Flexible Image Transport System

General description
FITS is a heavily-used and well-standardised format for storing astronomical data. It can store both images and tables, and has basic but flexible support for metadata.

The format is fundamentally rather simple; indeed it is simple enough that a basic reader can be constructed with a relatively small amount of effort, but this is rarely necessary, since there are readers and writers for a broad range of scientific programming languages.

The metadata is stored as a list of key-value pairs, with the keys being up to eight characters long, and the values up to 70 characters. There are some standards, and several conventions, for the choices of keyword. These conventions generally interoperate well, but readers should be aware that collisions are possible.

The format has been standardised in a sequence of papers in the journal Astronomy and Astrophysics, and a set of conventions for header metadata has been developed, over a number of years.

Other information
The format is summarised in Wikipedia.

For further information, see:
 * Version 3 of FITS is described in, and this document has been approved by the IAU's FITS Working Group. The document is also available online.
 * The =image/fits= and =application/fits= MIME types were registered in RFC 4047.
 * Further information, including libraries and file samples, is available at the FITS support office

Software
See the FITS support office.

Sample files
See the FITS support office.

Identification
FITS files start with the sequence of characters =SIMPLE =.

The common FITS file extensions are =.fits= and (less often) =.fts=.

The =image/fits= and =application/fits= MIME types were registered in RFC 4047.