Subject: CD matrix in a FITS header

From: landsman at stars.gsfc.nasa.gov (Wayne Landsman (301)-286-3625)
Date: 9 Jan 92 05:24:00 GMT

I'm interested in placing image astrometry in my FITS header using
the "coordinate description" CD matrix favored by the Space Telescope
group.   These are partial derivatives which relate instrumental coordinates 
to astronomical coordinates, and provide more generality than the
the CDELTi astrometry described in the FITS papers.    (For example,
they allow for non-perpendicular axis.)

    I have seen the CD matrix formatted as a FITS keyword in the form
CD1_1, CD1_2... and also in the form CD001001,CD001002...    I believe
that the first format was once deprecated because it contained an underscore.
But the NOST FITS document explicity allows underscores, so is there any
reason to prefer either of the above representations of the CD matrix?

    A more general question concerns the status of coordinate systems
within the FITS standard.   A couple of years ago I thought an effort
was being made to standardize more general coordinate systems within FITS.
However, the latest NOST FITS document makes no mention of this, and limits
references to astrometry to the CROTAi and CRPIXi keywords.    Is there
still hope of standardizing more general astrometry, or has this effort been 
scuttled for a while?

Wayne Landsman
landsman at stars.gsfc.nasa.gov

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From: bschlesinger at nssdca.gsfc.nasa.gov (Barry Schlesinger)
Date: 9 Jan 92 14:55:00 GMT

In article <9JAN199201244080 at stars.gsfc.nasa.gov>, landsman at stars.gsfc.nasa.gov (Wayne Landsman (301)-286-3625) writes...
> 
>   I'm interested in ... using
>the CDELTi astrometry described in the FITS papers....    
> 
>A couple of years ago I thought an effort

	The charge to the Technical Panel developing the NOST Standard
was to codify the existing IAU FITS Standard, eliminating
contradictions between and ambiguities in the original FITS Papers.
The Panel decided early in the process that it would include material in
the Standard only if there was either a basis in the FITS papers or
community consensus on the issue as verified by the IAU FITS Working
Group and reviewers' comments; it would not attempt to create new FITS
rules.  On the basis of these criteria, the Panel decided that it
would not be appropriate to include any World Coordinates
specifications in the Standard.  I agree that standardization of World
Coordinates is needed, but we do not want to include any such rules in
the Standard until the IAU Working Group has confirmed that there is a
consensus. 

	Maybe people would like to discuss the World Coordinates 
questions here.

					Barry Schlesinger
					Coordinator,
					NOST FITS Support Office  

						and

					Secretary,
					NOST FITS Technical Panel