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Thanks once again for your replies - everything helps to learn more.
I only have one camera - the sensor looks very clean, no smudges.
The seeing conditions have been poor - I can hardly remember when there were 2 consecutive clear nights! Possibly was high level (virtually invisible) clouds. I have taken photos in moisture ridden skies - usually the stars get big and fuzzy? I haven't seen a deep blue sky for a long time - always a slight haze - but I do get some half decent images - but nothing like most of you guys.
I looked more closely at the individual subs - I can't see this ring affect on them. However I did notice that a lot of the images had a lot of elongation from drift. My mount has (or rather had) a lot of play in the RA gear - I have now tightened it. This possibly gave poor polar alignment (the celestron all star polar alignment is usually very good - I usually get less than 1 arcminute drift/hour. I know you probably get better - but I have to do a full set up every time.) Last night the drift was more like 1 arcminute/4 minutes which is hopeless without guiding.
Possibly (?)- deepskystacker caused these star rings by trying to stack/process poor images - probably not, but at least now I have removed this. I also uninstalled and re-downloaded DSS - just in case there was a bug, also unlikely but I have to eliminate as many things as I can think of.
Next time I will concentrate on manual focusing - maybe my bhatinov mask is giving an inaccurate diffraction pattern? Also, now I have tightened the gear I will get the autoguider back in action.
I'll keep posting until I resolve the problem - maybe what I learn will help others.
Regards,
Tony.
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