[Tony_ (Tony)]

I seem to be having trouble getting a decent image lately (9.25 SCT. CGEM mount, Canon 550d). The attachment show the problem I am having with stars. (Don't worry about the quality - I haven't processed it much) I know SCTs don't give stars as sharp as a good refractor - but I rarely used to get stars like this now I get it every time.
I use a Bhatinov mask to focus - and I am sure I get this quite close.
It's not mirror flop. Collimation is quite good.
I suspect it has something to do with seeing conditions? It is cloudy most of the time with just a few clear "gaps" (1 night) - perhaps this problem is caused by disturbances?
The problem is that the stars are either a little circle or a star with a circle around it.
Does anyone know what causes this and how to fix it? I used to use a CG-5 mount now I have a CGEM which tracks better - I haven't made any other changes.

Regards,
Tony.