Rokinon Lenses: see link.
AT10RC, F8, CCD, CFW-10: use 2-inch extension tube, set focuser scale near 2.5 cm.
AT10RC, F4.8, CCD, CFW-10: just a whisker out from full-in.
AT10RC, F8, 6D: Short, wide camera adapter behind a 2-inch mirror diagonal, no other extension tubes. (Try Feathertouch and an extension or A-P snout w/o diagonal.)
AT10RC, F6, 6D: Put AT 2-inch flattener / compressor on short, wide-mouth adapter and use minimal out-travel. Images suck, however.
AT10RC, F5.3, 6D: see here but recheck practicality of flatting and inspect PSF's.
AT10RC, F8, video camera: (tk)
AT10RC, F15, video camera: (tk)
AT65EDQ, 6D: Near 5.5 cm.
AT65EDQ, ST2000XM: (tk, untested)
TMB92SS, 6D w/Orion flattener & 10mm extension (OTA long): near 4 cm.
A-P, 5" F6: tk
A-P, 5" F4.5: tk
10" F4 Newtonian, 6D: insert Feathertouch adapter, then Baader corrector. Focus is about 1/8 inch out from full-in on the Feathertouch (leave the stock focuser at full-in).*
* Is there enough in-travel to use the Feathertouch adapter on the 10" F4 Newt? That would solve so many problems! If necessary, try turning the knurled ring off the 2-inch adapter for a few extra mm of in-travel. [No, there was not room for the Feathertouch adapter. Close, but no. Yes, I could turn the knurled ring off the 2-inch threaded adapter to let it move farther in, did, and yes, now the 6D comes to focus some small distance out from full in while using the Feathertouch adapter. So! Focuser issues solved on the Newtonian! Almost: at full-in, the bottom of the focusing tube intrudes into the light path as revealed by a notch in out of focus stars. This may not be an issue at the actual focus position, but it probably is, and if it is, it's nothing a chop saw or the lathe can't fix. Lop it off. Lopped it off because I could. There. I've been working on this since August 2014. Gracious.]
The 6" F5.9 achromat is really a visual instrument. The Baader Fringe-Killer helps its photographic performance but not as much as it helps its visual performance. In photos, blue-bloat both with and without the Fringe-Killer is objectionable. Focusing with the stock focuser is really not up to top-shelf astrophotography either. It's a nice RFT (remember that that's why it's in the kit!). It's easy to get spoiled, no?