Subject: FITS basics and information sources (monthly posting)

From: bschlesinger at nssdca.gsfc.nasa.gov (Barry Schlesinger)
Date: 19 Jun 92 21:13:00 GMT

This basic FITS information will be 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 may be
different.  A FITS file consists of a sequence of Header Data Units
(HDUs). The header consists of keyword=value statements, which
describine 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 that includes support for many platforms 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, and a list of the current 
extension type (structure) names registered with the IAU FITS Working 
Group.  It also contains, in LaTeX form, the text of the proposal for 
one of these new extension types, IMAGE.  A README. file describes the 
contents of the directory.  There is a further subdirectory, SOFTWARE, 
described by README.FIRST, containing 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.

	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