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.
Transfer Scan Mode (TRANS)
- 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.