FGS data from the HST Data Archive are stored and distributed in archival "waiver" FITS format, and must be converted back to the machine dependent GEIS format before calibration processing (please see Chapter 2 of the
Introduction to the HST Data Handbooks for more details on FITS and GEIS file formats). It is important to use the
strfits task in
stsdas.fitsio (or in
tables.fitsio) to convert FGS data from FTIS to GEIS formats, as the pipeline which extracts FGS science data from the telemetry stream employs a special convention for mapping GEIS files to FITS format. More detail on using
strfits is presented in Section 2.3.1 of the
Introduction to the HST Data Handbooks.
The strfits parameter file (
fiostrfis.par) specific to FGS GEIS data is displayed below. Note that the scale parameter must be set to "no".
Two separate IRAF/STSDAS tasks are involved in the reduction and calibration of FGS science data. The first,
calfgsa (also known as the "pipeline") takes as input the GEIS files generated by
strfits. The second,
calfgsb, processes the output products of
calfgsa. Note that
calfgsa is used both for Transfer Mode and Position Mode data, while
calfgsb is only useful on Position Mode data.
Before executing calfgsa, all FGS data from an HST visit should reside in the same directory. This includes the six GEIS files for the three FGSs
(the *.aih and *.aid files, where i = 1, 2, or 3) and the support schedule file (the *
.dmh file). The location of the reference files (available from the
FGS Web site) can either be entered in response to prompts from
calfgsa or specified in the
calfgs.par parameter file.
As an example, if an FGS visit to be analyzed comprises five individual astrometry observations, then the local directory where
calfgsa is executed should contain the files shown in
Table 3.1, where PPP and SS are place holders for the actual program_ID and visit ID, respectively. For this example, there should be a total of 35 files (5 observations, each with 7 files).
