See:
http://www.astrosurf.com/altaz/600_e.htm
Photos and description of a 40" f/3...
Clear skies,
Shneor Sherman

"Chuck" wrote in message ...
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 ...