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E.P. Nelan, R. Makidon (STScI), G.F. Benedict, Q.B. McArthur (University of Texas at Austin)
In addition to enabling HST to be pointed with 2 mas precision, HST's FGS1r is used as a science instrument for both narrow and wide angle astrometry. Narrow angle observing is supported by Transfer mode, whereby an object is repeatedly scanned to yield the fringe's morphological details to high SN provide, thereby providing sub-diffraction limited resolution. Wide angle astrometry is enabled by Position mode observing which tracks, in a sequential fashion, the fringes of a scientific target and nearby reference stars distributed within a few arc minutes around the target. Narrow angle observing has resolved binary systems down to 7 mas, while wide angle performance has yielded parallaxes with accuarcies approaching 0.2 mas, for object's as bright as V=3 or as faint as V=16.5.
We discuss some recent science results that have come from HST FGS observations. This includes the determination of M-dwarf masses to 2 mas, and investigations of WR stars. We also discuss some of the ongoing HST FGS programs and the observing techniques being employed by FGS GOs.