From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 02/01/2016 01:13 PM, Phil Hobbs wrote:   
   > On 02/01/2016 11:54 AM, Quadibloc wrote:   
   >> 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   
   >>   
   >   
   > Optics nomenclature is what it is mostly for historical reasons, so I'd   
   > guess that you're  quite right.   
   >   
   > The names often don't match up exactly in different domains, either.   
   > For instance, "defocus" is something quite different in the ray and wave   
   > pictures--the ray one you can fix by twisting the focus ring, the wave   
   > one one you can't, exactly--in the wave picture, an axial displacement   
   > contributes terms of all even orders, whereas "defocus" in the wave   
   > picture is 2nd order only.   
      
   Even orders in the wave picture, that is.   
      
   >   
   > Cheers   
   >   
   > Phil Hobbs   
   >   
      
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC   
   Optics, Electro-optics, Photonics, Analog Electronics   
      
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   Briarcliff Manor NY 10510   
      
   hobbs at electrooptical dot net   
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