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Hello Peter,
yes I think you are right about it being collimation. If it was just tilt I would expect the elongation at opposite corners to be at right angles to each other. Using a Paracorr with the CCD beyond focus results in sagittal elongation (ie. in line with the centre of field), and having the CCD inside focus results in tangential elongation (at right angles to a radial line going through the field centre). So, given that the centre of field is in focus, if tilt was causing the distortion one corner would be inside focus and the opposite corner outside focus.
I'm a little skeptical of CCDInspector's conclusions though - I did a rough
collimation a while back and it told me I was within 1" collimation error, which I found hard to believe. Even your 7" doesn't sound like enough to cause the focusing error that you have? Also, I'm not sure how it can unambiguously determine tilt, given that different optical systems distort in different ways in / out of focus? Maybe someone else can shed some light?
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