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

[Don Wells]

Peter Bunclark writes:
 > .. 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..

The radial terms in TAN+poly will carry the difference between TAN and
ARC (x^3/3 + 2x^5/15 + 17x^7/315 +..). It may indeed be better to
_fit_ positions of catalog sources in Schmidt imagery using an ARC
projection model, but the BENDXY experiment tells us that the
resulting ARC+distortion description can be accurately transformed to
TAN+poly for _interchange_, at least for the UKST. I know of no reason
to think that any other Schmidts will be more difficult for TAN+poly
than is UKST. In summary, I am not aware of any reason to add the
polynomials to any projection other than TAN.

-Don
-- 
  Donald C. Wells      Scientist - GBT Project        dwells at nrao.edu
                    http://www.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
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