[Robbos30 (Peter)]

Hi, Can anyone help me?
I was talking to a few chaps at Astrofest on the weekend and was pointed to ice in space.
I wanted to know what size secondary mirror I would theroricialy need to turn my f (4.818) Dob into a photographically scope? For visual it has a 4.5 inch secondary currently.
I've been looking at an equatorial platform from Tom O (USA) as well and wondering if the f ratio 4.818 is going to be a major disadvantage? The EQ has a provision for a plug in guider eg ST4.
Just playing with the figures at present.
Thanks.