This is my implimentation of Pavel Bahtinov's design for a focusing mask. I used the pattern generator found here http://astrojargon.net/MaskGen.aspx to generate the mask for my particular OTA. This program outputs a drawing in scalable vector graphic (SVG) format. I downloaded a free program called Inkscape, found here, http://www.inkscape.org/download/ to open and print the mask.

The mask is real sensitive to focus and not affected by seeing conditions. There is a real subtle difference between the diffraction pattern in the two images below. Pay close attention to the central diffraction spike---it moved from just left of center (out of focus) to center(in focus). As you rake your focuser in and out, the central spike will travel between the outer spikes.