A Brief Introduction to FITS

FITS is the standard computer data format widely used by astronomers to transport, analyze, and archive scientific data files.

History of FITS

The following paragraphs are reproduced from the Introduction to the 1999 FITS Standard document.

The Flexible Image Transport System (FITS) evolved out of the recognition that a standard format was needed for transferring astronomical data from one installation to another. The original form, or Basic FITS, was designed for the transfer of images and consisted of a binary array, usually multidimensional, preceded by an ASCII text header with information describing the organization and contents of the array. The FITS concept was later expanded to accommodate more complex data formats. A new format for image transfer, random groups, was defined in which the data would consist of a series of arrays, with each array accompanied by a set of associated parameters.

These formats were formally endorsed by the International Astronomical Union (IAU) in 1982. Provisions for data structures other than simple arrays or groups were made later. These structures appear in extensions, each consisting of an ASCII header followed by the data whose organization it describes. A set of general rules governing such extensions and a particular extension, ASCII table, were endorsed by the IAU General Assembly in 1988. At the same General Assembly, an IAU FITS Working Group (IAUFWG) was formed under IAU Commission 5 (Astronomical Data) with the mandate to maintain the existing FITS standards and to review, approve, and maintain future extensions to FITS, recommended practices for FITS, implementations, and the thesaurus of approved FITS keywords. In 1989, the IAUFWG approved a formal agreement for the representation of floating point numbers. In 1994, the IAUFWG endorsed two additional extensions, the image extension and the binary table extension.

FITS was originally designed and defined for 9-track half-inch magnetic tape. However, as improvements in technology have brought forward other data storage and data distribution media, it has generally been agreed that the FITS format is to be understood as a logical format and not defined in terms of the physical characteristics of any particular data storage medium. In 1994, the IAUFWG adopted a set of rules governing the relation between the FITS logical record size and the physical block size for sequential media and bitstream devices. The IAUFWG also approved in 1997 an agreement defining a new format for encoding the date and time in the DATE, DATE-OBS, and other related DATExxxx keywords to correct the ambiguity in the original DATE keyword format beginning in the year 2000.

Brief highlights of the history of FITS:

• 1979: Initial use and interchange of FITS files
• 1980: Random groups convention developed
• 1981: Published original (single Header Data Unit) definition paper
• 1981: Published Random Groups definition paper
• 1982: FITS format is formally endorsed by the IAU
• 1988: Defined rules for multiple Header Data Units (HDUs) in a FITS file
• 1988: FITS Working Group established by IAU (IAUFWG)
• 1988: FITS definition extended to include ASCII (American Standard Code for Information Interchange) TABLE extensions
• 1990: FITS definition extended to include IEEE (Institute of Electrical and Electronics Engineers) floating-point data
• 1994: FITS definition extended to multiple image arrays in IMAGE extensions
• 1995: FITS definition extended to binary tables in BINTABLE extensions
• 1997: Adopted a Y2K-compliant date format
• 2001: Reiterated existing standard in one paper (NOST (NASA/Science Office of Standards and Technology) document)
• 2002: Approved conventions for generalized world coordinates and celestial coordinates.

How FITS Evolves

The IAUFWG was given authority over FITS matters by the 1988 IAU General Assembly. When the developer of a data set finds that it does not fit well into the primary HDU or a standard extension format, a new design may be developed. No change can be made that would cause existing FITS files to be out of conformance -- the "once FITS, always FITS" rule. Because software to read FITS files uses the type name of an extension to determine whether or not the software can read the extension, extension type names must be unique. The IAUFWG maintains a list of extension type names that have been registered; the list is at the FITS Support Office. A unique name for any new extension type, even a developmental extension or one that will be used only locally, must be registered with the IAUFWG. After astronomical community discussion, a formal proposal is distributed. This proposal is discussed by the community and may be further modified. Tests are run using the new format to confirm that it can be practically used for data transport. If the astronomical community reaches a consensus that the proposal should be adopted as standard FITS, and if successful data transfer using the proposed extension can be demonstrated, it is submitted for ratification to the regional committees — the European FITS Committee, the Japanese FITS Committee, the Australia and New Zealand Regional FITS Committee, and the American Astronomical Society Working Group on Astronomical Software (WGAS) FITS Committee. Following approval by the regional committees, it is submitted to the IAUFWG. Approval by the IAUFWG establishes it as a standard extension.

In addition to the formal rules, a number of conventions are widely observed. Some conventions are used throughout the community others only within a particular discipline such as high energy astrophysics or single dish radio astronomy. Usually, the originator(s) of a convention will circulate an initial proposal for comments among a small group in the same discipline or installation. After this proposal has been refined based upon these comments, it is put out for public comment, usually by announcement to the sci.astro.fits newsgroup of a URL from which it can be retrieved. Comments at this time may lead to additional changes. If the affected community accepts the convention, installations will start using it in their FITS files. Since failure to use a convention is not a violation of the FITS rules, FITS readers unaware of it must not terminate with an error or give incorrect results when encountering it.