Hi, what's the shortest focal length Dob that's practical? As the aperture
gets larger (greater than 20") what is the downside of making an F3 dob
rather than an F4.5 (more common). Thanks ...

I own a 12.5 inch F4.06 DOB. It does work and it works quite nicely with a
Paracorr, it is also very touchy about collimation and it is hard on eyepieces.


If you look around, you will see essentially no amateur scopes that are less
than F4, in my view that is with good reason.

In my view, if F3 were workable, all large DOBs would be F3 because it really
would make the scope more manageable.

If you really want to go ahead with this project, I would not process until you
have found someone who owns such a scope and take the time to not only talk to
them about it but also look through it.

jon