2.1 File Name Format

HST data file names encode a large amount of information about the files themselves. Datasets retrieved from MAST consist of multiple files in FITS format, each with a name that looks like this:

Figure 2.1: HST file name structure.

  • 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. For more information on the rootname composition, see Section 2.2.
  • 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 seven 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. The suffixes for HST files vary between instruments, and the data handbooks for each HST instrument list the suffix types and their meanings.

The identifier referred to here as a “suffix” has often been called an “extension” in the past. However, the individual pieces of multi-extension FITS files are also known as “extensions” (see Section 3.2.1). For clarity, this handbook will use the term “extension” when referring to a component of file and the term “suffix” when referring to the three character identifier in a filename.