Focusers

I recently saw that a manufacturer had advertised that they put a "low profile focuser", which some people have identified as Vixen focusers, on their Newtonian reflectors. What are "low profile" focusers?

Is there such a thing as a high profile focuser?

Yes, Meade specializes in high profile focusers!

But seriously, a low profile focuser is a physically shorter mechanical aid to position the eyepiece at the focal plane. Since the eyepiece will be closer to the secondary mirror, the secondary mirror can be downsized for a smaller central obstruction (CO). For instance, a f/10 planetary Newtonian might use a low profile focuser to minmize CO size.

Many helical focusers are low profile by design, but a Crayford can be short too. Prime focus photography may be possible with a low profile focuser replacement for a "taller" rack and pinion unit.

See Dave Keller's excellent Newt software application for design considerations, including focuser profile:

http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm