Largely because the format has evolved incrementally, there are a number of inconsistencies and ambiguities among the FITS papers, especially the first four. To provide a document with a clear and unambiguous specification of the rules of FITS, the NASA/Science Office of Standards and Technology (NOST) has developed a formal FITS codification, the Definition of FITS (NOST 1995). This NOST standard was developed by technical panels of astronomers chaired by R. Hanisch (STScI), chair of the Working Group on Astronomical Software of the American Astronomical Society. The FITS Support Office coordinator served as secretary of the panel, and the FITS office was responsible for writing the original text and for the bulk of the editing during the revision process.
Development of the standard follows a procedure developed at the NOST that follows the common practices of other standardization bodies. The technical panel develops an initial Draft Standard. When the panel has reached consensus on this Draft Standard, the document is made available to the astronomical community for comment, typically for a period of two months. Availability of the document for review is announced to the sci.astro.fits newsgroup and to the electronic mailing lists described in section 6.5. The American Astronomical Society Executive Office also is notified. Special efforts are made to ensure review by members of the IAUFWG and the regional FITS committees. After the period for review is over, the technical panel reviews the comments and revises the standard in reponse. Replies are sent to all reviewers, describing the changes made by the technical panel in response to the review, and explaining the reasons when the recommended revision was not made. This cycle is repeated until the number of revisions recommended by the reviewers is so small that it is evident that the astronomical community has reached consensus.
The document that results from this process is called a Proposed Standard. This document is then submitted to a NOST accreditation panel, composed of the NOST Executive Board (the NOST Secretary, two other members from the Space Science Data Operations Office, and one from NASA Headquarters) and an outside astronomer. The role of this panel is to review the process of developing the standard: whether the steps taken to publicize the availability for review were adequate to reach the entire interested community, whether the technical panel replies addressed all the reviewers' points, and whether the resulting Proposed Standard represents a consensus of the community.
Version 1.0 of the standard covered the rules in the first four FITS papers and the floating point agreement. There were three technical panel review cycles. On June 18, 1993, the accreditation panel voted to approve the Definition of FITS as the NOST standard for the use of FITS. Questions about the specifications of units led to the development of a revision, version 1.1. There was only one review cycle; the number of comments was small and the community members present at the 1995 business meeting of the Working Group on Astronomical Software agreed that this lack of comments reflected general satisfaction with these changes. Version 1.1 was approved by the accreditation panel on June 14, 1995. The NOST Definition has been submitted to the chair of the IAUFWG as the start of the process of seeking endorsement of the NOST document as the international standard for FITS. In the ongoing electronic discussions of FITS, FITS rules are frequently quoted in the form in which they are expressed in the NOST document.
A technical panel has been formed to add the IMAGE and BINTABLE extensions and the blocking agreement to the NOST standard. Clarifications arising from community discussion of the FITS rules also will be made. The result of this process will be Version 2 of the Definition of FITS (now available from the FITS Support Office or in html at http://archive.stsci.edu/fits/fits_standard/).
The standard is available in both printed and electronic form. Electronic copies are available in several forms: flat ASCII, uncompressed PostScript, compressed PostScript,and LATEX. Style (nost.sty) and index files are provided for use with the LATEX form. The three forms have identical content, except that the ASCII version may not preserve the font usage in the other versions. The standard may be obtained from the anonymous ftp site in files of the form fits_standard.* through links starting at the FITS Support Office home page. When a Draft Standard is announced for community review, it is available in the LATEXand PostScript forms; a text form is sometimes available.
The FITS Support Office maintains and updates this User's Guide, which is designed to provide a better introduction to FITS for the novice than the precise but dry standard. In addition to providing the rules, it explains them in more detail than the standard and discusses their motivation. It discusses recommended and discouraged practices and provides hints on how to use particular structures. Sample FITS headers illustrate the different formats. Some widely used conventions that are not specified in the formal rules of FITS are described. The Guide also discusses current FITS developments, including proposals for new rules or conventions. Copies are available in printed form and electronically in compressed or uncompressed PostScript and LATEX, in files of the form users_guide.*. The style file guide.sty is provided for use with the LATEX form.