From: rrllff@yahoo.com   
      
   On Wednesday, August 28, 2013 11:54:26 AM UTC-4, alfi...@udayton.edu wrote:   
      
   First, forget about the third lens and model the beam expander by itself.   
      
      
   Set the ratio of the two focal lengths to be -3.   
      
   Set the distance between the the two lenses to be   
   the difference between the sum of the two focal   
   lengths.  e.g. (-6) + (+18)   
      
   Define the pupil as Entrance Pupil Diameter and   
   set it to a sensible size e.g. 2   
      
   Define field type as Angle   
      
   Set flag for Afocal Image Space   
      
   Put in two dummy surfaces, one at the beginning   
   and one at the end.   
      
   You should now be all set, with a 3X beam expander.   
      
      
   I didn't run the program, but this should be   
   correct.  Let me know if there is a problem.   
      
   http://www.richardfisher.com   
      
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