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Thanks.
I use a bahtinov mask and I always focus anti-clockwise to avoid mirror flop - which seems to work well. I take a fair time with the focus to get the diffraction spike centered and this always works well.
As I have said - I have never seen anything close to this before with my images. My stars are usually fairly round (apart from PE issues) across the whole field.
The only thing different to usual was a new camera (60d) - but I doubt whether that could cause this affect - it is similar enough to the 40d and 550d in overall construction.
It was cold (about 1-2 degrees) and I had the dew heater higher than usual and probably higher than it needed to be. I think this may have caused a temperature gradient from the outside of the corrector plate to the middle while I took this image. Since I focused at the centre this would be least affected - the outer part is probably way out of focus causing the coma. By the time I got to the second image the temperature had probably stabilized. Also, I think I turned the heater on after I focused - so the change in temperature may have affected the focal point. (I usually turn the heater on sooner).
Regards,
Tony.
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