3.1 Preparing FGS Data for Processing

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".

Image Reduction and Analysis Facility
PACKAGE = fitsio
   TASK = strfits
fits_fil=               *.fits  FITS data source
file_lis=                1-999  File list
iraf_fil=                       IRAF filename
(templat=                 none) template filename
(long_he=                   no) Print FITS header cards?
(short_h=                  yes) Print short header?
(datatyp=              default) IRAF data type
(blank  =                   0.) Blank value
(scale  =                   no) Scale the data?
(xdimtog=                  yes) Transform xdim FITS to multigroup?
(oldiraf=                  yes) Use old IRAF name in place of iraf_file?
(force  =                  yes) Force conversion from fits?
(offset =                    0) Tape file offset
(Version=          23-Mar-1998) Strfits version
(mode   =                   ql)

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 Website) 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).
Table 3.1: GEIS Files in an FGS Dataset

When this simple set up is complete, calfgsa can be executed.