The optical path through an FGS is complex because the beam passes through multiple optical elements. The relative alignment of all these components and the wavelength dependencies introduced by their reflective surfaces and refractive optics impose the resolution and magnitude limits of the FGS. Most of the FGS calibration procedure consists of empirical and semi-empirical subtraction of the instrument's signature, necessitating observations of standard stars in various spectral ranges and all modes of observation.
The three FGSs on board HST occupy three of the four radial bays. Normally two FGSs are used for pitch, yaw, and roll control of the telescope, leaving one available for astrometry. The telemetry from the guiding FGSs is captured and processed by the ground system and stored in the Hubble Data Archive. The processed FGS data can be retrieved for any HST observation in the form of OMS files (see Appendix B), which provide information on spacecraft jitter. However, the data files associated with an FGS astrometry observation, which are used in the FGS calibration pipeline, record jitter information at a much higher time resolution (40 Hz as compared to 0.33 Hz).
An FGS can be used for astrometry in two different modes:
FGS 3 is currently designated and calibrated as an astrometer. This FGS can measure point-source angular positions of 1-2 mas over the brightness range of 3 to 17 magnitudes and can resolve the components of binary systems with separations as small as 15 mas. For many scientific studies, this FGS continues to exceed ground based efforts in sensitivity, dynamic range, and resolution.