HST Data Handbook for WFPC2

Chapter 2:
HST File Formats

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2.1 Historical Perspective
2.2 FITS File Format
    2.2.1 Working with FITS Image Extensions
    2.2.2 Working with FITS Table Extensions
2.3 GEIS File Format
    2.3.1 Converting FITS to GEIS
    2.3.2 GEIS Data Groups
    2.3.3 Working with GEIS Files
    2.3.4 The "waiver" FITS format

STScI automatically processes and calibrates all the data received from HST. The suite of software programs that performs this processing-part of a system known as OPUS-is frequently called the pipeline, and its purpose is to provide data to observers and to the HST Data Archive in a form suitable for most scientific analyses. Pipeline processing assembles data received from HST into datasets, calibrates the data according to standard procedures described in the instrument sections of this handbook, and stores both calibrated and uncalibrated datasets in the Archive.

Pipelines of older instruments (FOC, FOS, FGS, GHRS, HSP, WF/PC-1, and WFPC2) generate files in the so-called GEIS (stands for Generic Edited Information Set) format. Since GEIS is a machine dependent format, these files are converted to a specific kind of FITS file format, sometimes referred as "waiver" FITS, before being archived. We'll explain the structure of this "waiver" FITS format later in this chapter. Since the "waiver" FITS format is only designed for archival purpose, it is necessary to convert it back to the GEIS format before further data processing and analysis using IRAF/STSDAS tasks.

Instruments installed after the 1997 servicing mission (STIS, NICMOS, ACS, and most likely all future instruments) have pipelines which generate FITS files directly. They are ready to be used by relevant IRAF/STSDAS tasks and, unlike the "waiver" FITS files, do NOT need to (and indeed, should not) be converted to GEIS format. Sometimes FITS files for the newer instruments are referred to as "FITS with extension" or "extended" FITS files. But this can be misleading, since a "waiver" FITS file also has one (ASCII table) extension.

Much confusion has occurred about the two kinds of FITS files been archived at STScI. So we like to repeat one more time:

Older instruments (FOC, FOS, FGS, GHRS, HSP, WF/PC-1, and WFPC2) generate files in GEIS formats, but are stored and delivered as "waiver" FITS format in the archive, and need to be converted back to GEIS format before processing. Newer instruments (STIS, NICMOS, ACS) generate and store files in FITS format and should not be converted to GEIS.

This chapter describes these two HST file formats, first giving some historical perspective on the reasons why they were selected, then explaining the FITS and GEIS formats in more detail. STIS,ACS, and NICMOS observers should pay particular attention to the section on FITS files, which shows how to identify and access the contents of these files and covers some important conventions regarding header keywords. Veteran observers with the other instruments will find little new in the section on GEIS files, but newcomers to the older HST instruments should consult the material on data groups and conversion from FITS to GEIS before proceeding to chapter 3 of the HST Introduction.