WCS questions

[Stephen Walton]

[Sorry if you see this twice;  the first post didn't propagate
very far, apparently.]

At the recent RISE 96 workshop at HAO in Boulder we discussed the 
possibility of adopting the standard in the solar community that the
linear part of the WCS of a solar image result in (x, y) coordinates
such that x**2 + y**2 = 1 at the solar limb, with (x=1, y=0) at the
solar West limb and (x=0, y=1) at the solar North limb.  This makes a
good deal of sense from the perspective of solar astronomers, who often
want to work with the radial distance from the center of the disk.  At
the workshop, I was asked to write a small sample program, probably in
Fortran, to show how to add WCS information to a solar image whose
geometry is given in the header in some other format.

However, I am now unsure how to proceed.  It is clear from the WCS
draft spec that the units of (x,y) are to be degrees, which is far less
convenient for solar (and I presume planetary) astronomers, for whom
the center and the limbs of the object being observed are the logical
reference points.  On the other had, we also need the ability to map
pixel coordinates to latitude and longitude, which is the intent of the
AZP mapping.

How much violence would it do to the WCS draft to have a new CTYPE
pair, perhaps "PLAT----" and "PLON----" (Planetary latitude and
longitude), for which the units of (x, y) would be normalized distance
from object center rather than degrees?  I'd prefer SLAT and SLON for
"solar," of course, but these seem to have been reserved for other
purposes :-) .

A related comment, and the reason this is cross-posted, is which matrix
to use, the PC matrix or the CD matrix.  I ask because IRAF, at the
moment, has the most complete and easy-to-use WCS implementation of
which I'm aware, including automatic update of the WCS when images are
magnified, rotated, or subsections copied, but is based on the CD
matrix.  This convenience within IRAF, and the recent WCS-based  image
matching tasks there, are, in my opinion, one of the major reasons for
adopting a WCS convention.  But I want to remain compatible with other
software such as IDL, FITSIO, WCSLIB, and so on.

It is also quite unclear from the WCS specification "how much" of the
coordinate transformation should be in the PC matrix, and how much in
the CDELT values.  It seems as if the idea is that an image rotation,
for example, would only require changing the PC matrix, but this
should be made more precise in some manner.

Sorry for the length of this.  Thanks to those of you who've read to
the end and care to comment.

--
Stephen Walton, California State University, Northridge
"Be careful what you wish for;  you might get it."	swalton at csun.edu