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

[Peter Bunclark]

On Wed, 12 Jan 2000, Mark Calabretta wrote:
> third-order is required to account for differential refraction, etc.
> Lasker et al. (1990) used a gnomonic projection for Schmidt plates for
> the GSC (and so DSS) and refers to previous workers who did the same.
> It appears to be the normal procedure in photographic astrometry.
> 
Like many normal procedures, it's still sub-optimal for Schmidt plates.
We established this in practice in the early days of APM (Automatic Plate
Measuring machine in Cambridge, UK).
I'll bet than none of these `normal' procedures involve doing the fit
in ARC and transforming to TAN + radial for _interchange_.

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 
interchange; so much so that producers of reduction packages incorporating
astrometry (I'm thinking of IRAF and STARLINK in particular) are waiting
for this !"$%^&*() WCS to finalise before they'll cut yet more code;
and FITS is becoming a data-reduction normal format, and most folk
will use either packages or things like wcslib building blocks, so my 
point is, doing a lsq fit in ARC and transforming to TAN via an analytical
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.

Pete.