Space Telescope Science Institute
Intro to HST Data Handbooks 8.0 May 2011
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Introduction to the HST Data Handbooks > Chapter 2: HST File Formats > 2.1 HST Data Files

HST Data Files
The STScI pipeline automatically processes and calibrates all the data received from HST and assembles them into a form suitable for most scientific analyses.
Data from WFPC2, ACS, COS, NICMOS, STIS, and WFC3 are made available to observers as files in Multi-Extension FITS (MEF) format, which is directly readable by most PyRAF/IRAF/STSDAS tasks.
For first generation instruments (FGS, FOC, FOS, GHRS, HSP, and WF/PC-1), the data are in a format known as Generic Edited Information Set (GEIS). Byte ordering for binary data in GEIS files is machine-dependent. Therefore, for purposes of archiving and file transfer, GEIS data are converted to a modified FITS format known as “waivered FITS.” It is necessary to convert waivered FITS files back to GEIS format for data processing and analysis using IRAF/STSDAS tasks.
All WFPC2 data are now available in either waivered FITS or MEF formats. The user may specify either format when retrieving that data from the HDA. WFPC2 data, in either GEIS or MEF formats, can be fully processed with STSDAS tasks.
For older instruments, Heritage Instruments, (FOC, FOS, FGS, GHRS, HSP, WF/PC-1, and WFPC2) data are provided in waivered FITS format, which needs to be converted back to GEIS format before processing. Note that WFPC2 data is also available in MEF format from the HDA. Newer instruments (ACS, COS, NICMOS, STIS, and WFC3) generate and store files in FITS format and should not be converted to GEIS.
Table 2.1: HST Instrument File Formats
Format for STSDAS Analysis

HDA now also stores WFPC2 data in MEF format.

This chapter describes GEIS and MEF formats, in more detail. ACS, COS, NICMOS, STIS, WFPC2, and WFC3 observers should pay particular attention to the section on MEF files, which shows how to identify and access the contents of these files and covers some important conventions regarding header keywords. Users of the heritage instruments should consult the material on data groups and conversion from waivered FITS to GEIS format found in Section 2.3.1 before proceeding to Chapter 3.
Throughout this document, references to FITS files will always mean HST Multi-Extension FITS format files. References to waivered FITS files will always be explicitly stated.

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