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  • ...In the good old days....

    From the "ankle" of my Chesterman height gage.

    Break out the pencil and paper.



    • Optics,
      Don't believe all you read

      The famous Mars ruler ?


      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


      • Maybe there's more to that than simple incompetence, the old German inch, the "zoll", was 1.037 imperial inches. Works out to 26.3398 mm. Nah, I just think they screwed up.



        • I wonder tho how you deal with stray light with the open construction.
          It's dark at night here. Well, except for now when it doesn't get completely dark at all. Not a productive time for astronomy. At most times of year it's dark at night. No streetlights and I live on a dead end road at the dead end. No headlights and the back field is well removed from the sight of anything but sky. If circumstances required it the scope could be either draped or a sock put on.

          The design of an instrument to be unaffected by temperature is called passive athermalization.
          Interesting. However, from what I can find it isn't a general term. It seems only to apply to optical systems and in particular telescopes.

          It also seems that the term is synonymous with passive temperature compensation, which is how I described it.

          This patent application is related to copending, commonly-assigned U.S. patent application Ser. No. 860,936, filed in the names of O'Brien et al on Mar. 31, 1992, and entitled "COMPACT, PASSIVELY ATHERMALIZED OPTICAL ASSEMBLY", now U.S. Pat. No. 5,120,650 the disclosure of which is included herein by reference.


          The present invention relates to means for effecting temperature-compensation of focal length in an optical assembly.

          Certain optical assemblies such as those found in laser output scanners require a stable monochromatic collimated light beam provided by a laser diode and a collimating lens. For adequate optical performance, the beam source must maintain a predetermined beam quality over a wide ambient temperature range. In conventional apparatus, the beam source and lens are mounted in a mechanical structure that attempts to maintain the beam focal length while the apparatus undergoes temperature-induced structural changes. Hence, the athermalization (i.e., thermal compensation) is effected either passively or actively.

          Passive compensation systems typically rely on the differences in coefficients of thermal expansions of the various elements in the optical system such that there is minimal net focus shift with temperature.

          [my emphasis]
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