Having come into possession of a GSO RC6, I would like to move into (or at least try out) the world of OAG. I don't think there are any particular backfocus issues with the RC6/CCDT67, but if I'm doing OAG I also want to run it on my short refractor. That uses a 55mm-backfocus flattener or reducer.
My current train is:
M42:QHY183 (17.5+/-) - 21mm spacers* - 16.5mm M42-M48 adapter = 55ish mm
* the 21mm spacers are useful for swapping in and out a 1.25" L-Extreme filter. The filter is the scopestuff filter holder sandwiched between two 10mm M42 spacers. When I swap that out, the 21mm assembly gets capped on both ends (so I never have to touch the filter itself) and replaced with a 20mm spacer plus a 1mm ring.
I spent much of this afternoon sorting out what additional spacers would be required to make a QHY OAG-M or OAG-S work (it's complicated, but I think I have it). I also considered the Orion TOAG, but almost everyone seems to add a helical focuser to that (unless you actually have the Orion guide camera) - if I'm going to do that I might as well just go with the QHY that already has a focuser - unless there is some reason why the Orion is objectively better than the the QHY OAGs.
Then there's the ZWO, and here's a specific question I hope the forum can answer for me. The ZWO says it comes with an M48-M42 adapter that uses 0 backfocus.
With that adapter, does the ZWO OAG essentially screw directly into the M48 threads of the flattener? If so, the 16.5mm optical thickness of the OAG is just about exactly right.
That would be nice, as the money saved could be used on the likely eventual upgraded guide camera.