Bahtinov Focusing Mask

Bahtivov Focus Mask Project

This webpage documents construction of a plastic Bahtinov focusing mask for a C8. The only design requirement was to avoid using cardboard or paper because this would absorb moisture if stored in my observatory. The Bahtinov mask was fabricated from free on-line templates and an old plastic paint bucket lid, and the project required a single afternoon.

Focusing for Astrophotography

I took focusing for granted and never worried too much about it until I began experimenting with webcams for astrophotography. Focusing for visual observation is pretty straight forward, but I was surprised by how difficult it was to get a webcam in focus on a simple target such as a planet. There are many different focusing methods that range from pretty simple (by eye) to computer aided methods. A description of 17 different methods, arranged by accuracy, can be found at Astropix.com.

Bahtivov Mask

The Bahtinov mask is a plate with three sections of parallel slits that fits over the end of the telescope and produces diffraction spikes on the focal plane (below left photo); this is just like the diffraction spikes formed from the spider vanes on a Newtonian telescope. Each section of parallel slits is orientated at a slightly different angle, giving three different diffraction spikes for each bright object. The below center and right photos show Sirius through the Bahtinov mask. The Bahtinov mask forms a long cross, composed of two diffraction spikes, centered on the bright object. The third diffraction spike lies within the cross. As the focus is adjusted, the center diffraction spike appears to move. When the center diffraction spike is symmetrically placed within the cross formed by the other two diffraction spikes, then the object is in focus (below right photo).

Bahtivov Mask Template

Free Bahtinov mask templates are available on-line. Some websites provide finished templates, while others have a template generation program that allows specification of aperture, focal length, slit width, etc. Most Bahtinov templates are generated in SVG (scalable vector graphics) format, but there are also free programs available to manipulate and print SVG files (Inkscape, GIMP). I used the template generation program on the Astrojargon Mask Generator Page to generate my template (below left photo). There was a problem with the template generator and it would not produce the center cut out that fits around the C8 secondary holder, so I modified the template myself (below right photo). The black marker on the finished template is from the process used to transfer the pattern onto the plastic blank.

Cutting the Template

A cardboard or paper Bahtinov mask would have been much easier to fabricate, but this would absorb moisture if stored in my observatory. Instructions for a cardboard Bahtinov mask can be found on the Sky at Night Magazine website. I used a plastic paint bucket lid because the plastic was strong enough for the open slit design, but still soft and flexible. I didn't want to be placing a hard or sharp mask anywhere near my C8 corrector plate. I covered the plastic lid with masking tape, taped the template to the lid, and used a large permanent marker to transfer the design onto the masking tape (below left photo). I first tried cutting the plastic with scissors and a razor knife, but it was too difficult to cut and retain any degree of accuracy (meaning cut straight slits). I found that a small hobby cutting tool and a scrap of metal as a straight edge worked very well to cut the plastic (below right photo). I used the metal edge as a guide, giving a very straight cut.

It worked best to rough cut the slits and then make the end cuts with a razor knife. I then broke each slit into two sections that could more easily be removed; that is why each slit is drilled with a 5 mm hole (below left photo). After all the slits were rough cut, I used the straight edge and saw to fine cut the slits to the proper width. It worked best to do the fine cutting after most of the slit plastic was removed because there was less resistance, allowing better control of the cutting tool. The final Bahtinov mask is shown below (below right photo). I tried coloring several plastic scraps black, but paint and permanent marker would not hold onto the soft plastic and either cracked or smudged.

The completed Bahtinov mask on the C8 (below photo).