Subject: What FITS Is and Isn't

From: bschlesinger at nssdca.gsfc.nasa.gov (Barry Schlesinger)
Date: 3 Jan 92 14:57:00 GMT

This posting is in response to inquiries that appear from time
to time about the "FITS image format." 
	FITS (Flexible Image Transport System) is a data format 
designed to allow convenient exchange of astronomical data.  A FITS 
file consists of a series of Header Data Units (HDUs), each of which 
consists of a header made up of keyword=value type statements 
describing data that follow.  The data section of the HDU may contain 
a digital image, BUT IT DOESN'T HAVE TO.  Other possible formats 
include tables and multidimensional matrices that are not images. 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, in which standard support for many platforms
exists as part of the package for decoding the image transported.
Separate software must be developed or obtained for converting the
data from the FITS file into a form that can be readily displayed.  As
noted by Don Wells in his alt.sci.astro posting, 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 with floating point data 
or more than two dimensions, is not guaranteed. 

					Barry Schlesinger 
					NSSDC/NOST FITS Support Office