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28322Re: 203mm TMB achromat refractor

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  • tmboptical
    Oct 3, 2005
      --- In tmboptical@yahoogroups.com, "Donald Rosenfield" <cardar@c...>
      wrote:
      > Thomas, how does this instrument compare with the BO/TMB doublets
      > that have eight times the color-correction of an achromat? That
      > subsumes that this doublet is corrected better than an ordinary
      > achromat?
      >
      > Thanks,
      > Donald Rosenfield

      Hi Donald,

      The TMB designed achromats do not have any better color correction
      than what BK-7/F-2 glass can give you. The difference is the TMB
      achromats are fully optimized (you would be surprised how many
      achromats on the market are either not fully corrected, i.e.,
      zero coma, the C and F color crossings are not at the 70.7%
      zone, and the peak visual is not nulled at the correct wavelength
      (555nm). All these things are corrected in the TMB achromats. Add
      a high quality wavefront, smooth surfaces, and a great multicoat,
      and it all adds up to an achromat that seems to be better than
      what an achromat should be. It is in the details.

      How does the BO/TMB SD doublets that have eight times the color-
      correction compare to the TMB achromats in color correction?
      Eight times better.

      Regards,

      Thomas Back
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