The Bahtinov Mask is a variation on the Hartmann Mask which uses three separate grids. 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 centered on the star and symmetrically positioned between the other two spikes. Small deviations from optimal focus are easily visible.
Creation of a Bahtinov Mask is very similar to that of the Hartmann Mask. You should use the astrojargon Bahtinov template generator to create a template for your telescope. You should print the generated template according to the instructions provided.
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 round. A cereal box 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.
Cutout the template grid pattern with a sharp knife and ruler. To cut out the short edges I used an old flat head 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 center. This will create a series of tabs that will attach the mask to the telescope. Fold each on 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.