FITS (Flexible Image Transfer System) files

A FITS file is the standard file format for astronomical images. They are typically a few MB (in this activity, 2 MB) though larger cameras and higher resolutions (such as those from the Hubble Space Telescope) can make these up to a few hundred MB in size.

FITS images can be used in conjunction with image display and photometry packages such as LT Image or Makali'i as well as FITS viewers such as DS9. In addition to the image itself, a FITS file will contain data in the FITS header. This part of the file contains a wealth of information about how and when the image was taken as well as the instrumentation used and the weather conditions at the time.

This information can be gained from Makali'i by selecting 'Image Info' and 'FITS Header'.

Figure 1 is a screenshot from the FITS header as produced by Makali'i. Much of the information here is not required for simple image viewing or photometry, but some of the fields that may be of use are described below. It's worth remembering that not all telescopes (or indeed, cameras) return identical FITS headers or levels of detail.

  • DATE, UTSTART, MJD describe the time at which the image was started (3rd December 2005).
  • EXPTIME gives you the exposure time for that particular image (10 seconds).
  • FILTER1 and FILTER2 give you the filter information, in this case Bessel-V band.
  • The camera used is given in INSTRUME, in this case, Dillcam which has a GAIN (required when you are discussing uncertainties in your measurements) value of 2.62.
  • Other information found in a FITS header of use (but not shown here) might include.
  • TELESCOP identifies the telescope used for this exposure.
  • RA, DEC identifies the co-ordinates of the object at the centre of the image.
  • L1SEEING records a value (in pixels) for the seeing at the time of the exposure. This is a measure of how good the atmospheric conditions with acceptable images have a value of less than 6 and very good images perhaps less than 4 (4 pixels corresponds to 1" or arcsecond).


Figure 1: A FITS header as shown in Makali'i.
Credit: Fraser Lewis, Makali'i (NOAJ)

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