Hi Mike,
You'll need to determine what the back focus distance of your scope is, i.e. how much space there is from the back of the scope's tube until where an image is formed in focus.
For example, on my Canon DSLR lenses there's only 44 mm, so I can (just barely) attach my CCD, filter wheel, and lens adapter - but no OAG. On the other hand, my GSO RC8 scope has over 150 mm so there's heaps of room for an OAG.
Eyepieces often need more back focus than cameras at prime focus, so some OTAs are designed such that eyepieces work great at the minimum focus distance - but the ideal focus distance for photography is actually inside the tube (i.e. not enough inward travel)!
You basically just need to figure out how much distance there is between (1) where your camera is in focus, and (2) how far "in" you move the focuser from that position. If that distance is greater than the back focus requirements of your OAG, then you're set.
Many manufacturers produce OAGs, some are even integrated into the CCD camera system to reduce the back focus requirements. Here's one that only requires 9 mm of back focus:
http://www.teleskop-express.de/shop/...-fuer-CCD.html
The Astrodon MOAG seems to be well rated, though pricey:
http://www.astrodon.com/products/har...f-axis_guider/
Good luck!
