7.1 Data Processing Overview

FGS Astrometry observations are analyzed at three distinct levels, the exposure-level (individual observations), the visit-level (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 website at:
http://www.stsci.edu/hst/instrumentation/fgs.

Epoch-level analysis of FGS data is not, by its nature, a procedure which lends itself to generic pipeline processing. However, tools to provide the observer with some of the more common manipulations encountered in data analysis of FGS astrometry observations are being made available to the general FGS user. Currently, these tools are not STSDAS tasks, but a collection of stand-alone scripts and executable files to achieve plate solutions for Position mode observations and the deconvolution of binary star transfer functions from Transfer mode observations.

The processing and analysis applied at each level is discussed below. Interested readers are encouraged to monitor the STScI newsletter or visit the FGS web page for updates to the status of these tools. More detailed discussions can be found in the FGS Data Handbook version 4.0 or later.