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

[Peter Bunclark]

On Thu, 13 Jan 2000, Mark Calabretta wrote:

> 
> 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
> (be my guest).  Fitting ARC to Schmidt plates and then converting to
> TAN+poly is no different to doing a model fit for refraction etc. and
> then converting.
I'm not asking for the inclusion of a new `model approach', but the WCS
proposal already has ARC, and it already has the poly,
just not at the same time - but the combination of these is the
simplest, lowest power, description of a Schmidt plate.

> 
> 
> You could perhaps look at it not as a problem but as an opportunity for
> photographic astrometrists to compare their instruments via plate
> solutions represented in a common interchange format.
As I said before, I believe FITS WCS is a tool for the astrophyicist;
it won't do at all for comparing astrometric instruments, except
superficially.
> 
> >Don, you keep using the string _interchange_ - good try, but FITS is
> >just so phenomenally successful it is in fact used for far more than 
> 
> Sorry, but we have never offered anything more than data interchange.
Well of course, in truth, I know that.  Community practice, though,
has a will of its own...
> >patch to the polynomial terms is in practice several orders of magnitude
> >harder than working in a coordinate system that appears in the FITS
> >standard.
> 
> Several orders of magnitude?  Let's be realistic!  For ARC vs TAN it's a
> simple analytic expression.  For more complicated cases I have a program
> using LAPACK which I can make available.  It consumes trivial CPU time.

I guess I didn't explain my point - what I meant was, there are very
few folk who think they understand the mathematics of astrometry.  There
are vanishigley few who in fact actually do understand it. Hence, for most
of us, we rely on a standard, meeting our needs, for which some clever
person has written some rigorous software.  In fact, early in my career
I have written the plate-solution stuff with nothing more than
a spherical-astronomy book and a Fortran compiler, but more recently,
I wouldn't dream of not using the far more trustworthy astrometric
libraries available, or better, complete packages.

Cheers,
	Pete.