Hy Murveit replied to the topic 'Plate solving and WCS?' in the forum. 3 years ago

Magnus,

I've recently run across this too, in looking at polar alignment issues. I believe (but am not positive) that CD1_1, CD1_1, CD1_1 & CD1_1 are the elements of a 2-D matrix which defines the angle of rotation and pixel scale of the image. They are the "modern" way of doing this (i.e. as of a long time ago) but we seem to use the previous setup, where we use older versions of the keywords (that should still be respected).
It seems that we use CDELT1, CDELT2, CROTA1, CROTA2.

If you research the proper values of CD* given CDELT1,2 CROTA1,2, then I'd be happy to try and submit a merge request adding those keywords.
You might start with www.atnf.csiro.au/people/mcalabre/WCS/ccs.pdf

Hy

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