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    Re: Another Davis Instruments Mark 15 Question
    From: John Kabel
    Date: 2003 Mar 1, 17:16 -0500

    I agree with this post.  I started out with a plastic Davis 20, and
    could only get about 70 % of the sights under 10 miles (on a stationary
    pier in cold weather) with about 50 % of the sights at or above 5
    miles.  I have continued to play with it off and on, and have never
    gotten accuracy below 3 miles on any set of sights.  I do an index
    check after every group of five sights on one body.
    
    With my Astra IIIB I can routinely get under a mile, with many sights
    below half a mile under the same conditions.
    
    John Kabel
    London, Ontario, Canada
    
    
    > Hello group, new member, first post.
    >
    > I would be surprised if a plastic Davis sextant would have an accuracy of +/-
    > 0.5 minutes throughout the arc. An accuracy of this, 30 seconds of arc, might
    > be compared to the aluminum frame Astra IIIC, quoted at 20" of arc, as were
    > some of the Freibergers also quoted at 20" of arc even though priced at $1000
    > in 1998. Using Google I ran onto a post by someone who taught cel. nav.
    > saying that in his experience plastic sextants " Having taught a Celestial
    > Nav program for some years I've gone through a lot of sextants and I've found
    > that with the plastic ones the biggest errors were usually in the filters.
    > Up to 7-8 ' of arc wasn't that out of the ordinary. Another poster cautioned
    > that " The problem with plastic sextants comes from leaving them in the sun,
    > or in their box in the sun.  The plastic warps, it doesn't take much, and
    > they are virtually rubbish at that point.  Given good care, a plastic sextant
    > will work just fine..." snipped part about excellent plastic sextant accuracy
    > of this second poster, which first poster countered.
    >
    >  I would much rather believe +/- 5 minutes over the arc is much more
    > realistic than +/- 0.5', and only then with frequent IC checks. The
    > Celestaire catalog says that the accuracy of the Mark 15/25 is
    > "Unpredictable".
    >
    > But that is only the opinion of someone centrally located between the
    > Atlantic and Pacific, and ~ 500 miles from the nearest marine vista.
    >
    > And a similar question - what about the cheap brass reproduction sextants
    > sold on Ebay - what of their accuracy?
    >
    > Respectfully,
    > Marvin
    >
    > Marvin Sebourn
    > osugeography---.com
    >
    
    
    

       
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