How to make a simple Bahtinov mask
Focusing mask which was invented by Pavel Bahtinov is considered to be the easiest, fastest and a very accurate tool for manual focusing (you may follow the following link for original bahtinov mask generator and additional information).
During focusing procedure the mask is placed in front of telescope's aperture. The telescope then should be pointed to a bright star. Bahtinov mask produces 3 diffraction spikes. Two of them form an "X" shape, and another spike crosses it, forming a "Ж" shape, and it moves left or right, according to focus position. When a precize focus is achieved - the pattern becomes symmetrical.
This method is very sensitive, and the displacement of diffraction spikes is apparent even with slightest focus displacement. Bahtinov mask takes guessing out of the equation, and it can be used also on "regular" camera lenses for astrophotography. Bahtinov mask is highly superior to Hartmann focusing mask, however it is slightly more complicated to make.
The most elegant way of making it is laser cutting, or CNC. I've also read reports on printing the mask with a laser printer, and transfering it to a glass with hot iron.
I've made myself a Bahtinov mask using materials commonly avaliable in any office supply store.
Required tools and materials:
- Binder made from hard and thin plastic
- Stanley knife and scissors
- Contact glue
- Ruler and a marker pen
- Printer (A3 if possible for larger scopes)
First step was to generate a mask using this bahtinov mask generator, and then print it. You can use Mozilla Firefox to open the generated file. Since I printed the mask on A4 paper - it didn't fit on one page - and I had to make two halves on two pages.
Then I cut one half of the plastic binder, and attached to it my printed mask . I used a masking tape, making sure it's smooth and held firmly in place. It is important to keep the lines straight and parallel (while it won't affect the focusing accuracy, it will make the spikes thinner and easier to "read").
Then the annoying task - cutting the plastic. I did it by placing a ruler over each slot, and sliced with a knife, without too much pressure, several times over each cut.
after the slots were cut out and the paper removed - the mask looks like this:
Next step was to measure the telescope outter diameter, and to draw a circle on plastic. Then I cut the plastic with scisors around, while leaving 12 small sections around the circle, for attaching the mask to OTA. These sections should be bent 90° inside after cutting:
Then I cut a few strips of plastic, to fit the 12 cutouts on the mask (2.5cm in my case). I glued the strips to bent sections using a strong contact glue:
The final result: