NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Hewitt Schlereth
Date: 2010 Oct 25, 08:17 -0400
You asked: "Did you have any special procedure? For example, some people suggest dealing with the variability of plastic sextants by taking IC observations immediately before and after each sight and then averaging."
Yes, we checked index error before and after and averaged. I also encouraged the technique of setting the sun a little above or below the horizon (depending on whether it was rising or setting) and swinging the arc till the sun came to the horizon.
In doing my lunars I checked the index error at the beginning of each session, but not at the end because I found it the same from day to day. The method I used to find IE was to compare measured moon semi-diameter with the value in the Nautical Almanac.
In taking the sight I set Jupiter a bit away from the moon and then waited till I could no longer detect a gap. It became apparent to me that I was not able to see a gap less than 1'. What you say about metal sextants makes sense. One of these days I may get hold of a metal sextant again - the gods and eBay willing. :-)
-Hewitt
Hewitt, you wrote:
"But after five days practice ashore and five days at sea, using our one and only David Mk 15, they were consistently getting sun LOP intercepts of 1', and often even better."Wow. That's pretty good! Did you have any special procedure? For example, some people suggest dealing with the variability of plastic sextants by taking IC observations immediately before and after each sight and then averaging.
Let's compare two instruments with and without telescopes. Suppose I have a Davis plastic sextant and a good metal sextant, let's make it a new Astra IIIB just to be definite. I use both of them without a telescope for some kind of standard observation that I know I can trust, like Moon-Jupiter lunars. I could expect errors (at the 1 s.d. level) of about 1 minute of arc for the Astra sextant, and, if we use your Davis instrument (or whatever procedure you use to make it work as well as you do) I could expect errors of about 1 to probably 1.4 minutes of arc for the Davis sextant. Now, the Astra sextant is really only limited at this point by the resolution of human vision. So if I now attach a 7x telescope and repeat the observations, I can expect errors (again at the 1 s.d. level) of about 0.14 minutes of arc, and improvement of a factor of 7 in direct proportion to the magnification. This assumes I've also re-done the IC test with the telescope to get the same level of accuracy there. But by contrast, with the Davis sextant, magnifying probably won't help much at all. It's very likely already close to its limit. We might reduce the error from 1.4 to 1.0 (at 1 s.d.) but we can't take out that residual error that arises from the instability of the instrument due to its plastic construction.
-FER
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