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

[Peter Bunclark]

On Mon, 10 Jan 2000, Don Wells wrote:

> Dear FITS WCS ad hoc task force,
> 
> issue (1) , Mark has a Web page on his numerical experiments:
> 
>     Notes on the TAN projection 
>     http://www.atnf.csiro.au/~mcalabre/TAN/index.html
When you click on the BENDXY link on this page, it says:
``Forbidden

Your client is not allowed to access the requested object.'' 

> 
> other distorted cameras need to be checked against TAN+poly before we
> approve the notation?  I would like to see comments from the various
Since I really wanted to get this whole thing finalised, I didn't bring
up the following at ADASS; however, there seems to be enough loose ends
that another might as well be added; the point I want to make is that
Schmidt cameras are concentric optical systems to quite high order
(Seidel perfect) and hence are fundamentally ARC-projection systems;
by starting with TAN you have to have a higher-power, and hence less
stable, polynomial correction.  I would prefer, therefore, to be able
to apply the polnomial to any projection, rather than restrict it
just to TAN.

> It will be appropriate for 'fitswcs' people to solicit comments from
> astrometrists they know in order to assure that Mark addresses their
> concerns in the text of paper-2.
Fundamental astrometry is done with transit circles, Hipparcos & GAIA.
Will FITS WCS describe raw datastreams from any of these types of 
instruments?  Whereas output catalogs and maps will have already
been reduced to geometrically perfect projections.
Differential astrometry, eg parallax & proper-motion studies, when done
from direct imaging, uses the technique of differencing in (x,y) space from
matched images at different epochs.  Hence much intrumental distortion
and projection effects cancel out, and indeed an exremely accurate
reference frame is not even required; again, FITS WCS is not of
much concern.
	I contend that FITS WCS has to stop somewhere; to cover an
infinite number of cases, it would have to be infinite.  So we should
concentrate on the end-user; it needs to be accurate enough to correlate
catalogs, images & maps, and do astrometry down to the requirements of
target acquisition, fibre positioning, etc.  It serves the astrophysicist
rather than the astrometrist.


Peter.