Bahtinov mask and manual focusing

How to focus with a Bahtinov mask

1. 1

   Pick a bright star and center it

   Choose a bright star near your target if you can, so the mount and lens are pointing close to where you'll image. Center it in the frame. Brighter stars make a clearer pattern with a shorter exposure.

2. 2

   Fit the mask over the front of the lens or telescope

   Slip it on like a lens cap, or screw it onto the filter thread. Make sure it sits flat and doesn't wobble. Some people mark the 'up' side so it goes back the same way each night.

3. 3

   Set up a short test exposure

   Use live view zoomed in on the star, or take 2 to 5 second test exposures at a high ISO. You want to see the three spikes clearly without the star core turning into a big blob.

4. 4

   Turn the focus slowly and watch the middle spike

   As you turn the focus ring or knob, the middle spike slides sideways across the point where the other two cross. Move in small steps and let the image settle after each one, because touching the lens makes it shake.

5. 5

   Stop when the middle spike is centered

   When the middle spike sits exactly in the middle between the two tilted spikes, with all three meeting at a single point, you're in focus. If you pass it, the spike appears on the other side. Go back until it centers.

6. 6

   Lock the focus, then take the mask off

   Tighten the focus lock or tape the ring. Remove the mask gently so you don't knock anything, and take a normal test frame to confirm the stars look small and round.

7. 7

   Check again as the night cools

   Optics shrink as the temperature drops, which shifts the focus. Refocus after a big temperature change, after a meridian flip, or any time stars begin to look softer.

Reading the pattern

• Middle spike off to one side

  You're out of focus. The further off the middle spike is, the further you are from focus. The side it's on tells you which side of focus you're on.

• Middle spike crosses to the other side

  You've turned past focus. Come back slowly. You'll cross through the right spot as the spike passes through the middle.

• Spikes meet at one point

  All three lines pass through the star in the same place, and the middle line is equally spaced between the other two. That's focus.

• Pattern looks smeared or wobbling

  Wind, a flexing tripod or a hand on the lens can blur it. Wait a couple of seconds after each adjustment. A remote or phone control for the camera helps.

Tips that save a night

• Use a bright, not saturated, star

  If the center of the star is blown out white, the spikes are hard to judge. Shorten the exposure or lower the ISO until you see color in the core.

• Focus at the temperature you'll shoot at

  Let the gear cool for 15 to 30 minutes first. Focusing right after you carry it out of a warm house means it will drift.

• Don't leave the mask on for the real photos

  It blocks most of the light and adds the spikes to every star. Its only job is to find focus.

• Watch for dew

  A foggy lens or corrector makes stars bloat no matter how well you focused. A dew heater or shield is a cheap fix.

• Wide lenses are forgiving, long ones aren't

  On a short lens, small focus errors barely show. On a long lens or telescope, a tiny turn matters. Take your time.

Other ways to focus

• Live view and the stars themselves

  With no mask, zoom live view on a bright star and turn focus until it's as small and sharp as it gets. It works, but it is harder to tell when you've hit the best point, because the star shrinks gradually.

• Software measurements

  Capture software such as NINA, SGP or ASIAIR can measure star size (HFR or FWHM) and find best focus by itself. This is the most repeatable method, and the one most people move to once they have a computer or controller on the rig.

• Other mask designs

  Variants such as the Tri-Bahtinov and the Carey mask make the pattern easier to read or more sensitive. The idea is the same: line up the spikes.

Questions

What is a Bahtinov mask?

A flat cover with a pattern of slots that fits over the front of your lens or telescope. The slots bend the light from a bright star into three spikes. How those spikes line up shows how close you are to focus. It was invented by Pavel Bahtinov, a Russian amateur astronomer.

Why do I see 'Bhatinov mask' written both ways?

The correct spelling is Bahtinov. People often type it as Bhatinov by mistake, so you'll see both online.

Do I need one?

No, but it's one of the cheapest upgrades in the hobby. Without it you are judging star size by eye, and that gets harder on a small screen in the cold. Many people also print their own to match their lens.

Does the mask have to match my telescope or lens?

Yes. It needs to cover the opening, and the slot spacing is sized to the aperture. A mask for a 70 mm refractor will not work well on a 200 mm telescope. A mask made for a camera lens fits that lens' front diameter or filter thread.

Can I use it with a camera lens on a small tracker?

Yes. Fit it to the lens, zoom live view on a bright star and focus by the pattern. Take the mask off once you are done, and check that the focus ring hasn't moved.

How often should I refocus?

Once at the start, then again after any big temperature drop, a meridian flip, a lens or filter change, or if the stars look soft. On a cold night that might be every hour or so.