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basics.txt
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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:<MSDOS.GRAPHICS>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