If you flip a transparent image also the visible orientation will flip. However the actual orientation in the sky can not be flipped. Any inverting telescope or camera does not change the orientation in the sky so the angle is better reported unflipped.


The CROTA2 is only part of the solution. In is a pair with CDELTA2. You can't specify the orientation of an image with one value like CROTA2. You need two values.

If you want an uniform report by all solvers, then you should use something like this:
if (cd1_1*cd2_2-cd1_2*cd2_1)>=0 then sign:=+1 else sign:=-1;
rotation:= -arctan2(cd2_1,sign*cd1_1)*180/pi; //arctan2 returns arctangent of (y/x)

See www.hnsky.org/astap.htm#viewer_angle

Han