Mask Experiments     To test some of the ideas on the previous page: A single vertical vane was made from paper card. This produced a spike on both sides of the star. Theory correct so far...   A double vane was made. The animation shows two spikes moving across the star, as predicted.   I was interested in what a single series of slits would produce.               A 30 second exposure with the Sigma 500mm lens         This is a 0.25 second exposure to show more detail near the centre       There is clearly some colour shift.  Blue is displaced above the central line, red is below. This is most probably due to chromatic aberration in the lens (although it is supposedly an 'apochromatic' lens).     This is a single series of slits with the reflecting telescope that should have zero chromatic aberration. Note that there is still a small amount of displacement of red and blue, but it is not symmetrical about the centre.       On march 9th I was contacted by Ferry Zijp (from Nuenen, The Netherlands) who has been doing work on the Bahtinov Mask. This is a superb animation he has made. The simulation is for a D8" , f/4.4 Newtonian and monochromatic light at lambda=550 nm. It covers the defocus range from -250 micrometer to +250 micrometer in steps of 25 micrometer. The intensity plots are "clipped" in order to show the small intensities in the high diffraction orders Is 40 degrees the best angle between slits?   The standard Bahtinov mask has slits at an angle of 40 degrees to each other. This image is using an experimental mask with slits at only 10 degrees. Is it now easier to judge when the central spike bisects the angled spikes?   A new mask - the 'Carey Mask' Even more mask variants!