FITS Documents

Table of Contents

Documents Endorsed by International Astronomical Union

The International Astronomical Union (IAU), through the IAU FITS Working Group (IAUFWG), has endorsed the following documents as the standard for FITS:

Published Papers

Often referred to as the FITS Papers

Wells, D. C., Greisen, E. W., and Harten, R. H., FITS: A Flexible Image Transport System, Astronomy and Astrophysics Supplement Series, 44, 363-370, 1981.

Greisen, E. W. and Harten, R. H., An Extension of FITS for Small Arrays of Data, Astronomy and Astrophysics Supplement Series, 44, 371-374, 1981. (NOTE: The random groups format described in this paper has been used almost exclusively to transport radio interferometry and has now largely been replaced by binary tables. Writing data other than radio interferometry data using this format is not recommended.)

Grosbøl, P., Harten, R. H., Greisen, E. W., and Wells, D. C., Generalized Extensions and Blocking Factors for FITS, Astronomy and Astrophysics Supplement Series, 73, 359-364, 1988.

Harten, R. H., Grosbøl. P., Greisen, E. W., and Wells, D. C., The FITS Tables Extension, Astronomy and Astrophysics Supplement Series, 73, 365-372, 1988.

Ponz, J. D., Thompson, R. W., and Munoz, J. R., The FITS Image Extension, Astronomy and Astrophysics Supplement Series, 105, 53-55, 1994. (LaTeX version of draft)

Cotton, W. D., Tody, D. B., and Pence, W. D., Binary Table Extension to FITS, Astronomy and Astrophysics Supplement Series, 113, 159-166, 1995. (Final proposal)

Floating Point Agreement

Specifies the use of IEEE-754 floating point and describes its application to FITS. The IEEE standard is copyrighted and must be ordered from IEEE.

Blocking Agreement

Available in text form only.

Formulated for fixed-block sequential media, variable block sequential media, and bitstream devices.

Many controllers and devices for high density storage media such as optical disk can access data only in physical blocks of fixed length, typically 2**n bytes. These rules prescribe the number of 2880-byte FITS logical records in a physical block and how to proceed when the block size is not an integral multiple of 2880 bytes. Also, they extend the rules for 1/2-inch magnetic tape in the original FITS papers to other variable block length sequential media. Finally, they provide a rule that can be used for pure bitstream files, such as files on a computer.

Year-2000 DATExxxx Agreement

The original standard form for dates in FITS, DD/MM/YY, will become ambiguous after the turn of the century. Out of FITS community discussion, Peter Bunclark of the Royal Greenwich Observatory developed a proposal for a new standard form for values of the DATExxxx keywords to provide for a four-digit year. This format, based on ISO-8601, also allows both time and date to be given in the keyword value. Some amendments, dealing primarily with definition of the time system, were formulated by Arnold Rots. Files using this format were successfully exchanged between AIPS and ESO-MIDAS. On November 10, 1997, the amended proposal was approved by the IAU FITS Working Group. An appendix that is not part of the formal proposal recommends conventions for time system specifications. The new date syntax DATE-OBS='1997-11-13' will replace the existing standard date syntax DATE-OBS='13/11/97' (which will remain valid for 1900-1999). In order to give major package writers adequate time to revise their software, FITS writers should commence writing the new format between 1999-01-01T00:00:00 and 2000-01-01T00:00:00. The agreement discusses the transition further.

NOST Definition of FITS

How to Get a Copy

What It Is

The Definition of FITS is a codification into a formal standard, by the NASA/Science Office of Standards and Technology (NOST), of the FITS rules endorsed by the IAU. In developing this standard, some contradictions and ambiguities in the original FITS papers have been eliminated. The current standard, version 2.0, was approved as a NOST standard on March 22, 1999. It covers the material in the first four FITS papers, the Floating Point Agreement, the binary table and image extensions, the Y2000 DATE and DATE-obs keyword changes, and the blocking agreement.

