Subject: FITS basics and information (periodic posting)

From: bschlesinger at nssdca.gsfc.nasa.gov (Barry Schlesinger)
Date: Mon, 20 Jul 1992 21:46:00 GMT

This basic FITS information is posted and updated periodically
for the benefit of new readers and the reference of old readers. 

	FITS (Flexible Image Transport System) is a data format
designed to allow 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 contain other
information such as information about the instrument or the history of
the data or data set.  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 with standard support for many platforms included
as part of the package for decoding the image. Separate software must
be developed or obtained for converting the data from the FITS file
into a form that can be readily displayed.  As has been discussed in
this newsgroup, 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, but support for all FITS files where the data
are in the form of an image, in particular those where the data matrix
members are in IEEE floating point format (BITPIX<0) or the matrix has
more than two dimensions (NAXIS>2), is not guaranteed. Discussion of
FITS - image converters appears in this newsgroup 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).  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.  This document is available only in hard copy form. 

	NASA is sponsoring development of a formal standard for FITS.
The goal of this process is a document consistent with 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 current draft, 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 stored in the data
matrix defined in the first of the four papers.  The basic agreement
appears verbatim in the User's Guide, and the substance is
incorporated in the Draft NASA Standard. 

	The NOST maintains a file of FITS information available by
anonymous ftp from nssdca.gsfc.nasa.gov or DECnet copy from NSSDCA, in
the directory FITS. It includes copies of the current draft NASA
Standard in flat ASCII, PostScript, and LaTeX.  Style and index files
are provided for the LaTeX form.  A current list of the extension type
(structure) names registered with the IAU FITS Working Group is
maintained.  Also available, in LaTeX form, is the text of the
proposal for one of these new extension types, IMAGE.  A README. file
describes the contents of the directory.  A SOFTWARE subdirectory,
described by an included README.FIRST file, contains a program in C to
read and list the headers of a FITS file and another file with
information on publicly available FITS software packages. The ERRTEST
subdirectory contains several versions of the same FITS file, a valid one and
several with different kinds of header errors, for use in testing
software to read FITS files.  An included README.FIRST file contains
details. 

	Paper copies of many of the documents listed above can be
obtained from the NOST Librarian. Paper copies of the User's Guide and
either paper or electronic copies of the Draft Implementation
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 addresses of
the NOST are as follows: 

  (Postal) NASA/OSSA Office of Standards and Technology
	Code 933
	Goddard Space Flight Center
	Greenbelt MD 20771
	USA

(Internet) nost at nssdca.gsfc.nasa.gov
(DECnet) NCF::NOST

	Please mention this posting in your request.

					Barry M. Schlesinger
					Coordinator,
					NASA/NSSDC
					NOST FITS Support Office
		
(301) 513-1634				fits at nssdca.gsfc.nasa.gov
					NCF::FITS