primarily intended for work on double stars but also usable for angular diameter determination-has proven more stable. Even the deleterious effects of the thermally induced solar array jitter have mostly been overcome. A large collection of reduction and analysis software is available to support this measurement technique (Lattanzi, Bucciarelli, Holfeltz, and Taff 1992) and its calibration is proceeding apace. Interesting scientific work from the FGSs on bright Hyades binaries and the highly eccentric double star ADS 11300 has already been published (Franz et al. 1991, 1992).
Finally, a new FGS Instrument Handbook (Taff 1992a) has been prepared which should minimize the complications of dealing with the General Observer Proposal Forms.