How the NOST Standard Was Created

To write the standard, NOST appointed a technical panel of astronomers knowledgeable in FITS. The Chair of the current NOST FITS Technical Panel is R. J. Hanisch (STScI), who is also the Chair of the American Astronomical Society Working Group on Astronomical Software (WGAS). The FITS Support Office Coordinator is Secretary to the panel. For each version, the panel first produced a Draft Standard, releasing it for comment to the astronomical community when the panel determined by consensus that release was appropriate. The panel then reviewed the community comments, either incorporating them into the standard or deciding why comments should not be incorporated. Responses were sent to all who sent comments. This process was repeated until the number of community comments was so small as to indicate that community consensus had been reached. The panel then released the result to NOST as a Proposed Standard.

NOST then created an accreditation panel, consisting of the NOST Executive Board and an astronomical community representative, to verify that the technical panel had followed the proper procedures in the development of the standard. In particular, it ascertained whether the community had been given a satisfactory opportunity to review the standard and whether the technical panel had properly considered and responded to all comments. For each version, upon verifying that the proper procedures had been followed, the accreditation panel approved the Proposed Standard as a NOST Standard.

User's Guide

How to Get a Copy

What It Is

A User's Guide for FITS is commissioned by NASA Headquarters and maintained by the FITS Support Office. Its contents are as follows:
The Origin and Purpose of FITS
Why FITS was created, who uses it, and the philosophy behind it
History
How and why the rules developed
FITS Fundamentals
The current standard and recommendations for good practice
World Coordinate Systems
Mappings beween an array of data and the real world, especially projections of the celestial sphere -- how they are done now and how they will be done in the future
Advanced FITS
Conventions, proposals, and applications -- they're not part of the formal rules but are widely understood; some may become standard in the future
Resources
Where to get more information
Appendixes
Sample FITS files and a guide to IEEE formats
The current version, 4.0, was completed in April 1997.

List of Registered Extensions

How to Get It

What It Contains

It includes a brief description of the structure of the extension each type name identifies, the developer or responsible organization, and the status of the extension (e. g., standard, under discussion, local). Also described is the procedure for registration of extension type names. This list is updated as new extension type names are proposed and extensions with reserved names progress through the process required for IAU FITS Working Group approval. It should be used as the primary reference on registered extensions rather than the appendix in the Definition of FITS or the corresponding section in the User's Guide.

Why It Exists

Extension types in FITS must have unique names, e. g., TABLE, IMAGE. The reason is to permit users to define local or developmental extensions and place them in FITS files, while making it possible for software that cannot read the extension to skip over it and continue reading the next extension in the file. To ensure uniqueness for all extension type names, even for local extensions used only at one installation, there must be an official list. This registry is under the IAU FITS Working Group and is maintained by the FITS Support Office.

World Coordinates

The preprints of Representations of world coordinates in FITS and Representations of celestial coordinates in FITS are available from the astro-ph Web site.

List and description of other WCS documents

Directory where documents can be obtained in .Z-compressed form

World coordinates issues are those of the transformation between the FITS array and the physical space it represents, most often the celestial sphere. The draft text of conventions for world coordinates currently under community review proposes rules for describing the physical coordinate values attached to each member of a FITS data array, with detailed discussion of projections from the celestial sphere to the array plane. It has evolved from the 1983 Astronomical Image Processing System (AIPS) Memo #27, which describes some world coordinates conventions implemented in AIPS at that time. These conventions have been widely used elsewhere as well.

The latest description of the World Coordinate System are contained in Paper I, Paper II, and Paper III.

Proposed Conventions

Checksum

ftp directory with documentation

R. Seaman and W. Pence have proposed a scheme for embedding a checksum within a FITS header. This checksum could be used to verify that the data in a file were transported without errors. IAU FITS Working Group Chair D. Wells has recommended that this proposal be considered by the regional FITS committees.

Hierarchical Grouping

Hypertext copy of proposal

This convention, proposed by D. Jennings, W. Pence, M. Folk, and B. Schlesinger, provides a means to logically group together FITS HDUs that are are physically located in different files or are in one file but physically separated. It would also facilitate HDU-FITS conversion.

Other FITS Support Office pages

The FITS Support Office is hosted by the HEASARC (High Energy Astrophysics Science Archive Research Center)

(Electronic Mail) fits @ fits.gsfc.nasa.gov
(Telephone) +1-301-286-4599

Last revised: 12 September 2002

Responsible NASA representative: Dr. William D. Pence