Now, about back-focus distance.. and absolute confusion around Meade/Celestron 4000 Series FR/FF.
In data sheet that came with reducer, nothing was mentioned about that, they supplied only drawing of un-specified adapters and spacers and cameras..
Standard SCT - 2" adapter + EOS camera adapter (ebay, Alibaba etc.) is 69 mm long.. which means the back-focus for EOS camera is 114.5 mm, measuring from SCT flange.
I reduced this distance to widely accepted/recommended 105 mm value by using low profile EOS adapter.
BUT..
here (post #21) Meade says backfocus distance should be:
"The backfocus distance on this 6.3 reducer is pretty forgiving. The customer should use a 85mm back focus when measured from the last lens."
I am pretty sure here the intended information was lost in translation.. and "Last lens" actually means "last side" of the FR.. because measuring from "last lens" means measuring from glass surface.. which can result in glass surface damage. And, optical distances are usually referenced to "optical centre" which is a point usually (but not necessarily) located in the space between glass elements.
Anyway.. 85 or 105 mm, that needs to be determined experimentally, and criteria for optimal distance value is not f/0.63 but shape of the star images.
Does anybody have some recommendation about this?