Thanks guy's. F/7.5 is what my setup happens to land at when I put the F/6.3 reducer on (calculated using Astrometrica to plate solve the FOV).
I think I've mentioned before how I need one of these things:
http://www.ziplink.net/users/lester/FLR.html
Because I have my NGF-C w/RoboFocus between scope rear cell and the F/6.3 reducer, the focal length changes quite dramatically. And I get problems with distortions etc (you can see the slightly illongated stars). I also have problems with the camera hanging way off the back of the scope, but only as far as it limiting how far south I can point.
It's even more of a problem if I wanted to put the 350D on there prime focus, I think I'd end up with tones of distortion and huge problems with overhang.
One of these days I'll decide I can spare the $200 for the thing linked above
Roger.