Part I: Introduction to Reducing the HST Data
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| Part I: Introduction to Reducing the HST Data | |||||
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2.1 FITS File FormatChapter 2:
HST File Formats
2.1.1 Working with FITS Image Extensions
2.1.2 Working with FITS Table Extensions
2.2 GEIS File Format
2.2.1 Converting FITS to GEIS
2.2.2 GEIS Data Groups
2.2.3 Working with GEIS Files
2.2.4 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. For the older instruments (FOC, FOS, FGS, GHRS, HSP, WF/PC-1, and WFPC2) pipeline processing assembles data received from HST into datasets, calibrates the data according to standard procedures described in the instrument sections (Part II) of this handbook, and stores both calibrated and uncalibrated datasets in the Archive. For newer instruments, calibrated data is no longer stored in the archive. The OTFR System generates calibrated data as it is requested from the archive.
Pipelines of the older instruments generate files in GEIS (which 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 to 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, and ACS) 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 archived at STScI. So we would like to repeat this warning one more time:
This chapter describes these two HST file 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.
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
