From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 06/03/2018 02:28 PM, Michael Koch wrote:   
   > 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.   
   >   
      
   The pupil of the eye will be the limiting aperture if the system is   
   designed for comfort, but it may or may not be in a test setup.   
      
   In the absence of vignetting, I think they'll be pretty similar since   
   both are basically occurring in the pupil (of the telescope).  If the   
   eyepiece were optically closer to the image, the effect would be less,   
   because the phase errors would have mainly a local effect, and of course   
   right at the image, it would hardly do anything except change the FOV.   
   (It would be a field lens and not an eyepiece.)   
      
   Cheers   
      
   Phil Hobbs   
      
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   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC / Hobbs ElectroOptics   
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