Astronomical Data Formats

In the astronomy labs you will encounter many different data file formats, echoing the often-bewildering chaos of the internet. You can often tell the format of the data file from the extension to the file name. The extension is the 3 or 4 letters (sometimes more under UNIX systems) following the final "." in the filename (such as ".txt" indicating a text file or ".Z" indicating UNIX compressed file). Beware that UNIX systems are case-sensitive, although Windows and VMS systems are not.

A summary of many scientific data formats can be found in the Scientific Data Format Information FAQ


Most, if not all, of the data you will deal with in this course are in FITS format (extension .FITS, .FIT, or .FTS). FITS, the Flexible Image Transport System, is the standard format used for archiving and transporting astronomical data. Most astronomical data are archived in this format. Images obtained with the SBIG CCD camera on the 14" telescope are saved in this format.


IRAF and STSDAS have an internal data format which is FITS-like, but incorporate separate files for the headers and the data. These data are generally transported and archived in FITS format, and utilities exist to transform between the formats. The latest version (2.11) of IRAF can work directly with FITS files. IRAF data files use .imh and .pix extensions.

Image Formats

Sometimes images are tranferred in GIF (extension .gif), JPEG (extension .jpg, .jpeg), or TIFF (extension .tif, .tiff) formats. These are not true scientific data formats, but are designed for image compression. The data are byte-scaled, and the intensity encoding may be convoluted. However, it is possible to use these data for simple analyses. IDL has built-in routines for reading these formats.

Compressed Data

Data are often compressed for transmission over the network, or for storage. Common compression formats are .Z (UNIX compression) or .gz (gzip compression). The extension is appended to the full filename (e.g., data.fits.gz) on UNIX systems, or under Windows when long filenames are recognized. Methods for uncompressing data are discussed here.

Other Formats

Other standard formats and their extensions used on the internet are listed here, along with directions of how to read or execute the files.

If you encounter file formats you do not recognize, and that you believe are important for the successful completion of the lab, please contact your TA or instructor.

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