Space Telescope Science Institute
Intro to HST Data Handbooks 8.0 May 2011
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Introduction to the HST Data Handbooks > Chapter 5: HST File Names > 5.1 File Name Format

HST data file names encode a large amount of information about the files themselves. Datasets retrieved from the HDA, as described in Chapter 2, consist of multiple files in Multi-Extension FITS format, each with a name that looks like this:
Rootname: The first part of the file name (ipppssoot) is the rootname of the dataset to which the file belongs. All files belonging to a given dataset share the same rootname.
Suffix: The part of the filename between the “_” and the “.fits” is called the suffix (sfx), and it indicates the type of data the file contains. All science instruments except for COS, have data file suffixes with three characters. COS data file suffixes are between three and eleven characters long.
Format: The identifier .fits indicates that this file is in FITS format.
For example, a STIS data file named o8v502010_x1d.fits is a FITS file that belongs to the dataset with rootname “o8v502010”, and its “_x1d” suffix indicates that it contains calibrated science spectra.
GEIS Files
In order to use PyRAF/IRAF/STSDAS tasks to work with data from the instruments FOC, FOS, GHRS, HSP, WF/PC-1, and WFPC21 you will want to convert these waivered FITS files into GEIS format. See Section 2.2 for instructions on how to convert FITS files to GEIS files using strfits. Like MEF format files, the names of GEIS files also derive from a file’s rootname and suffix, and they have the form:
Generally the suffixes of GEIS files end either in “d”, indicating a binary data file, or “h”, indicating an ASCII header file. The two GEIS files x3l80101t.d0h and x3l80101t.d0d together contain the same information as the single waivered FITS file x3l80101t_d0f.fits.
The identifier referred to here as a “suffix” has often been called an “extension” in the past. However, the individual pieces of FITS files are also known as “extensions” (see Section 2.2.1). For clarity, this handbook will use the term “extension” when referring to a component of a MEF format file and the term “suffix” when referring to the three character identifier in a filename.

Note that the HDA also distributes WFPC2 data in MEF format files and those files do not need to be converted to GEIS format.

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