How To Make and Use a DIY Bahtinov Mask to Aid Focus

The Bahtinov mask is a device used to accurately focus astronomical telescopes. It is named after its inventor Pavel Bahtinov.

It can be really tricky to get precise focus, especially when using a DSLR viewfinder. Often you won't notice something just out of focus until you put it on a big screen to start post-processing. Unless you want to capture an image, transfer it to a laptop to check, make adjustments, and check again, you need a better way to achieve perfect focus.

This is where the Bahtinov Mask comes in. Like the Hartmann Mask which uses three large circles, the Bahtinov Mask uses a series of slits aligned in a grid. The grids are aligned in such a way that they produce three angled diffraction spikes on the focal plane of the instrument for each bright star or planet. As the instrument's focus is changed the central spike appears to move from one side of the star to the other. Optimum focus is achieved when the middle spike is centred on the star and symmetrically positioned between the other two spikes. Small deviations from optimal focus are easily visible.

The creation of a the is very similar to that of the Hartmann Mask. You should use the Bahtinov Mask Drawings Generator  to create a template for your telescope. You should print the generated template according to the instructions provided.

I also have some generic templates which you can also use to print out.

1. Generic Bahtinov Mask Template for 60mm aperture
2. Generic Bahtinov Mask Template for 80mm aperture
3. Generic Bahtinov Mask Template for 100mm aperture
4. Generic Bahtinov Mask Template for 127mm aperture
5. Generic Bahtinov Mask Template for 6 inch aperture

How To Make a Bahtinov Mask

Find a piece of cardboard with enough space to accommodate the mask template with a little space extra to allow us to create some tabs. I recommend about an inch all around. A cereal box unfolded works well for most telescope sizes. Draw around the telescope or dew shield so you know the area to work in.

Attach the printout to the cardboard, centered on the telescope. Once you start cutting, you do not want the template to move, but you still need to remove the remainder of the template when you are done.

Cut out the template grid pattern with a sharp knife and ruler. To cut out the short edges I used an old flathead screwdriver and hammer to punch through the card. A steel ruler and Stanley knife made light work of cutting the rest of the slots.

Next, cut out the template neatly, leaving an inch extra around the circumference. Snip all around the circumference towards the centre. This will create a series of tabs that will attach the mask to the telescope. Fold each of the tabs inwards.

Cutout a strip of cardboard about an inch wide, and long enough to wrap around the outside of the telescope or dew shield aperture. You can join two pieces together if you can find anything large enough. Make sure that it is a snug fit, but not too tight. Wrap this strip around the telescope and secure both ends together.

Place the mask onto the telescope, positioning all the little tabs onto the cardboard strip. Fasten the mask to the strip using sticky tape all around.

When you remove the mask from the telescope, the shape of the circumference will be held by the cardboard strip, which is now on the inside. I found that without this inside strip, over time the tabs would contract and make the mask unusable.

You can now decorate the mask if you wish, or just paint it black.

How to Use a Bahtinov Mask

To quickly get the best possible focus, follow these steps to use your new Bahtinov Mask.

1. Set your camera ISO or gain high and turn on live view.
2. Aim your telescope at a star. If you cannot see stars in the live view screen and everything is connected properly, it could be that the ISO or gain needs increasing further, the telescope is so out of focus that they are blurred - try spinning the focuser to see if anything becomes visible. It could also be that the star you are pointing to isn't bright enough. Not much light gets through the mask so be sure to focus using a very bright star such as Vega, Sirius or Arcturus.
3. Adjust the focus until the central spikes on the star show an X with a line through the centre.
4. Use shutter priority or bulb mode to capture the star and diffraction spikes. Live view is good, but for the most accurate focusing it's always best to try some test images.
5. Repeat the previous two steps until you have perfect focus.