What you want is to be able to get the scope near the zenith (maybe up to about 70deg.) without having to use a stepstool if possible. For most that means that their eyeball is up around 66" and when you build a dob right, the eyepiece will end up at the zenith at about the focal langth of the primary mirror. If you are 6'6" in height then go for that longer scope or if you don't mind carting a foot high stool around (you can make it into a storage cablnet for the telescope stuff tho) then you will get additional focal length without any problems.
A 12"F6 is going to be at least 6' high at the EP so be aware of this as you figure out what you really want to build. Once you make your decision, go for it as you have rationalized already what you are willing to put up with.
I built a 4.25" F45 refractor just for the fun of it. The thing is LONG but it does give great views of the planets. It is a little bit of a problem moving it about to view parts of the sky but it is an interesting scope nevertheless.

• f/ratio-I should've been more specific - mattb - May 31, 2003 - 23:55 UTC
  • Re:F/ratio-I Should've Been More Specific - optics - June 1, 2003 - 12:10 UTC