Here is my Bahtinov focusing mask made out of cardboard and some black paint:

Bahtinov Focusing Mask

I have used the mask generator at http://astrojargon.net/MaskGenerator.aspx and Inkscape for printing the generated SVG file.

It works like a Scheiner mask but is much more precise. It helps a lot when you try to find the focus with an astro camera (a Philips SPC900NC in my case). Your naked eye is much more forgiving than the machine eye of the camera because your eye is a camera with a very powerful image processing device attached to it. ;-)

It is very easy to see when you have found the focus with this one. I am going to test my custom made mask a bit more. If it keeps on working this good I will buy a CNC machined one from an astro shop.

Here is a video that shows how you focus with a bahtinov mask: