George, I just finished reading the information on your website...It is excellent! The graphics and animations are superb teaching tools. I taught physics for 29 years, and still learned a few things! I would point out that the color produced in the diffraction pattern of a mask is not due to chromatic aberration, but is a product of diffraction itself. Diffraction, like refraction, is wavelength dependent. That is, different wavelengths diffract differently. So diffraction can be used to produce a spectrum (think diffraction grating) like a prism can, but through a different mechanism. The color you see in the reflection from the working surface of a CD is caused by this wavelength-dependency of diffraction. Reflection is not wavelength dependent (all wavelengths follow the law of reflection) so reflectors don't suffer from chromatic aberration. Again, your work shows with great clarity just how and why the Bahtinov or Carey mask works. The only question that remains for me is about the formula for determining the width of the bars and slits in the Bahtinov mask. The prescription is for the width to be 1/150 to 1/200 of the focal length of the scope on which is it to be used. I can't think of any reason for this from a physics perspective. Any ideas?

Thanks for the work you did!

Steve