With a white-light interferometer (Fine Guidance Sensor 3) on the Hubble Space Telescope (HST) we have secured fringe scanning and fringe tracking observations to measure distances, orbits, and, hence, masses, for several nearby low-mass stars. We have made progress towards a more precise Mass-Luminosity Relation (MLR) for the lower Main Sequence. However, the MLR is a map whose low mass region is complicated by relative and absolute age and whose high-mass end is very poorly determined. To begin to disentangle these effects, and to obtain high-precision mass determinations throughout the Main Sequence, we will participate in the Space Interferometry Mission (SIM) to observe binary stars of all masses in five star clusters with a large range of well-known ages and chemical compositions. We will also observe a sample of stars throughout the Main Sequence. The unparalleled angular resolution and limiting magnitude of SIM will allow us to obtain masses precise to 1%.
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 FGS's photometric dynamic range, fringe visibility, and fringe tracking ability allow the instrument to exploit the benefits of performing interferometry form a space-based platform. The FGSs routinely provide HST with 2 milli-seconds of arc pointing stability. The FGS designated as the Astronomer, 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 faint 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 mas 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.
G. Benedict, Barbara McArthur, Edmund Nelan, William Jefferys, Otto Franz, Larry Wasserman, Darrell Story, Peter Shelus, A. Whipple, Arthur Bradley, R. Duncombe, Q. Wang, Paul Hemenway, William van Altena, Laurence Fredrick
Space-based interferometry already exists. We describe our experiences with on-orbit calibration and scientific observations with Fine Guidance Sensor 3 (FGS 3), a white- light interferometer aboard Hubble Space Telescope. Our goal, 1 millisecond of arc precision small-field astrometry, has been achieved, but not without significant challenges. These included a mechanically noisy on-orbit environment, the self-calibration of FGS 3, and significant temporal changes in our instrument. Solutions included a denser set of drift check stars for each science observation, fine- tuning exposure times, overlapping field observations and analyses for calibration, and a continuing series of trend- monitoring observations. HST FGS 3 will remain a competitive astrometric tool for faint targets in crowded fields and for faint small-separation binaries until the advent of large- aperture, ground-based and longer-baseline space-based interferometers.
KEYWORDS: Stars, Control systems, Prisms, Servomechanisms, Calibration, Monochromatic aberrations, Signal attenuation, Space operations, Mirrors, Interferometers
The three fine guidance sensors on-board the Hubble Space Telescope are the first white-light amplitude shearing interferometers to be used for space platform guidance, control, and astrometry. Two fine guidance sensors (FGS) under fine lock control now maintain spacecraft pointing precision to within 7 milliseconds of arc rms over the majority of each orbit. Fine guidance sensor control optimization techniques have yielded significant improvement in tracking stability, integrated performance with the pointing control system, loss-of-lock statistics and astrometric accuracy. We describe the optical interferometer, based on the Koester's prism design. We include a discussion of the instrument calibration status, the FGS fine lock performance design enhancements, pointing control system design enhancements, and ground software techniques appropriate to jitter removal in astrometric data. The combination results in marc sec precision astrometry.
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