From: etaloncn@gmail.com   
      
   Dear Samlon and Anorton,   
   Thanks for your kindly reply!   
      
   Here I am in a inspection room of an OEM and have not many options and   
   accessories, even a retro sphere.   
      
   I understand Anorton's suggestion that weak refractive index gap contribute   
   less on final wavefront. But as OEM, measuring is the top priority. I will try   
   Anorton's configuration on flat mirror, which should bring less astigmatism   
   than my own.   
      
   The thinking to decrease aperture of measuring beam comes from below points.   
   1, The small aperture bring much less Spherica aberration, which may bring   
   much less overlapping effect. 2, The IRR happens in central area.   
      
   The IRR fringe is so like a discontinuous function. I just can not imagine it   
   caused by polishing process. Maybe it comes from uneven UV curing.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)