The fine guidance sensors aboard the Hubble Space Telescope, the scientific capabilities of these interferometers

The Fine Guidance Sensors (FGS) aboard the Hubble Space Telescope (HST) are optical white light shearing interferometers that offer a unique capability to astronomers. The FGSs's photometric dynamic range, fringe visibility, and fringe tracking ability allow the instrument to exploit the benefits of performing interferometry from a space-based platform. The FGSs routinely provide HST with 2 milli-seconds of are pointing stability. The FGS designated as the Astrometer, FGS3, has also been used to (1) perform 2 mas relative astrometry over the central 4 are minutes of its field of view, (2) determine the true relative orbits of close (20mas) faint (m(v)=15) binary systems, (3) measure the angular diameter of a giant star, (4) search for extra-solar planets, (5) observe occultations of stars by solar system objects, as well as (6) photometrically monitor stellar flares on a low mass M dwarf. In this paper we discuss this unique instrument, its design, performance, and the areas of science for which it is the only device able to successfully observe objects of interest.