Hi Terry,
A C925 Edge HD is what I have and usually I mount an Orion CT-80 on top as the guide scope. Please see pics. The CT-80 (FL-400mm) is not great for photography but is more than adequate for guiding. With a 0.7X FR, the C925 goes from 2350 to 1645 mm FL, so a 400mm guide scope fits into that thumb rule of guide to main scope FL ratio of 20-25%. It would probably be OK without the focal reducer too.
But on top of the C925 I have a Losmandy DC 9.25 dovetail made specifically for that scope. And I also use the DVR 108/66 scope rings. This ring set has two to hold the guide scope, and another smaller one to hold onto the extensions for the guide camera, so you don't have to worry about flexure of the focuser rack?causing guiding error when the mount changes position over the course of an evening. This was a nice idea the Losmandy came up with.
Hope this info gives you some ideas.
Regards,
John
