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This is interesting. I'm setting up an all-SX imaging train for my astrograph and that mmeans either a straight MPCC (55mm) or one of those 90mm jobs Baader also sell.
At the moment, my best calculation of the physical backfocus from MPCC flange to imaging train in 57mm and I recall reading from Baader that the MPCC tolerances are only +/- 1mm. But from this thread, I see that my actual optical distance could be more than that 57mm because of the filters in between. Have I got that right?
Peter
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