So I bit the bullet and bought CCD Inspector.
Looks like my collimation needs some work - but not too much. The collimation error is between 8 and 13 arc seconds on my RC8.
Also analysing a succession of images from the last few nights, it seems that the aspect ratio shifts between images with a max aspect ratio of 32% on 1 image and only 14% 2 images later. I am assuming that aspect ratio measures the elongation of stars - tell me if I'm wrong here. This says to me that its not a consistent problem ie tilt or collimation which confirms my initial diagnosis that its guiding problems or an inconsistent flexure issue.
Please tell me if you disagree.
I guess this is progress of sorts
Pete
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