I think what everybody forgets at some point is that they are not made for astrophotography. And hence why should they change their design when it isn't designed for that in the first place. The ED 80 from skywatcher is made for AP and hence you need almost 100+mm of back focus to gain focus for eye pieces.
Visual and Astrophotography are 2 completely different things. They require different aspects, Visual requires a low Central obstruction, AP anywhere up to 50% i have seen and just little things like that. If you go out to do AP first up the scope you will get will more than likely be none of the Skywatcher dobsonian range for a start irrespective of how good value for money they are.
That being said sometimes you need to modify to do what you want it to do, I have done this to my Skywatcher 10" Dobsonian, enough not to call it a Skywatcher Dobsonian any more. Accept what it is that you have and make it work from there. if you don't like to modify. purchase something already made for the job at hand its easier that way!
There is so much knowledge in this forum that a quick question will land you a answer if not within a day within the week to what would best suit.
Just my 2 cents worth
