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Subject: Re: Another Davis Instruments Mark 15 Question
From: John Kabel (jjkabel@XXX.XXX)
Date: Sat Mar 01 2003 - 17:16:54 EST
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@XXX.XXX
>
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