"Chuck" wrote in message ...

Hi, what's the shortest focal length Dob that's practical?

Steve Mock at my astro club has a 25" F/3.5 Dob. It is a little cranky,
takes only expensive eyepieces, and certainly isn't optimized for high
power, but the images are really quite acceptable. And at 25", the
difference between F/3.5 and F/4.5 is the difference between a short
stepladder and a rather scary drop down to Earth.

A propos of the diffraction-limited circle, it is worth remembering
that in most locations, the atmosphere won't allow a 25" scope to
operate anywhere close to diffraction-limited anyway.

Also worth remembering that most pro scopes these days are F/2
or thereabouts.

- Tony Flanders