From: astroelectronic@t-online.de   
      
   > Well, you could do a Foucault knife-edge test.  Not as easy as a star   
   > test, of course, but you don't need additional optics.   
      
   My friend tested the mirror with a Bath interferometer from the center of   
   curvature. Spherical aberration was subtracted in software. I don't remember   
   if he also made a Foucault test.   
   I tested the same mirror with a Zygo interferometer in autocollimation against   
   a calibrated reference flat. Both results are in good agreement. We are quite   
   sure that the mirror is good.   
   The problem is that in the star test some eyepieces show overcorrection and   
   some show undercorrection.   
      
   Michael   
      
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