Shown above are Ronchi displays generated by the Ronchi DOS program and captured with the screen capture program PrintKey. Starting from the top left we are outside of  focus near the center of curvature of a parabolic mirror. As we move the Ronchi grating toward our 10 inch mirror the vertical bars spread and curve. For the three displays nearest to the center of curvature it is necessary to use a 1000 line per inch grating. Of course one can do anything in the mathematical world.  In the real world a 1000 line per inch grating would be hard to use.   BACK