Don't forget that the critical focus zone (for imaging) gets much,
much smaller, so it can be a bit of a bugger to get perfect focus on faster systems.
If you get the CCDCalc program, you can see what the critical focus zone is for a particular scope combo will come out like.
http://www.newastro.com/newastro/boo...camera_app.asp
Using a dob for long exposure photography can be problematic, especially truss tube designs as they are not as rigid. Not only that, you've got to get them tracking, which on the massive aperture you're talking about, means some pretty tricky gear.
Turbo