How can I get accurate focus for DSLR astrophotography?

Question

I struggle to nail focus for astrophotography with a DSLR. My current method is to use live view at high magnification, manually focus on a bright star or the moon, then leave the lens in manual focus and recompose to my target. The problem is that on the camera screen, a star can look almost the same when it is slightly out of focus, and sometimes I think the focus ring gets bumped while moving the camera. What techniques help improve focus accuracy and make it easier to confirm in the field? I have heard about diffraction masks such as Bahtinov masks, but I do not know how they work. I cannot tether this camera to a laptop, though general advice for others is still useful.

bahtinov-mask live-view dslr astrophotography manual-focus

Originally by rfusca. Source · Licensed CC BY-SA 4.0

rfusca

15y ago

voithos · Accepted 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!

How can I get accurate focus for DSLR astrophotography?

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