The following software packages display or manipulate the relatively simple class of FITS data files that containing 2-dimensional images, often of celestial objects in the night sky. It should be noted that FITS is a very general data format that is used for many different types of astronomical data sets, so these packages are not necessarily capable of reading every type of FITS file. Developers of new image display programs should be aware of the special requirements for effectively displaying FITS images.

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Notes Regarding the Display of FITS Images

An application intended to render a FITS image for viewing by a user has significantly more responsibility than an application intended to handle other standard image formats (e.g., JPEG or GIF images). FITS data arrays contain elements which typically represent the values of a physical quantity at some coordinate location. Consequently, they need not contain any pixel rendering information in the form of transfer functions, and there is no mechanism for color look-up tables. An application should provide this functionality, either statically using a more or less sophisticated algorithm, or interactively allowing a user various degrees of choice. (See also: a short introduction to astronomical image processing).

Furthermore, the elements in a FITS data array may be integers or floating point numbers. The dynamic range of the data array values may exceed that of the display medium and the eye, and their distribution may be highly nonuniform. Logarithmic, square-root, and quadratic transfer functions along with histogram equalization techniques have proved helpful for rendering FITS data arrays. Some elements of the array may have values which indicate that their data are undefined or invalid; these should be rendered distinctly.

The data array in a FITS image must have a dimensionality between 1 and 999, the boundaries inclusive, indicated by the NAXIS keyword. The extent of any coordinate axis in a FITS data array may, however, consist of only a single element. Hence an algorithm designed to render two-dimensional images will be capable of displaying a three- or four-dimensional FITS array when one or two of the axes consist of a single pixel.

Three-dimensional data arrays (NAXIS = 3 with multiple elements along each) are of special interest. Inspection of the World Coordinate System (WCS) keywords in an image with NAXIS = 3 or more may indicate that one of the axes is temporal. Writers of viewer applications should consider presenting such an image in a fashion akin to that used for an animated GIF. Even in the absence of WCS indication of a temporal axis this time-lapse display technique can be effective, and application writers should consider offering it for all three-dimensional arrays.

A FITS image with NAXIS=1 is a one-dimensional entity such as a spectrum or a time series. Writers of applications intended to handle these one-dimensional FITS images should consider presenting such an image as a graphical plot rather than as a two-dimensional picture with a single row.