Definition of the Flexible Image Transport System (FITS), version 3.0

Abstract

The Flexible Image Transport System (FITS) has been used by astronomers for over 30 years as a data interchange and archiving format; FITS files are now handled by a wide range of astronomical software packages. Since the FITS format definition document (the "standard") was last printed in this journal in 2001, several new features have been developed and standardized, notably support for 64-bit integers in images and tables, variable-length arrays in tables, and new world coordinate system conventions which provide a mapping from an element in a data array to a physical coordinate on the sky or within a spectrum. The FITS Working Group of the International Astronomical Union has therefore produced this new version 3.0 of the FITS standard, which is provided here in its entirety. In addition to describing the new features in FITS, numerous editorial changes were made to the previous version to clarify and reorganize many of the sections. Also included are some appendices which are not formally part of the standard. The FITS standard is likely to undergo further evolution, in which case the latest version may be found on the FITS Support Office Web site at http://fits.gsfc.nasa.gov/, which also provides many links to FITS-related resources.Key words. instrumentation: miscellaneous -methods: miscellaneous -techniques: miscellaneous -reference systemsstandards -astronomical databases: miscellaneous ForewordSince its inception in the late 1970s, the Flexible Image Transport System (FITS) data format has enjoyed wide acceptance in the astronomical community. Indeed, FITS is used as the archival format for most digital archives of astronomical data, FITS is one of the most common run-time file formats for analysis applications, and FITS is seeing continued utility as a component of data formats for the Virtual Observatory. After the initial FITS definition papers appeared (Wells et al. 1981; The FITS standard has evolved in many significant ways during the last decade. In 2005 the IAUFWG approved use of the 64-bit integer data type in FITS images and tables and added support for variable-length arrays within tables. But some of the most significant extensions to FITS did not appear in the standard document per se, but in a series of papers (Greisen & Calabretta 2002;Calabretta & Greisen 2002;Greisen et al. 2006) that described formalisms for the support of world coordinate systems (WCSs), i.e., the mapping from a position in a data array to a physical coordinate system such as a position on the sky or within a spectrum. Although the IAUFWG officially recognized the content of these papers as a part of the standard, they were not formally mentioned in the FITS standard document itself.In late 2006 the IAUFWG appointed a technical panel to address specific ambiguities that had been identified in the standard over the years, to clarify the meaning of some portions of the text, and to consider how to incorporate all the approved elements of the standard into one document. This work was completed by mid-2007, and ...

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