Home made Bahtinov mask + templates
Focusing mask which was invented by Pavel Bahtinov is considered to be the easiest, the fastest and an accurate tool for manual focusing. During the focusing procedure the mask is placed in front of telescope's aperture and then the telescope 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 diagonal "X" spikes) according to focus position. When precize focus is achieved - the pattern becomes symmetrical.
This method is very sensitive, since the displacement of diffraction spikes is apparent even with the 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 much easier. Bahtinov mask is highly superior to the Hartmann focusing mask, its predecessor, however it is more complicated to make.
I have generated printable PDF templates for the most popular telescope apertures using the original bahtinov mask generator from "Astrojargon" website. You should use Adobe PDF or a similar software for printing. In printer settings make sure to select the correct page size (A3 or A4), 600dpi resolution and set the scale to 100% (or select "actual size"). Note that for larger apertures you'll need to print a "mosaic" of several pages, and then connect them together. Each page has 1 inch overlap and markings which indicate how to connect each piece.
Making the mask
The most elegant way of building the mask is laser cutting, CNC or even 3D printing. I've also read reports on printing the mask on paper with a laser printer and then transfering it to a glass using a hot iron - method which is suitable for making a small mask for DSLR or a refractor.
I've made the mask for my 8" reflector using materials commonly avaliable in an office supply store. This is the same method I use for making aperture solar filters as well.
Required tools and materials:
- Binder made from hard, thin plastic (must be non transparent)
- Stanley or a hobby knife and scissors
- Contact glue
- Ruler and a marker pen
- Printer (A3 if possible for larger scopes)
First step was to print the mask - you can use the provided templates, or use the original bahtinov mask generator, and then print it (any modern browser can open the generated .svg file, however chrome works better). In my case I had to print the mask 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 (to ensure that diffraction spikes will be bright and thin)
Then the annoying task - cutting the plastic. I did it by placing a metal ruler over each slot, and slicing several times over each edge, without applying too much pressure.
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 mask out using scissors, while leaving 12 small rectangular sections around the circle. These sections should be bent 90° inside after cutting:
Then I cut a few strips of plastic, to fit these 12 cutouts (2.5cm wide in my case). I glued the strips to the bent sections using a strong contact glue. You should follow the instructions written on the glue tube, which usually tell you to spread the glue on both surfaces and then let it dry for 10-15 minutes before attaching them together:
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. You can also make the inner diameter of the mask slightly larger then required (3-4mm) and stick a thick soft tape inside - this will hold the mask much firmer.