From: astroelectronic@t-online.de   
      
   Phil,   
      
   I'm not sure if I have understood all this correctly. Let's use another   
   example:   
      
   We are star testing a lens with 10cm diameter and 100cm focal length.   
      
   Case 1: The lens has 10 waves of astigmatism (over the full 10cm aperture).   
   The spot size in the best focal plane will be much larger than the Airy disk.   
      
   Case 2: The lens is perfect, but we put an astigmatic window exactly in the   
   middle between the lens and the focal plane. The window introduces 10 waves of   
   astigmatism (over 5cm aperture) into the light cone.   
      
   In both cases the light cone has 10 waves of astigmatism when it arrives at   
   the focal plane.   
   But I think case 2 will have a smaller spot size than case 1. Do you agree?   
      
   Case 3: Let's assume the astigmatic window is placed at distance x in front of   
   the perfect lens. In this case the spot size is independant of distance x,   
   right?   
      
   Are there any general rules how to estimate the spot size, depending on the   
   position along the light path where the aberration is introduced?   
      
   Thanks,   
   Michael   
      
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