I use an OAG that includes a purpose built focuser
http://www.aeroquest-machining.com/page9.html
Its large pick off prism is outside the actual focuser. I do wish there was a coma corrector in the form of a small segment of one to go over just the pick off prism to correct the comatic stars I get.
I use the MPCC to get rid of coma in the images, but the pick off prism is separate.
Scott