The Hubble Space Telescope guidance and astrometric instrument, the Fine Guidance Sensor, is reviewed. We will discuss (1) the interferometric design of the instrument and its usefulness to astrometry, (2) the astrometric observing modes and the software commands used to implement them, and (3) the expected state of the spacecraft during an observation with respect to the line-of-sight jitter, drift, and guidance mode.
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 spacebased platform. The FGSs routinely provide HST with 2 milli-seconds of arc pointing stability. The FGS designated as the Astrometer, FGS3, has also been used to (1) perform 2 mas relative astrometry over the central 4 arc 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.
The observed and measured on-orbit performance of various aspects of the fine guidance sensors is presented and discussed in the light of the original requirements and predictions. The fine guidance sensors are shown to meet or exceed the original requirements concerning dynamic pointing errors, photometric repeatability, and moving-target tracking capability. Calibration accuracy has been sufficient for observations to date, and fine-lock acquisitions are approaching a 100% success rate. Improvements to the fine-guidance-sensor tolerance of telescope spherical aberration, the South Atlantic anomaly, and solar-panel vibrations have been made, and further improvements are expected.
The Hubble Space Telescope (HST) is an orbiting astronomical observatory, designed to operate as close as possible to ground based instrumentation, given the limitations of operating in a low earth orbit. The spacecraft design had to accommodate an absolute pointing accuracy of 0.01 arc seconds, a relative pointing stability of 0.007 arc seconds rms, the capability to maneuver 90 degrees in 18 minutes, and operate autonomously in a safemode control scheme for up to 72 hours. Furthermore, the design had to provide for a flexible, stored command methodology, and real-time command capability. This paper briefly reviews the spacecraft engineering hardware and software design. A detailed critique of the on-orbit performance of the spacecraft is provided. Enhancements and work-arounds, which have enabled HST to continue implementation of a successful science plan, are explained.
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