This is a useful tool that will help astro-photographers achieve critical focus. Marketed as Sharpstar2 by Lonely Speck, it is a filter designed to fit standard filter holders. You can read all about it on the Lonely Spec website.
This YouTube video from PhotoShelter gets into the physics of diffraction, 'sunbursts' and how diffraction also limits the sharpness of your lenses at small apertures.