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I am imaging with a refractor so do not know if my comments are valid for your scope. I too use CCDIns to check every image. I have noticed a few things that lead me to believe that you cannot depend on a single result from CCDIns to draw much of a conclusion, rather a general conclusion can be made from a group of images. For one thing the various measurements do not seem to be independent of one another. Strangely, I have noticed that when FWHM is lower it is often accompanied by somewhat higher aspect distortion. I think it might have something to do with distortion having a possible larger effect on on a smaller diameter object...that is, distortion possibly shows a higher percentage the smaller the circumference. This is just my theory....no idea if I am correct! However, I have also noticed that aspect map changes when I change filters, even after careful focusing and that results are very difficult to achieve that are in close agreement. At best, I think the tool is very useful to give general trends and clearly a conclusion can be drawn only if a lot of images are taken and the results averaged out. There is discussion about this in the manual for CCDIns along with specific instructions on just what sort of image can be used to evaluate field curvature, etc.
Pete, re guiding. What do your images look like if you take really short exposures...say 10 sec. Do you see much distortion? What you see in a short exposure is probably optical...with longer exposures it guiding or flexture.
Peter
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