I've been doing a bit of astrophotography and the problem that I almost always have is nailing focus. I'll manually focus on a bright star or the moon (which hopefully isn't an option!) by magnifying on live view, leave the lens on manual focus, and then rotate the camera to the target.
My problems happen two fold:
Evenly greatly magnified on the live view screen, there's very little difference between in focus and slightly out of focus of a bring point light source. It's extremely difficult to tell in camera if its right, but painfully obvious once on the computer after the exposure.
I think sometimes the focus ring gets bumped while moving the camera to the target. So my focus ends up way off.
What are techniques or methods to help here? I've seen some reference to some sort of diffraction mask that could help, but don't know anything about it. My particular DSLR doesn't tether to a laptop, so computer aided focus is out for me (but it may be worth mentioning in general for a wider audience with similar issues).