From: astroelectronic@t-online.de   
      
   Hello all,   
      
   I have tested a few TeleVue Ethos eyepieces with a Zygo interferometer at a   
   very fast f/2.8 light cone (tested with a f/1.9 transmission sphere, and   
   aperture limited in software to 67.9%).   
   Three tested eyepieces have quite different amounts of spherical aberration:   
   21mm Ethos  Z8 = 0.80 waves (undercorrected)   
   13mm Ethos  Z8 = 0.33 waves (undercorrected)   
   8mm Ethos   Z8 = -0.48 waves (overcorrected)   
      
   Now let's compare two cases:   
   Case 1: A f/2.8 paraboloid mirror is perfect, and the eyepiece has 1 wave of   
   spherical aberration.   
   Case 2: A paraboloid mirror has 1 wave of spherical aberration, and the   
   eyepiece is perfect.   
   When we make a star test with these two telescopes, will we see the same error   
   in both cases? Or can we argue that the error in the eyepiece is less severe,   
   because it occurs closer to the observer's eye?   
      
   The backgound of my question is that a friend of me has figured a 14" f/2.8   
   mirror, and we think the mirror is quite good, verified by two independant   
   tests.   
   However when making the star test the mirror isn't as good as expected. We are   
   trying to figure out what's going on.   
      
   Thanks,   
   Michael   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)