This basic FITS information is updated and posted to Usenet, periodically, typically the third week of the month. FITS (Flexible Image Transport System) is a data format designed to provide a means for convenient exchange of astronomical data between installations whose standard internal formats and hardware differ. A FITS file is composed of a sequence of Header Data Units (HDUs). The header consists of keyword=value statements, which describe the format and organization of the data in the HDU and may also provide additional information, for example, about the instrument or the history of the data. The data follow, structured as the header specifies. The data section of the HDU may contain a digital image, but, except for the first, *it doesn't have to*. Other possible formats include tables and multidimensional matrices that are not images. The first HDU must contain a multidimensional matrix or no data at all; the data in subsequent HDUs, called extensions, may be of any type, consistent with certain rules. The "Image" in the name comes from the original use of the format to transport digital images, but it's not just for images any more. FITS is not principally a graphics format designed for the transfer of pictures; it does not incorporate "FITS viewers", packages for decoding the data into an image. Users must develop or obtain separate software to convert the data from the FITS file into a form that can be readily displayed. As has been discussed in the Usenet newsgroup sci.astro.fits, and in alt.sci.astro.fits before it, the Extended Portable Bitmap Toolkit (pbm+) can be used for converting many FITS files to such a format. However, support is not guaranteed for all FITS files where the data are in the form of an image. In particular, there may be problems when the data matrix members are in IEEE floating point format (BITPIX<0) or the matrix has more than two dimensions (NAXIS>2). Archie Warnock and Ron Baalke have announced release of version 7.8 of the IMDISP program. IMDISP is an interactive image processing program that runs on an IBM PC computer and supports FITS input. IMDISP 7.8 is available via anonymous ftp at ames.arc.nasa.gov [128.102.18.3] in a file called imdisp78.zip in the pub/SPACE/SOFTWARE subdirectory and at hypatia.gsfc.nasa.gov in the pub/software/imdisp subdirectory. It is also available through Simtel-20 [192.88.110.20] at PD1:IMDISP78.ZIP. Additional discussion of FITS->image converters appears in sci.astro.fits from time to time. The fundamental references on FITS are the following four papers, often referred to collectively as the "Four FITS Papers". These papers are the formal standard for FITS, endorsed by the International Astronomical Union (IAU). 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 format described in this paper has been used almost exclusively to transport radio interferometry and is likely to be replaced by other formats in the future. Writing data other than radio interferometry data using this format is not recommended.) Grosbol, 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., Grosbol. P., Greisen, E. W., and Wells, D. C., "The FITS tables extension, Astronomy and Astrophysics Supplement Series, 73, 365-372, 1988. A User's Guide for FITS, commissioned by NASA Headquarters, is maintained by the NASA/OSSA Office of Standards and Technology (NOST) FITS Support Office. This Guide is intended to be a tutorial for new FITS users. In addition to presenting the rules of FITS, it provides some of the history and reasoning behind the choice of the rules, adds recommendations on good practices, and discusses current developments in FITS. The current version, 3.0, was issued in January 1993. This document is at present available only in printed form, but steps are under way to generate a PostScript version that will work on many systems and a flat ASCII version. The NOST is sponsoring development of a formal standard for FITS. The goal is a document codifying FITS as endorsed by the IAU, eliminating some contradictions and ambiguities in the original FITS papers, that can be endorsed by the IAU FITS Working Group as the FITS standard. The document is being developed by a Technical Panel chaired by Dr. Robert J. Hanisch (STSci), with review by the astronomical community. Only minor revisions are expected to the draft currently available, version 0.3b, but the form of the standard is not final, and it does not supersede the four papers and Floating Point Agreement endorsed by the IAU as the official standard for FITS. The IAU has endorsed the Floating Point Agreement, which defines how floating point numbers are to be expressed in FITS. The basic agreement appears verbatim in the User's Guide, and the substance is incorporated in the Draft NASA FITS definition. For information about the contents of this directory, retrieve and read the AAREADME.DOC file. The README files with names other than AAREADME.DOC are now pointers to the AAREADME.DOC files; the names have been retained for the convenience of users who may have learned about the directory from material that uses the old name. Additional material can be obtained by anonymous ftp from the National Radio Astronomy Observatory, from fits.cv.nrao.edu, in the directory FITS. The Documents subdirectory (case is significant) contains copies of the BINTABLE Binary Tables extension proposal, which is now under consideration by the FITS committees, and a draft describing a proposed method for incorporating data compression under FITS. A number of additional documents are available in ASCII text form, including the proposal on physical blocking of FITS files on media other than tape, and material on FITS, its history, and the FITS community. Its FITS_wcs subdirectory contains material serving as the basis for continuing discussion of world coordinates issues, some of which appears on this newsgroup from time to time. One is a draft proposal for world coordinates conventions being developed by E. Greisen and M. Calabretta. Two AIPS memos describing projections from a sphere onto a plane are also included, in PostScript form. A copy of an earlier paper by D. Wells and R. Hanisch summarizing conclusions of a workshop on World Coordinates held in Charlottesville in 1988 mremains in the Documents subdirectory. Unless otherwise noted, documents are available from NRAO in both LaTeX and PostScript forms. There is an AA.README file for the Documents subdirectory and a README file for its FITS_wcs subdirectory. Printed copies of many of the documents listed above can be obtained from the NOST Librarian. Printed copies of the User's Guide and either paper or electronic copies of the Draft NOST Standard, for those without ftp access, are available. Because of restrictions set by the copyright holder, NOST can send copies of the four FITS papers only to non-profit organizations. The NOST can be reached as follows: (Postal) NASA/OSSA Office of Standards and Technology Code 633.2 Goddard Space Flight Center Greenbelt MD 20771 USA (Internet) nost@nssdca.gsfc.nasa.gov (DECnet) NCF::NOST Telephone: (301)286-3575 8 a. m. - 5 p. m., U. S. Eastern Time If the Librarian is unavailable, a phone mail system takes the call after four rings. Use the FITS office electronic mail address below for replies or questions. It is monitored by other NOST staff memebers when I am away from the office and provides a greater certainty of rapid response. Barry M. Schlesinger Coordinator, NASA/NSSDC NOST FITS Support Office (301) 513-1634 fits@nssdca.gsfc.nasa.gov NCF::FITS