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).
Answer
A very simple, yet effective method to achieve almost perfect focus is to use a Bahtinov mask. I believe that this is the "mask" that you were referring to. It is a diffraction mask that is placed on the aperture of the telescope, creating three diffraction spikes. When the image is in focus, the three spikes line up perfectly. If it is even slightly out of focus, it is very easy to tell. There are online generators that you can use to make your own, or you can buy more well-made ones from commercial suppliers.
An earlier, and less effective device is the Hartmann mask. It relies on similar principles of diffraction, but it is generally thought that a Bahtinov mask is more precise and easier to use. Besides these, there are other methods of focusing, as you know, but for simple, amateur astrophotography, I'd use a Bahtinov mask.
Just don't forget to take it off before you actually start taking pictures!
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