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: Marvin Sebourn
Date: 2003 Feb 28, 22:31 EST
From: Marvin Sebourn
Date: 2003 Feb 28, 22:31 EST
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
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
