Posted 21 May 2020 - 01:44 AM
Honestly, the response doesn’t make much sense to me. Why would an oil spaced refractor not work? Generally speaking, the radius of curvature for any doublet or triplet refractor is about ⅓ of the focal length. So all refractors of a given focal length should need essentially the same flattener. That’s why, for example, my AP 105mm Traveler from 1994 (oiled) and a current AP 92mm Stowaway (Air spaces) share the same flatteners and reducers. They have the same focal length.
APM, it sounds like, is trying to use the same reducer/flattener for a large number of telescopes of different focal lengths by varying the back focus. This will, of course, change the amount of focal reduction, but OK. If they have given the correct back spacing for another refractor of the same focal length as your 130, I would have expected that to work reasonably well for ANY refractor of that focal length. Obviously, any given reducer/flattener is designed for one specific radius of curvature, so it will only be perfect at that one focal length.