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D. Jong, E. Nelan, R. Makidon (STScI)
The Fine Guidance Sensors (FGS) support the Hubble Space Telescope (HST) and its myriad of instruments by providing highly accurate guiding. However, the FGS are themselves capable of very impressive science through observations in two modes, POSITION and TRANSFER, for wide angle and narrow angle astrometry, respectively. In POSITION mode, a star’s interferometric fringes are tracked to precisely determine its location in the detector’s field of view. Using this method, parallax and proper and reflex (if the star has a massive companion) motions can be measured to a per-observation accuracy of about 1mas. Over multiple epochs, the accuracy approaches 0.2mas. TRANSFER mode observations are executed by repeatedly scanning an object’s fringe pattern to obtain the necessary S/N, resulting in an interferogram with sub-mas sampling. Post-observation, these scans are analyzed and compared to point source calibration fringes to identify binary systems and to measure the angular separation, position angle and relative brightness of their components. Close binary systems can be detected down to about 7mas, for V < 12, and 12mas for V=15.
Observations taken in the two modes can be combined for deriving the total and fractional masses of binary systems, and subsequently the mass-luminosity relationship of the binary components. The FGS are also capable of non-point source observations. In the past, they have been used to determine the angular diameter of a stellar disc, and have also observed asteroids and planetary moons, measuring their angular sizes down to about 8mas. Furthermore, the FGS are also an excellent 40 Hz photometer, offering milli-magnitude accuracy for relative photometry at 1 second intervals (V=10). This technique was used to study the thermal structure of the Neptunian moon Triton when it occulted a star.
During SM2 (1997), FGS1 was replaced with a refurbished sensor, FGS1r. Its improvements include an internal mechanism capable of tip/tilt articulation, to compensate for the Telescope’s spherical aberration. Because of this feature, FGS1r was designated as the HST astrometer in 1999, replacing FGS3. Its performance is expected to be as good or better than that of FGS3, and preliminary results support this conclusion.
Thanks to Space Telescope Science Institute for providing the necesary funding.
The author(s) of this abstract have provided an email address for comments about the abstract: jong@stsci.edu