HST Fine Guidance Sensors

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


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

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.