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    Re: Plastic vs Metal
    From: Lu Abel
    Date: 2005 Jun 11, 19:54 -0700

    You will get absolutely no argument from me that a metal sextant will
    always be a more accurate instrument than a plastic one (unless the
    former has been severely abused by an ignorant owner).
    Remember what started this whole thread of discussion:  I posed a
    question of what navigation instruments one might buy for a long
    offshore voyage in a recreational boat given a budget of $1000.   My
    (amended) proposal was for three $200-ish GPSs (one for use as primary
    navigational instrument and the other two sealed in a waterproof, metal
    box (to protect against lightning strikes) plus one of the "other" GPSs
    -- a gray plastic sextant.
    Others on the thread said they would rather forgo one of the extra GPSs
    (or maybe even both of the spares) to purchase an Astra, mainly because
    "plastic sextants aren't accurate."
    I'm sure that for high-accuracy surveying a plastic sextant would be
    totally inadequate.  But in a heaving small boat at sea where, as others
    on the list have noted, 10 mile positional accuracy is more than
    adequate, I wonder if having a one or two minute scatter in sights due
    to the sextant itself would even be noticed when one is likely to have a
    five minute or more scatter just due to boat motion?   And let's not
    forget that the sextant has been identified as a *backup* navigational
    tool, not the primary one.  Under my proposal there are a couple of GPS
    sets and a ton of batteries in that waterproof can, too!
    I don't currently own a metal sextant, so I can't directly take on your
    challenge.  But if I have a chance this summer I might take a few runs
    of sights just with my Davis to see if I get the kind of scatter you
    mention.  (It's been a long time since I've taken a long run of sights
    and I don't recall what the scatter looked like)
    Lu Abel
    Frank Reed wrote:
    > Lu Abel, you wrote:
    > " I'll agree "metal is  more reliable," but by how much?  I
    > have co-taught USPS's celestial  courses for many years and about 90% of
    > our students have used Davis  sextants.  In years of checking sights I've
    > never run across one that  was off because the sextant was off -- any
    > errors have always been traced to  student error.  My own Davis Mark 15
    > is almost two decades old and has  never given me a problem."
    > So how do they compare? Can you do some tests?  Try this: get out your Davis
    > plastic sextant and any decent metal sextant. Take  a couple of dozen sun
    > sights alternating between the two instruments and compare  the results. When I
    > have done this, I generally find a scatter of about two to  three minutes of arc
    > around correct, calculated values with a Davis Mark 15  plastic sextant and
    > about 0.5 minutes of arc with a metal sextant. If  few  minutes of arc error
    > doesn't bother you, then a plastic sextant is just fine. I  own one plastic
    > sextant and three metal sextants (though two of these are  temporary investments).
    > -FER
    > 42.0N 87.7W, or 41.4N  72.1W.
    > www.HistoricalAtlas.com/lunars

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