What is a Bahtinov Mask?
A Bahtinov mask is a device used to help focus telescopes but can also be used to achieve focus on any camera with a telephoto lens. It consists of grids positioned in a way so that they create a diffraction spike when focused on a bright star in the night sky. Focusing can be on any sufficiently bright star-like object, even a planet.
In this article, I’ll show how to make a super inexpensive and easy to make Bahtinov mask out of some window screen, a couple of plastic container lids, and glue.
How a Bahtinov Mask Works
A Bahtinov mask uses at least three grids positioned so that they produce angled diffraction spikes. When you’re focusing through this design of Bahtinov mask, you will see a “+” and an “X” diffraction pattern. As you focus your lens, these diffraction patterns will move back and forth and start to overlap each other. When you get the “+” and the “X” centered on top of each other, your lens is correctly focused!
It’s as easy as 1,2,3!
- Put the Bahtinov mask on the front of your lens.
- Turn on live view and use the magnify feature to zoom in on a bright star or planet.
- Adjust your focus until you have a perfectly aligned diffraction spike. See my example below.
Tip #1 – If you’re having trouble finding stars in live view, rotate your focus ring. If your focus is too far off you might not see any stars at all.
Tip #2 – A Bahtinov mask will not work to focus on the Moon. If you want to photograph the Moon, point at a nearby bright star or planet and focus on that, and then re-center your camera on the Moon.
Will it Work for Me?
Although this homemade Bahtinov mask can achieve perfect focus, it does have its limitations. From my testing, I’ve determined that this mask design works best at focal lengths of 70mm or longer on a full-frame camera. If you’re using an APS-C crop sensor camera, be sure to consider your camera crop factor to your lens focal length. For wide-angle focal lengths, such as 15 – 50mm, I would recommend using a Carson LumiLoupe. To learn how to check out this video!
To purchase the Carson Lumiloupe on Amazon and help out this website through your purchase, click here!
What You Need:
- A small section of a plastic window screen. A metal screen will work, but it’s harder to cut than fiberglass or plastic mesh.
- A piece of thin plastic that is slightly larger than the front diameter of your lens. Such as a coffee container or storage container lid
- Superglue. Or hot glue.
- Utility knife. Scissors work well too.
How to Make It:
- Place the front end of your lens on the plastic and trace it. I would advise leaving the lens cap on to avoid the risk of scratching the front element. For this example, I cut out two pieces of plastic and will sandwich the window screen in between them. However, you can attach the screen to one sheet of plastic if that’s all you have.
- Cut out the circle. I took the extra step of using a drawing compass to make sure my traced circle was perfectly round, but this is optional. Just remember to cut your plastic a little larger than you need in case you make a mistake.
- Next, we’ll be cutting out the middle portion of our plastic leaving a border around the outside and a strip directly across the center. Leaving a border allows room to attach the screen. To do this, I used a ruler to draw a straight line through the middle of my plastic circle.
- Then I used the same ruler to draw two parallel lines slightly above and slightly below my center line. Space these lines about 1/4″ (5 mm) away from the center line.
- Now using the drawing compass again, I draw an inner circle that will create my border. Using the compass is an optional step. You can just “eye-ball” the cut.
Measure Twice, Cut Once
- Before you cut, look at my example below. Do not cut the entire inner circle out. Instead, you want to cut out two “D” shapes. The size of the border is up to you. It needs to be just big enough to give you room to attach the screen, but not too big that it will block a significant portion of your lens.
Attaching the Screen Mesh
- Attach the bottom screen. For the bottom half, you want to align your screen so that the grid is perfectly vertical and horizontal. Meaning that one set of wires in the screen will be parallel with the middle strip on your plastic sheet, and not at an angle to the center line. This half of the mask creates the “+” look in the diffraction spike when you’re adjusting focus. Use a few dabs of superglue to hold the screen in place and let the glue set.
- Attach the upper screen. For this part, we want to align the screen at a 45° angle to the center line. This mesh creates the “X” pattern. Glue the screen down once it’s aligned. It’s important to have one part of the screen with a “+” grid and the other part with the “X” grid.
- Use scissors or a utility knife to trim the excess screen.
One or Two Plastic Sheets
- If you’re only using one piece of plastic, go ahead and add more superglue on the borders to make sure it doesn’t come apart. If you’ve used two pieces of plastic, align the top mesh with the bottom one before adding more superglue to the border and attaching the second plastic sheet.
- So now let it dry. Plastic is not very porous and depending on the type of glue you used this could take a few hours to set up
Your Bahtinov Mask is Finished
- At this point, you made a Bahtinov mask! I went one step further and hot glued a plastic strip around the outside of my Bahtinov mask. All you need is something to hold it on your lens while you focus temporarily. A bit of gaffers tape will do the trick!
This style of Bahtinov mask is simple to make and made out of items you probably have laying around the house. It has completely changed the way I focus with my camera. If you’ve made this mask or have questions, leave a comment and let me know how it works!