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    Re: Plastic vs Metal Sextants
    From: Joe Shields
    Date: 1999 Aug 26, 10:38 AM

    Though I have never tried one of the more expensive Davis sextants with
    optics to know for sure, I have read several reports and posts and plus
    with my own extensive experience with the $30 Davis Mark III sextant I
    have come to the conclusion that:
    if you are going to do plastic then cost/benefit-wise the Davis Mark III
    is the best bet.  Accuracy listed by Davis is no better in the more
    expensive models then for the Mark III, and if the "10-watt light bulb
    [vs] a 100-watt light bulb" statement below is true, then naked eye
    through a sight tube (which is what you have with the Davis Mark III) is
    probably simpler and clearer to use.  No sense in kidding yourself with
    one of the more expensive models with plastic optics.  With the Mark
    III, I can consistently get well within the expected +/-10'  accuracy
    range suggested in the Davis booklet.  With three successive shots
    averaged and reduced, I'm usually under +/-2'
    The Mark III (for me) was a great instrument to learn Celestial.  I've
    had mine for 10 years, taught myself Celestial well enough to challenge
    the ASA Celestial Nav. test and pass it with a 92%.
    My next sextant will be an Astra IIIb (deluxe), not one of the upscale
    plastic Davis models.
    Is there anyone out there who has done extensive testing with both the
    Davis Mark III and either the Mark 15/ Mark 20 models who
    agrees/disagrees with my conclusion?
    > ----------
    > From-         Richard B. Emerson[SMTP:navsys{at}PINEFIELDS.COM]
    > Reply To:     Navigation Mailing List
    > Sent:         Wednesday, August 25, 1999 3:38 PM
    > Subject:      Re: Plastic vs Metal Sextants
    > 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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