Subject: FITS Basics and Information (regular posting)

From: bschlesinger at nssdca.gsfc.nasa.gov (BARRY M. SCHLESINGER)
Date: 27 Jul 1993 09:00 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 has codified FITS as endorsed by the IAU into a
formal standard, eliminating some contradictions and ambiguities in
the original FITS papers, that can be endorsed by the IAU FITS Working
Group. as the FITS standard. This NOST Definition of FITS was
developed by a Technical Panel chaired by Dr. Robert J. Hanisch
(STSci), with review by the astronomical community.  On June 18, 1993,
it was approved as a NOST Standard by an Accreditation Panel
consisting of the NOST Executive Board and an astronomical community
representative; this review was to confirm that the community had been
given a satisfactory opportunity to review the standard and that the
Technical Panel had properly considered and responded to all comments. 
	The NOST standard has been submitted to the IAU FITS Working
Group for endorsement as the international FITS standard, to replace
the four FITS papers and the Floating Point Agreement, which are the
current endorsed standard.  While this committee could in principle
endorse a version that differs in some way from the NOST document,
changes are unlikely, because members of this committee were active in
the process of reviewing the standard and their comments were given
significant weight in the deliberations of the Technical Panel. 
	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 NOST 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 at this writing.  A proposed reorganization would
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 lacks
typeface formatting, 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. Because this software is still under 
development, it should not be run before reading separate
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 contains a number of
subdirectories.  A proposals subdirectory contain proposals, such as
the BINTABLE Binary Tables extension proposal, which is now under
consideration by the IAU FITS Working Group. A drafts subdirectory
contains drafts of designs not yet submitted, for example, a proposed
method for incorporating data compression under FITS.  The 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.  

	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