Flexible Image Transport System

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File Format
Name Flexible Image Transport System
Extension(s) .fits, .fts, .fit
MIME Type(s) image/fits
LoCFDD fdd000317
PRONOM x-fmt/383


General description

FITS (Flexible Image Transport System) 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.

PSRFITS is a particular variety of FITS for storing pulsar data, as used, for instance, in the SETI@Home project to analyze such data for possible evidence of extraterrestrial intelligence.

Other information

The format is summarised in Wikipedia, which includes some history of the format.

For further information, see:


See the FITS support office.

Also FITS Liberator, with source code here.

Sample files


Valid FITS files start with the sequence of characters SIMPLE=T .

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

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


  1. William D Pence, L Chiappetti, Clive G Page, R A Shaw and E Stobie, Definition of the Flexible Image Transport System (FITS), version 3.0, Astronomy and Astrophysics, 524, A42+ (2010) doi:10.1051/0004-6361/201015362, (ADS bibcode: 1981A&AS...44..363W)
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