I thought that I would share a link that I found that clearly discusses depth of focus, seeing, stopping down fast Newtonians and so on. It's a great read and probably a must read for anyone currently considering a fast newtonian.
I'm going to experiment with stopping my 15" down to 5" for planetary and lunar viewing if seeing doesn't allow decent viewing at full aperture. I have already made an aperture mask from protostar flock board and will try it out next time it looks like full aperture planetary viewing just isn't working out.
Certainly, I don't think I have ever owned a fast telescope that could top the planetary views that my original 10" f/6 offered. Anyway, for any who are interested, it's a really clear, good read:
http://www.brayebrookobservatory.org...thoffocus.html
Dean