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I've used a reducer with my C11 once and I believe I had it screwed directly into the primary cell, then the focuser came after. I don't recall any issues with backfocus. Then again it was a while ago. Will have to check. SCTs are usually very generous with back focus. I'm surprised you've hit the limit.
PS: Have you tried putting the reducer at the backfocus then grab an eye piece, look into it and go back and forth until you get focus? This will tell you where you need to be with spacers. No need to move the primary as you will find focus by moving out further. Once you've got it ball park then get closer to the back cell then find a good spot for the primary. Small travel of the primary translates to huge changes in back focus so you should be able to find it IMHO.
Last edited by multiweb; 06-12-2016 at 04:36 PM.
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