Subject: FITS basics and information (regular posting)

From: bschlesinger at nssdca.gsfc.nasa.gov (Barry Schlesinger)
Date: Fri, 4 Sep 1992 14:08:00 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 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 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; it does not incorporate 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 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.
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).
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) 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.  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 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
	 Goddard Space Flight Center
	 Greenbelt MD 20771
	 USA

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

Telephone: (301)286-3575 8 a. m. - 5 p. m., U. S. Eastern Time
	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