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    Re: Plastic vs Metal Sextants
    From: Chuck Taylor
    Date: 1999 Aug 25, 10:09 AM

    IMHO, there are five major differences between plastic and metal
    sextants:
    
    1. Thermal stability:  Metal sextants are less susceptible to changing IE
    due to thermal instability. This has already been discussed.
    
    2. Precision:  Metal sextants can be machined more precisely. They tend to
    have less "gear backlash". To see this, measure IE with your final
    adjustment of the drum clockwise. Then do it again with your final drum
    adjustment counterclockwise and compare. The difference is gear backlash.
    (You can compensate for this by always making your final adjustment in the
    same direction.)
    
    3. Optics:  Plastic sextants tend to have plastic mirrors and lenses, and
    these mirrors and lenses tend to be on the small side. Metal sextants tend
    to have glass mirrors and lenses; these mirrors and lenses tend to be
    larger in size and ground more precisely. To my eye, the difference in
    the brightness and clarity of a star's image is like the difference
    between a 10-watt light bulb and a 100-watt light bulb.
    
    4. Weight:  Plastic sextants are lighter and less tiring to use.
    
    5. Cost:  If money is an issue, plastic wins hands down. If you drop a
    plastic sextant, you don't feel so bad.
    
    Plastic sextants are certainly adequate for practical celestial
    navigation. Many navigators have successfully used them for crossing
    oceans and making landfalls. I don't mind using a plastic sextant, but a
    metal sextant is a pleasure to use.
    
    This is of course just one man's opinion; your mileage may vary. :-)
    
    ----------------
    Chuck Taylor
    SEASCAPE GB 32-2
    Everett, WA, USA
    

       
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