[fitswcs] Re: [fitsbits] new WCS papers

[Doug Mink]

Like Don Wells, I was surprised to see the PC matrix arise in the
post ADASS version of WCS-I, but I also agree that this is not
such a bad thing, as long as the CD matrix continues to be supported.
I spent the good part of a summer several years ago trying to get
an unambiguous rotation angle out of a CD matrix and found that it
is impossible analytically.  Separating the signed CDELTs out from
the rotation makes some processing easier.  My WCSTools software
will continue to support CDELTs, PC matrices, CD matrices, and even
CROTAs as it has since Mark put it in WCSLIB.  I will eventually modify
my WCS-fitting code to fit a PC matrix as well as a CD matrix, but
with the large modern catalogs I am using, fitting 10 parameters
instead of 8 should be no problem.

-Doug Mink
 Telescope Data Center
 Smithsonian Astrophysical Observatory