How to make a simple Bahtinov mask
Focusing mask which was invented by Pavel Bahtinov is considered to be the easiest, the 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. The central spike moves left or right (relative to the side X spikes) according to focus position. When precize focus is achieved - the pattern becomes symmetrical.
This method is very sensitive, as the displacement of diffraction spikes is apparent even with slightest focus shift. Bahtinov mask takes guessing out of the equation, and it can be used also on "regular" camera lenses for astrophotography. In my opinion this method is even superior to manually focusing while reading FWHM value. This is because the human brain interprets geometrical shape much faster then a numerical value, and is capable of evalutating its "average" position (sort of noise filtering) much easier. Also Bahtinov mask is highly superior to Hartmann focusing mask (its older cousing), however it is more complicated to make
The most elegant way of making it is laser cutting, CNC or even 3D printing. 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 an 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:
In a similar way - a daylight solar filter can be made, by glueing a sheet of astrosolar film between two circles of plastic. If you wish - you can make the inner diameter of the mask slightly larger then required (3-4mm) and stick some thick soft tape inside - this will make the mask hold tighter and easier to put on.