From: jsavard@ecn.ab.ca   
      
   What? We all know what a catadioptric optical system is - it's one that uses   
   both   
   lenses and mirrors!   
      
   Don't we?   
      
   However, in the area of astronomical telescope designs, there seems to be   
   something else at work.   
      
   A Maksutov or a Schmidt-Cassegrain telescope is catadioptric.   
      
   Even such designs as the Klevtsov-Cassegrain, where a sub-aperture corrector is   
   used for spherical aberration, are catadioptric.   
      
   But a Newtonian with a glass *coma corrector*, even if that coma corrector is a   
   permanent part of the telescope, is treated as a reflecting telescope.   
      
   Since spherical aberration is larger, and has to be corrected to make the   
   telescope usable, while coma is a more subtle issue, it seems as if only the   
   parts needed to form an image and to correct for spherical aberration are   
   treated   
   as fundamental - and parts that correct for coma are peripheral, not changing   
   the   
   "nature" of the telescope.   
      
   Am I just imagining things here?   
      
   John Savard   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)