Subject: FITS basics and information (periodic posting)

From: bschlesinger at nssdca.gsfc.nasa.gov (BARRY M. SCHLESINGER)
Date: 19 Aug 1993 16:19 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 (at least for the time being)
through Simtel-20 [192.88.110.20] 

Dominique Beauchamp, Universite Laval, Quebec City, has released a new
version (0.3 rel. 2) of ViewFITS, a FITS reader for OS/2 2.1,
presentation manager.  It is on ftp.cdrom.com in /pub/os2/incoming,
file name vf03r2.zip. This program gives the user the opportunity to
display FITS images (8,16 or 32 bits, integer or float), modifying the
gray shade palette, doing negation and zooming out. The program now
uses bitmap instead of direct drawing to the screen, with 100x faster
execution time.  It is still a beta version. If you try it, inform him
at beaucham at phy.ulaval.ca. 

	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. 	 

	The NOST has codified FITS as endorsed by the IAU into a
formal standard, eliminating some contradictions and ambiguities in
the original FITS papers.  This 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.  	There is an
experimental PostScript copy of the current version of the User's
Guide, generated by capturing the Macintosh MS Word original in
PostScript format. While copies have been printed without problems on
a variety of systems (a ghostwriter previewer could not deal with
certain fonts), because of the well-known problems in converting
Macintosh Word to PostScript, universal portability cannot be
guaranteed. Please notify the FITS Support Office by electronic mail
of any problems. The directory also contains a modified version of
this posting. An AAREADME.DOC file describes the contents of the
directory.  
	A SOFTWARE subdirectory contains a prototype for software in C 
that
will eventually validate FITS files, along with instructions. Because
this software is still under development, it should not be run before
reading the separate instructions file.  The current version investigates
required keywords for primary headers and prints sample data for
integer data arrays.  There is also C software to read and list the
headers of a FITS file.  Another file provides 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 or electronic copies of the
User's Guide and the 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