Appendix B:
HST File Names
B.1 Rootnames
B.2 Suffixes of Files Common to All Instruments
B.3 Associations
This appendix describes the syntax of HST data file names, which encode a large amount of information about the files themselves. Datasets retrieved from the Archive as described in Chapter 1 of the HST Introduction (Part I) consist of multiple files in 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 three-character second part of the name (sfx) is called the suffix, and it indicates the type of data the file contains.
- Format: The identifier .fits indicates that this file is in FITS format.
For example, an FOC data file named x3l80101t_d0f.fits is a FITS file belong to the dataset with rootname x3l80101t, and its suffix d0f indicates that it contains raw science data.
In order to use IRAF/STSDAS tasks to work with data from instruments other than NICMOS, STIS, or ACS, you will want to convert these FITS files into GEIS format. See Section 2.1 in the HST Introduction for instructions on how to convert FITS files to GEIS files using strfits. Like FITS files, the names of GEIS files also derive from a file's rootname and suffix, and they look like this:
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 FITS file x3l80101t_d0f.fits.
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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 2.1.1 in the HST Introduction). For clarity, this handbook will use the term "extension" when referring to a component of a FITS file and the term "suffix" when referring to the three character identifier in a filename.
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