How to make a bahtinov mask at home
What makes the focus mask invented by Pavel Bahtinov so useful, besides its astounding accuracy, is just how simple and inexpensive it is. The metal frame, laser cut bahtinov masks are nice when you need something sturdy that won't blow away in the wind, but with some cardboard and a box cutter you can actually get the same results.
First, let's go over how it is used.
Once your mount has been polar aligned, point your telescope or camera lens at a bright star like Sirius. It's better to use a star and not a planet, because we need a single point of light to do this effectively, but a reasonably sized telescope will resolve the disc of a planet.
With a star in view, make a rough focus manually, then place the mask over the front aperture. Using your camera's live view screen, adjust focus until you can see the 3 bright diffraction spikes.
Continue adjusting until the middle spike is centered in the X. Take a few test shots along the way to be sure you've got it. Now you're certain you have focus, and you've got the receipts to prove it. No more guessing! No more wasted nights.
Keep in mind, a bahtinov mask is really only useful for astrophotography. When doing visual astronomy you should focus to your eye. Everyone's vision is a little bit different, so a bahtinov mask would not necessarily provide you the clearest visual focus for your personal vision.
The advantage of a paper mask is how inexpensive and accessible they are. I once traveled almost 200 miles to a Dark-Sky Park before I realized I forgot my laser-cut mask at home. Fortunately, the hotel I stayed at on the way had a printer, so I was still able to get my shots.
When you need something more sturdy
Unfortunately, paper masks have a tendency to fly away in the wind (please be conscious of the pristine environments of dark sky locations) or get damaged by the morning dew. If you find yourself printing out the same mask over and over, consider getting a laser-cut metal mask or a sturdy plastic mask. We carry bahtinov masks for both camera lenses and telescopes that will fit nearly all sizes.
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