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Thank you John for taking the time for such a comprehensive reply.
I agree with you about GSO gear, I have had a 12" GSO dob for over 3 years now and think they are the greatest thing for the amateur observer, putting good optics and aperture in packages that almost anyone can afford.
With the images in the filters, your explanation was much as I suspected, and I was considering upgrading to better filters but was concerned that I would end up with the same problem. When I was only using the 12" I wasn't that concerned as the filter didn't make that much of a difference, I mainly only used an OIII to confirm PNs. Now with the 20" I notice that the DS filter makes a considerable difference to nebulosity. I tried out the filter on the Flame nebula the other night and it really popped!
Anyway, thanks again and I will have to think about my next step!
Malcolm
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