Home | Verifier | News | Docs | WCS | Samples | Libraries | Viewers | Utilities | Keywords | Conventions | Resources
FITS Extension Names
Registered with the IAU FITS Working Group
Extensions to the basic FITS format can be defined as specified in "Generalized Extensions and Blocking Factors for FITS", A&AS, 73, 359 (1988) and in Section 4.4 of the FITS Standard document. The following guidelines, as quoted from the first paper, should be considered before creating a new extension type:
The only restriction that will have to be placed on the freedom to create new extensions is that there should be only one approved extension format for each type of data organization. New extension types have to be created whenever the organization of the information is such that it cannot be handled by one of the existing extension types. It will be the function of each user who creates a new extension type to check with the standards committee to see if an extension already exists for that type of data organization and to propose one if it is really a new extension type. The contents of an extension and the optional keywords used, etc. will depend on the particular application. Thus one can use a standard table extension for all type of tabular information, without having to define a new extension type. With this restriction in mind, users should feel free to create new extension types when the need arises, but should always be aware that the use of non-standard extensions will inhibit the interchange of astronomical data.
Each extension type has a unique name given by the value of the mandatory XTENSION header keyword. The IAU FITS Working Group maintains this list of registered extension names. Anyone who wants to register a FITS extension name should contact the chairman of the IAU FITS Working Group.
Standard Extensions - These 3 extension types have been approved by the IAU FITS Working Group and are defined in Section 8 of the FITS Standard document as well as in the indicated Astronomy and Astrophysics journal articles.
- IMAGE - This extension type provides a means of storing a multidimensional array similar to that of the FITS primary header and data unit. Approved as a standard extension in 1994. The format of IMAGE extensions is defined in the A&AS, 105, 53 (1994) journal article.
- TABLE - This ASCII table extension type contains rows and columns of data entries expressed as ASCII characters. Approved as a standard extension in 1988. The format of TABLE extensions is defined in the A&AS, 73, 365 (1988) journal article.
- BINTABLE - This binary table extension type provides a more flexible and efficient means of storing data structures than is provided by the TABLE extension type. The table rows may contain a mixture of numerical, logical and character data entries. In addition, each entry is allowed to be a single dimensioned array. Numeric data are kept in binary formats. Approved as a standard extension in 1994. The format of BINTABLE extensions is defined in the A&AS, 113, 159 (1995) journal article.
Conforming Extensions - These conventions meet the requirements for a conforming extension as defined in A&AS, 73, p359-364, 1988 and in Section 4.4 of the FITS Standard document, however they have not been formally approved or endorsed by the IAU FITS Working Group.
- IUEIMAGE - This name was given to the prototype of the IMAGE extension type and was primarily used in the IUE project data archive from approximately 1992 to 1994. Except for the name, the format is identical to the IMAGE extension.
- A3DTABLE - This name was given to the prototype of the BINTABLE extension type and was primarily used in the AIPS data processing system developed at NRAO from about 1987 until it was replaced by BINTABLE in the early 1990s. The format is defined in the 'Going AIPS' manual, Chapter 14. It is very similar to the BINTABLE type except that it does not support the variable-length array convention.
- FOREIGN - This extension type is used to put a FITS wrapper about an arbitrary file, allowing a file or tree of files to be wrapped up in FITS and later restored to disk. A full description of this extension type is given in the FITS Registry of conventions.
- COMPRESS - Proposed in 1991 in a draft paper by Archibald Warnock, Robert Hill, Barbara Pfarr (all at GSFC), and D. Wells (NRAO) as a way of storing compressed images in FITS format. See the FITSBITS email list archive of postings for September and October 1991 for a discussion of image compression in general and this proposal in particular. This compression proposal was never implemented, but some of the same ideas were used in the tiled image compression convention that is described in the FITS Registry of conventions.
- FITS - Suggested by Perry Greenfield (STScI) in April 2002 as a way to hierarchically embed entire FITS files within other FITS files. See the discussion on the fitsbits email list in the April 2002 thread.
Other Registered Extensions - These extension names have been suggested, but the proposals have not been fully developed or implemented.
- DUMP - Suggested extension name for storing a stream of binary data (such as a telemetry stream) in a FITS file. This extension type was never implemented, but the FOREIGN extension type serves a similar purpose.
- FILEMARK - Suggested by Don Wells (NRAO) to represent the equivalent of a end-of-file mark on magnetic tapes. When the FITS format was designed in the late 1970s, FITS files were mainly written on half-inch magnetic tape where the end of a file could easily be identified by a tape end-of-file mark. Not all other media in use at that time supported such a clear hardware file delimiter, so the FILEMARK extension was proposed as a way of explicitly representing the end of the FITS file.
- VGROUP - This extension type was suggested by Don Jennings (GSFC) for possible use in supporting the analog of HDF (Hierarchical Data Format) group structures under FITS. This suggestion was later withdrawn, and the standard BINTABLE extension, in conjunction with the EXTNAME = 'GROUPING' keyword, was used for this purpose instead. See the description of the Hierarchical Grouping convention for more details.
Home | Verifier | News | Docs | WCS | Samples | Libraries | Viewers | Utilities | Keywords | Conventions | Resources
Last revised: Thursday, 19-Apr-2018 14:58:18 EDT
Contact us: fits @ fits.gsfc.nasa.gov
Hosted by: The HEASARC (High Energy Astrophysics Science Archive Research Center)
Responsible NASA representative: Dr. Tess Jaffe Privacy, Security & Accessibility Statements.