Bahtinov mask anyone?

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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

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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.