Subject: FITS basics and informations (periodic posting)

From: bschlesinger at nssdca.gsfc.nasa.gov (BARRY M. SCHLESINGER)
Date: 4 Jun 1993 16:54 EDT

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.  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 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).

     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
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/Science 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 NOST FITS definition. 

	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 at this writing.  A reorganization is planned that
will move the FITS material to a FITS subdirectory under a STANDARDS
directory. The FITS files include copies of the current NOST draft
standard in flat ASCII, PostScript, and LaTeX.  Style and index files
are provided for the LaTeX form.  Except that the ASCII form does not
have an index, the copies in the three formats are identical; only one
need be retrieved.  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 modified version of this posting also
appears there.  An AAREADME.DOC file describes the contents of the
directory.  A SOFTWARE subdirectory contains a prototype for software
that will eventually validate FITS files, along with instructions. The
current versions investigates required keywords for primary headers
and integer data arrays.  There is also a copy of 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.  Be sure to use binary transfer
for ftp access of FITS test files.  Both the SOFTWARE and ERRTEST
subdirectories include AAREADME.DOC files describing their content. A
modified version of this posting is now in the main directory. 

 	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 
remains 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/Science Office of Standards and Technology
	 Code 633.2
	 Goddard Space Flight Center
	 Greenbelt MD 20771
	 USA

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

Telephone: +1-301-286-3575 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. 
	Please mention this posting in your request.

	Use the FITS office electronic mail address below for replies
or questions. It is monitored by other NOST staff members 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
		
+1-301-513-1634				fits at nssdca.gsfc.nasa.gov
					NCF::FITS