DIY Focus mask

Spending 20€ versus self-made

Getting the stars in focus is not that easy. Sometime you find the statement “turn the focus to infinity”. However, at least for my telescope and my camera lenses “infinity” is even behind the focus. For that, there exist so called focus masks with the Bahtinov mask being the number one found in the internet. However, instead of buying one, it is pretty easy to make a simpler focus mask on your own.

It is pretty fast, to click the “buy” button in one of the many astronomy online stores and compared to the price of a telescope the Bahtinov mask is with approximately 20-50€ relatively cheap. But then you need another one for the tele lenses of the camera, …

The Bahtinov masks look pretty complicated and for each aperture another setup is required. So sadly I cannot just scale one mask up or down for usage with other diameters. There exists a formula, how to create those Bahtinov masks and also web pages providing templates, to make them on your own. Luckily I found another kind of mask, which is pretty simple to make on my own and totally sufficient for me: three equilateral triangles in one large triangle. And here is the instruction:

Required tools

  • compasses
  • set square
  • scissors/knife
  • dark and stable cardboard
  • rubber
  • paper-clip

tools

Howto

  1. Draw 3 circles:
    1. as large as the outer diameter of your telescope/lenses (forgotten here).
    2. somewhat larger than the outer diameter of your telescope/lenses (10mm here).
    3. Draw a smaller circle inside (for my 67mm lens I used a 57mm radius).

two circles

  1. You might remember, that you can draw a hexagon, by using the same radius like for the circle? Just punch in the compasses (with the same radius) somewhere on the inner circle, make a mark with it punch there in again and go round. You should end up where you started.

hexagon marks

  1. Use every second mark, to draw a equilateral triangle.

large triangle

  1. Measure the length of one of the triangle sides, subtract ~5mm, divide by two and make a mark on each triangle line with this distance from each corner.
  2. Connect those marks, that you end up with three small triangles inside the large one

marks on large triangle

  1. Cut everything: along the outer circle and the small triangles.

cut

  1. Use a nail to stitch holes for the three paper-clips in the (above forgotten) circle with the same diameter as the lens (don’t use the wrong one like I did here initially).

finished mask finished mask

  1. Fix it with a rubber to your lens. It should not be to tight, since you want to remove after finding the focus without shaking and moving the tripod.

marks on large triangle

  1. Wait until it gets dark, look for a light far away and try it.

Comparison to Bahtinov

In the following fotos, I took pictures with the bought Bahtinov mask on the telescope and the self made one on my tele zoom of the antenna lights on a high house (too small to call it skyscraper) ~2km away.

In my opinion the simple self-made focus mask is not worse than the bought Bahtinov mask. Especially considering the different focal lengths. My telescope with the Bahtinov mask has a focal length of 1500mm, whereas my tele zoom only has a focal length up to 300mm.

Addon 2021-11-19

I found another simple DIY focusmask on fotosvomhimmel.de: two round holes.

References


See also