FGS Astrometry observations are analyzed at three distinct levels, the exposure-level
(individual observations), the visit
(all observations within the HST orbit), and the epoch-level
(relating data from one visit to others). The astrometry data pipeline processes the observations up to and including the visit level. Epoch-level analysis requires tools beyond the scope of the FGS data pipeline.
The exposure- and visit-level corrections and calibrations are performed by the observer using calfgsa
and calfgsb. calfsga is a standalone executable (currently supported on Unix operating systems, including MAC OSX), while calfgsb
is implemented as tasks in STScI’s STSDAS system. These tasks are semi-automated and require little user input to process the individual exposures that comprise the typical astrometry visit. Reference files used by these tasks are maintained by STScI and can be found by following the links to the calibration sections of the FGS Web Site at