The Astrometry.net solver gives you only the four CD keyword for the solution orientation and scale (plus the reference pixel CRPIX1, CRPIX2 and position CRVAL1, CRVAL2) . They provide all the information. If you use them why can't do the same with ASTAP? Then fix all discrepancies. All solvers give you the correct and same solution. But you have to process at least two header keywords and not just one for orientation.

Stellar solver is using the Astrometry,net code. I don't know what Robert did for additional processing for orientation. Hopefully he can clarify this. But to standardize you better move/copy his code line which is a function(CD1_1, CD1_2, CD2_1, CD2_2) to Ekos and use it for all solvers. Then there will be no discrepancies.


Note:
The reason CROTA2 changed in ASTAP is that for flipped images it was wrong for the classic solution keywords (CROTA1, CROTA2, CDELT1, CDELT2). To simulate the old situation it would in principle require a new keyword something like ANGLE because I don't want to ASTAP to produce wrong classic keyword solutions. Since Astrometry.net doesn't provide any angle directly (the classic keywords are missing) it was much better and elegant to refer to the modern CD keywords. So the suggestion is just to use them and ignore CROTA2.


Han

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