Yes, I had a similar problem with a fixed-length eyepiece adapter extension - would only work with 25mm EP 0n 8" f5 Newtonian. I then bought extra tubes and acheived focus with 15mm and 9mm, however the camera was now half an arm's length out from the OTA and putting a big load on the standard focusser (fortunately it is a lightweight DSLR). I bought a strong replacement focusser but discovered that, being low-profile, it required all my tubes again to acheive back focus. I didn't think of trying the Powermate Barlow, also acquired recently, instead of an eyepiece, I suspect I will still require a lot of extension tube.
Still can't get my head around the need for so much back focus e.g. the new focusser has a built-in 50mm extension, but that is still 60mm short of what I need to get (visual) focus with any 1 1/4" EP. The original focusser has the eye lens 170mm out from the OTA, yet I read lots of threads dealing with how to get enough
inwards focus.
perhaps some IIS expert can explain
