Subject: Re: FITS format

From: [email-masked] (Barry Schlesinger)
Date: 17 Jan 92 15:18:00 GMT

In article <[email-masked]>, [email-masked] (Archie Warnock) writes...
>In article <[email-masked]>, [email-masked] (Travers Naran) writes...
>>How can I convert FITS pictures from the Hubble into something I can use on my  
>>Amiga? In particular, the FITS document I got from NASA's FTP site is kind of  
> 
>...  The images from the WF/PC 
>instrument on HST are in FITS Random Groups format - there are four 
>images in the data stream, one for each chip in the camera.
> 

	The first point to remember is that FITS is not a simple
format for pure pictures but a generalized data transport format in
which pictures can be stored in the form of a multidimensional matrix,
but such a matrix need not represent a digital image.  In order to
understand what a FITS file contains, it is necessary to read the
header (with software) and look at the values of the keywords and the
accompanying comments.  How to interpret headers is described in the
on-line NASA document.  Proper software can read the header and
extract the stored data that follows.  In the case of HST, it is
useful to read their accompanying documentation.  The NSSDCA anonymous
directory /FITS/software contains a C program for reading and listing
the header of any FITS file. 
	While the HST data are in what is called a Groups format, it 
is not the same as the Random Groups format described in the second of 
the four original FITS papers and the NASA Draft Standard for FITS.  
What HST does is to combine the four images into one matrix, with the 
first two axes being the two axes of the picture and the third axis
representing the separate chips.  The format of the primary array is a
standard BASIC FITS format, which can be (and has been) read by a test
prototype FITS reader that is capable of handling matrices only in
integer matrix format.  Interpreting the numbers requires
understanding the header keywords. 

					Barry Schlesinger
					NSSDC/NOST FITS Support Office