From: astroelectronic@t-online.de   
      
   Phil,   
      
   Yes, BEAM4 is a very nice program from the past and since a few years it's   
   available as freeware. I began wo work with BEAM2 back in 1994 or so.   
      
   There is also an easy expanation for this result. The lateral error in the   
   focal plane depends on two things:   
   1. The distance from the plane where the error is produced to the focal plane.   
   Smaller distance means smaller lateral error.    
   2. The angular deviation of a beam at the surface where the error is produced.   
   This depends on the wavefront error and the diameter of the light cone at that   
   plane. A 1 wave error over 40mm diameter will produce a larger angular error   
   than 1 wave over    
   200mm diameter.    
   These two effects compensate each other, so that the result in the focal plane   
   is always the same, regardless on which surface the wavefront error is   
   produced.   
   Always under the assumption that we define the Zernike polynomial over the   
   actual diameter of the light cone at that surface.   
      
   Michael   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)