In ASTAP version 0.9.314, I have added a hyperbole curve fitting routine to the inspector tab which find the best focus position for each area of the image using multiple images. The difference is expressed in focuser positions and should be a good representation of the curvature & tilt.
My focuser makes about 1500 steps per mm, so the found differences up to 150 steps are very small about 0.1 mm or 0.004 inch. So it would be good to have an other test set of a poorer performing telescope then my APO astrograph.
18:47:33 Finding the best focus position for each area using hyperbola curve fitting
18:47:47 median Focus=36000 , a=1,58025 , b=189,90935
18:47:47 center Focus=35950 , a=1,72525 , b=199,74451
18:47:48 outer_ring Focus=36000 , a=1,33893 , b=167,36795
18:47:48 bottom_left Focus=36000 , a=1,63600 , b=194,74364 offset: 50
18:47:48 bottom_right Focus=35863 , a=1,31063 , b=152,53939 offset: -88
18:47:48 top_left Focus=36100 , a=1,08150 , b=126,90394 offset: 150
18:47:48 top_right Focus=36000 , a=1,59440 , b=186,42191 offset: 50
Below the measured curve and the curve fitting for the CENTER area of the image.
Below the measured curve and the curve fitting for the OUTER area of the image. Note that the dip of the real curve is less deep, indicating off-axis aberration.