Originally posted by bobbotron Any good tricks on making sure you have the focus right?
I focus by using live view on a bright star. Put the star 1/3 of the way between center and a corner to allow for field curvature. Manually focus until the star shows up on live view, then activate live view zoom to fine tune focus.
Some autofocus lenses offer an imprecise manual focus ring that has too much play. I find that focusing too far in one direction, then slowly backing up the other direction, helps to get things into focus.
Periodically recheck focus by looking at your last photo. This is especially important when pointed near zenith since gravity can cause zoom/focus creep. An especially loose lens might need gaffers tape to prevent movement.
Originally posted by bobbotron Generally does stopping down your lens a bit help for astro?
"It depends." Stopping down offers more forgiveness in focusing, and can compensate for some lens limitations such as vignetting and chromatic aberration. You lose light, though, forcing more images to stack or using a higher ISO.
I recommend not stopping down to start. That simplifies everything else. If you find that you always gets soft images or other defects, then evaluate stopping down 1/2 stop and a full stop with careful comparisons of photos taken on the *same night*.