Mierle and I learned today that all of the papers describing the WCS conventions are pretty difficult! None of them give the complete formulae for converting pixel location to sky coordinates or the reverse. They describe parts of the transformations in words. None of them give fully worked-out examples against which code could be checked.
Furthermore, the WCS conventions themselves are ill-defined, in that there are mutiple different parameters/keywords that mean the same thing, and potentially conflict. So it is a mess. We decided to work internally in astrometry.net with a simplified, well-defined, minimal WCS parameterization, and push all this ugliness to the original reading
and writing
of WCS at the beginning and the end. We started a polemical and pedagogical paper on the subject.
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