HST Fine Guidance Sensors

If this section merely whets your appetite for FGS information, you will enjoy the HST FGS Instrument Handbook.

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

In this mode, FGS 3 scans over a star image, generating 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.
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 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.