AAS 198th Meeting, June 2001
Session 4. Instruments: Real and Proposed
Display, Monday, June 4, 2001, 9:20am-6:30pm, Exhibit Hall

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[4.05] Ultra High Angular Resolution and sub-mas Astrometry with HST's FGS1r

M. Lallo, R. B. Makidon, D. Jong, E. Nelan (STScI)

The 3 Fine Guidance Sensors (FGS) - white-light shearing interferometers - are critical to the mission of the Hubble Space Telescope (HST) by providing highly accurate guiding for the observatory. Moreover, FGS1r in particular is a capable science instrument. Its two observing modes - Position Mode and Transfer Mode - support wide angle and narrow angle astrometry respectively. In Position Mode, a star's interferometric fringes are tracked to determine its angular position relative to other stars in the detector's field of view. Using this method, a star's parallax, proper motion, and reflex motion can be measured with a precision of about 1 mas per observation, while multi-epoch observing programs have yielded astrometry with accuracies approaching 0.2 mas for objects as bright as V=3 or as faint as V=16.5.

Transfer Mode observations repeatedly scan an object's interferometric fringes to achieve sub-mas sampling of the fringe morphology with high signal-to-noise (conceptually analogous to imaging with a 1 mas pixel array). Post-observation analysis allows the measurement of angular separation, position angle and relative brightness of binary components, or a determination of the angular size of an extended object. Close binary systems with V < 12 can be detected down to 7 mas, while systems as faint as V=15 can be characterized to 12 mas, provided the magnitude difference between the components is less than about 2. (Wider systems with magnitude differences as large as 3.5 can be resolved.)

Both FGS observing modes can be utilized to derive the total and fractional masses of binary systems, and thus the mass-luminosity relationship of the binary components. The FGS have also been used to observe and characterize non-point source objects, including Mira variables, asteroids, and active galactic nuclei, yielding information on the structure of these objects on scales as small as 8 mas.

The FGS also function as 40 Hz photometers, offering milli-magnitude precision for relative photometry at 1 second intervals (V=10), a capability which has allowed the FGS to study flare stars and stellar occultations by the Neptunian moon Triton.

In this paper we present examples for each of these capabilities and compare FGS results to those of ground-based interferometers, astrometric observing facilities, and to HST's own WFPC2 imager.

STScI invites you to consider using the FGS as a science instrument in Cycle 11.

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