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    Re: Plastic vs Metal Sextants
    From: Richard B. Emerson
    Date: 1999 Aug 25, 1:38 PM

    Chuck Taylor writes:
     > 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.)
    
    Actually, it's been my experience that this is one area where plastic
    sextants don't seem to be too bad.  Granted that metal screw to arc
    fits are tighter, practically speaking, this isn't as big a problem as
    it might seem.
    
     > 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.
    
    This, aside from the thermal problem, is the second area where at
    least Davis sextants are simply inadequate.  The optics have poor
    contrast, insufficient field of view, and poor transmission /
    reflectivity.
    
     > 4. Weight:  Plastic sextants are lighter and less tiring to use.
    
    Having held onto a brass Plath for far too long on occasion, they can
    get heavy but generally a round of sights doesn't take long enough for
    fatigue to be a major issue.  Astras and aluminum Plaths, OTOH, are
    light enough to remove problems with fatigue.  I'd call this a wash
    most of the time.
    
     > 5. Cost:  If money is an issue, plastic wins hands down. If you drop a
     > plastic sextant, you don't feel so bad.
    
    Well, there is that.  'Course, it can also be argued that a good metal
    sextant will take knocks that might break a plastic sextant.  Better
    still, get a lanyard on the sextant and use it!  [g]
    
     > 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.
    
    Now here I disagree.  I think I've said elsewhere that learning to get
    good sights with a plastic sextant is rather like skating in sand: It
    can be done but there isn't a lot of progress made.  The problem is
    that even an Astra is a fair investment for someone who is just
    curious to see what's involved in using a sextant.  So either the
    student spends a lot of money or doesn't take the course because it's
    too costly or has a hard time with a barely adequate instrument.
    Tough choice...  Too bad all those WW II sextants are being bid
    through the roof on auction web sites.
    
     > This is of course just one man's opinion; your mileage may vary. :-)
    
    Likewise!  [g]
    
    Rick
    S/V One With The Wind, Baba 35
    

       
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