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    Re: Plastic sextants and prejudice. was: GPS shortcomings.
    From: Chuck Taylor
    Date: 2005 Jun 10, 19:32 -0700

    Just for fun I thought I would post a description of
    all the plastic sextants I know about.  Perhaps others
    can add to the list.
    
    Davis Mark 3
       gray plastic, solid frame
       "split" horizon glass, mirror + "air"
       vernier on the index arm, no gearing or fine
          adjustment
       vernier calibrated to 2' (that's two minutes of
    arc)
       no telescope (sight tube instead)
       plastic shades, frameless
    
    Davis Mark 12
       black plastic, hollow frame
       split horizon glass, mirror + clear glass
       vernier on the index arm with
          knob for "fine tuning", no gearing on limb.
          reminiscent of old "clamp-screw" verniers
          except that the "clamp" part is squeeze-
          and-release
       vernier calibrated to 2' of arc
       plastic optics in the scope
       plastic frames around the shades
    
    Davis Mark 15
       black plastic, hollow frame
       split horizon glass, mirror + clear glass
       drum vernier, calibrated to 0.2' of arc
       gear teeth on limb, worm gear in drum
       plastic optics in the scope
       comes with a sight tube also
       plastic frames around the shades
    
    Davis Mark 20
       same as Mark 15, except it has a light to read
          the vernier and limb, powered by batteries
          in the handle
       can't remember whether it is gray or black
          plastic and don't have one to look at
    
    Davis Mark 25
       same as the Mark 20, except for a whole-horizon
           mirror (called a "beam converger" by Davis)
       definitely gray plastic
       The newer Mark 25's have an LED instead of a
           bulb for lighting the scale
    
    Ebbco
       black plastic, mostly solid frame
       "split" horizon glass, mirror + clear glass
       drum vernier, calibrated to 0.2' of arc
       plastic optics in telescope
       framed shades
    
    I sort of like the Mark 12; I was getting
    intercepts of about 1.5' with it while at
    anchor a couple of summers ago.
    
    I've heard the Davis Mark 25 called a "GPS"
    (Gray Plastic Sextant). :-)
    
    The most striking difference between the plastic
    sextants and my Astra IIIb is, in my view, the
    quality of the optics. Another difference is that
    the index error tends to stay constant on
    the Astra.
    
    Best to all,
    
    Chuck Taylor
    North of Seattle
    
    
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