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ASA 2" .73 Back Focus for 8" f4.0 (800mm) Native Newt

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I wanted to ask what the recommended Backfocus for my 8" f/4 scope (native) with the ASA 0.73x reducer is - I don't find documentation for this on any of the pdfs/drawings. Doesn't it perfrom well with a 8" or what's the reasoning behind this? 

I can get down to an absolute minimum of 66.3mm which I fear is too much (basing on numbers for a f/3.8 system and different mirror sizes). Filters would add 0.7mm to the BF of the reducer. 

Any help is much appreciated! 




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