Hi,
Not exactly what you are asking but related. I own an Epsilon 160, excellent telescope and in order to pull all the performance out of it I am using OAG.
On way to achieve 55mm of backfocus with Camera+FW+OAG is using the SX FW and OAG. I have an Atik 460EX and I can reach 55mm. The trick is that the SX OAG can use different adapters/threads and there is one adapter for that threads directly to the Epsilon's Corrector, that way you eliminate extra back focus of step rings.
Feel free to contact me if you need more information.
CS,
Jose