Jon Isaacs wrote:

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.


Slightly OT:
With all the mud slung at SCT's for their central obstructions, this is
certainly a point in their favor when it comes to wide aperture scopes.
You get the aperture without an impractically long tube, and you can
use a focal reducer to get low power. With Dobs, wide aperture means
long tubes or fast optics.

-Larry Curcio.