wow looking at that rob your getting massive vingetting
. I recently looked at the raytrace programs and found that a increse in the secondary could virtually illuminate 100% the 40d sensor (58-70mm upgrade). but the MPCC causes some good vingetting.
In my flats now its kinda funny, i don't have a nice graduated gradient from the center to the outside. its a very abrupt line where it goes from 100% to 80% odd, this tells me that the MPCC is the culprit, i will do some tests removing the mpcc from the fray just to see what the difference is.
The only way around this is to go for a larger 2.5/3" corrector, they are made and they do cost! but they are quality objects.!
Myself i would like to go for a Wynne 3" 0.75x keller corrector they are about 1200 dollars but will take a F4.7 to F3.5 with a effective focal length of 895mm and effectively turning the scope into a newtonian apo with tight spot sizes! makes for some ultra wide field whooze action!