[fitswcs] Status of WCS negotiations

[Doug Mink]

Don Wells wrote:
> Traditional astrometry adds polynomial terms to the six term (affine)
> plate constant solution to represent various distortions of
> complicated cameras. The distortions can even include things like
> color (spectral index) terms, which represent lateral chromatic
> abberation (I hope that FITS is never required to convey such
> information). These terms are exactly equivalent to CDij keywords of
> higher order, which we could agree to define. My current opinion is
> that this would be a bad idea, but Doug Mink may want to argue in
> favor of it, because his current astrometric implementations are done
> in this style.

My main argument for using higher order CD terms is that this follows
the plate solutions which are currently being used by many of the people
I know who are currently trying to do astrometry with CCD images.  As
far as I know, most of them are fitting a plate solution, cataloging
objects in terms of that WCS, and throwing it away instead of writing
it back into the image header.  My current program, IMWCS in the
WCSTools package fits CRPIXn, CRVALn, and a CDn_n matrix first and
writes that to the header.  As an option, a polynomial, which replaces
the CD matrix, can be fit to the offset from the reference pixel CRPIXn,
CRVALn.  Usually, I fit six terms each for the x and y axes: a constant,
x, y, x^2, y^2, and xy.  I have experimented with x^3 and y^3 terms,
but that did not make a significant difference in the accuracy of the
fit in any of the images I have tested.  For now, I have used the
keyword COn_i for the ith polynomial term (in the list above) for
the nth axis, and implemented only for two dimensions., the goal
being to get something working so we could start getting good enough
astrometry of faint galaxies to feed the fiber spectrograph which
goes online next year on the upgraded MMT.  I presented some of the
results at the AAS Division on Dynamical Astronomy meeting in
Charlottesville last spring.  This polynomial code is included
in the current SAOimage, SAOtng, and SKYCAT image browsers.  I am
also working on support for the current IRAF polynomial method
(which I will let Lindsay Davis describe), and will support whatever
comes out of this discussion.

-Doug Mink
 Telescope Data Center
 Harvard-Smithsonian Center for Astrophysics