Optics
The Fine Guidance Sensors aboard HST are the only readily available white-light interferometers in space. The interfering element is called a Koester's Prism. Each FGS contains two, one each for the x and y axes, as shown in the optical diagram. In 1992 we carried out a series of tests on all three FGS and chose FGS 3 to be the primary astrometer.

Operational Modes

• Fringe Scanning or Transfer Scan Mode (TRANS)
  • In this mode, FGS 3 scans over a star image, generating a fringe (a transfer function scan) along each axis. A binary star will generate a linear combination of two transfer response functions. By comparing the TRANS data for a single star with that of a binary, we can determine separation, magnitude difference, and position angle information.
• Fringe Tracking or Position Mode (POS)
  • In this mode, FGS 3 locks onto and reports (40 times a second) the position of the null crossing point of the interferometer (the zero-crossing point in the fringe or transfer function).

HST Primary Mirror Misfigure
While the misfigure of the HST primary mirror has not rendered astrometry impossible with HST, it certainly has adversely affected performance. Compare the nearly perfect transfer function along the Y-axis with the corresponding X-axis transfer function. The additional structure seen in the X-axis makes it far more difficult to detect close binary stars. Additionally, slope changes near the null crossing render it a less precise POS mode device.

The New Fine Guidance Sensor (FGS 1r)
During the second HST servcing mission (December 1999) one of the FGS units was replaced with a refurbished and updated ground-test unit. This FGS has an articulated mirror which can minimize the effects of optical misalignments. It produces high-quality fringes on both the X and Y axes. FGS 1r should provide better POS and TRANS astrometry than did FGS 3.

• A Parallax of the Fundamental Distance Calibrator RR Lyrae
• A Parallax of the TV Columbae
• An Extrasolar Planet Search
• Cleaning Up the Hyades Color-Magnitude Diagram

Parallaxes
[6876] - Parallax of RR Lyrae
[6878] - Parallax of NGC 6853
[6875] - Parallax of delta Cephei

Binary Star Astrometry
[8728] - Companion to W922
[6885] - L726-8/UVCeti