like rumples I focus using the image on the screen, but this is not through any experience with Hartmann masks etc, I've just been too slack to make one up and try it
. I try and pick the most contrast rich transition on whatever I'm imaging - say the shadow of Saturn's globe on the rings and move backwards and forwards slowly trying to get a glimpse of the best focus on that spot when seeing snaps in place (so I know what I'm aiming for) then start at one side of that and using the electronic focusser (these are absolutely fantastic) nudge a step, wait, nudge again wait etc until I think I've passed the peak focus and then nudge back one. I also tend to "bracket" on focus taking an avi and then nudging focus either side and taking avi's that way I've a better chance of nailing a good one - requires a bit more disk space and post-processing time of course but worth it for a focus slacker like me
cheers,