[fitswcs] TAN+poly & astrometric discussions in WCS paper-2

[Mark Calabretta]

On Wed 2000/01/12 15:49:05 CDT, Doug Mink wrote
in a message to: fitswcs at NRAO.EDU

>I agree with Pete here.  It is not that hard to either specify or implement
>procedures for dealing with polynomials attached to many of the FITS
>projections, and I prefer to use the numbers which were fit rather than
>some transformation of them.  That's why I support the DSS WCS directly
>in my package.

We surely want to be able to continue to read DSS headers, but there's
no place for their syntax in CCS.  I stress the word "syntax" (rather
than semantics) because there's a straightforward translation to the
CCS syntax as shown in the example of Section 6.6.  There's no way that
DSS headers can be incorporated into a formal standard because they do
not conform to the precepts of the the fundamental FITS reference -
Wells, Greisen & Harten (1981).  Their pixels are numbered so that the
first has coordinates (1.5, 1.5), not (1,1), and that alone is sufficient
to disqualify them.  But the introduction of the keywords CNPIXn,
XPIXELSZ, YPIXELSZ, PLTRAH, PLTRAM, PLTRAS, PLTDECSN, PLTDECD, PLTDECM,
and PLTDECS was simply unnecessary when CRPIXn, CDELTn and CRVALn would
have done.

In the general case, you're never going to devise an interchange format
for the "model approach" to plate solutions, as opposed to the "empirical
approach" (unless by including on-the-fly compilable code in the header).
You can add polynomials to QSC if you like but it's not going to help.

Cheers, Mark