Thanks for your comments Bob & Matt..
Matt, the f/ratio I use is matched to my camera pixel size to try and capture all that the scope can deliver when I get good seeing.
Have a look at
http://www.telescope-optics.net/tele...resolution.htm for a good discussion on telescope resolution, particularly about half way down the page where there is a discussion on the 'edge spread' function (more representative of planetary imaging) compared to the 'point spread' function.
regards
Maurice