NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Another Davis Instruments Mark 15 Question
From: John Kabel
Date: 2003 Mar 1, 17:16 -0500
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@aol.com >