How many times have I gone home from a field trip only to find my shots a slightly out of focus? More times than I can feel comfortable with.
So, when it comes to focusing a telescope for astro-photography purposes, I don't look any further than a Bahtinov mask, a simple cheap and very effective solution.
This is what I use on my Meade LX-90:
My Bahtinov Mask for the 8"LX-90
Just slip it in front of the corrector plate and aim at a bright star.
Adjust the image focus until the three spikes at each end of the star image are equi-distant.
Betelgeuse in focus through my Bahtinov Mask
When the image is symmetrical, with the central spikes adjusted to dead centre, then the image is at its optimum focus.