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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant precision.
From: George Huxtable
Date: 2005 Jun 19, 18:14 +0100
From: George Huxtable
Date: 2005 Jun 19, 18:14 +0100
>I wrote, about determining shade error- >"one such shade on my sextant quite defeats any such attempts. It's >the very darkest shade, a very deep-blue one in my case, that's required >for viewing the sun. When I look through that shade, it's so dark that the >only object I can see is the sun. Without that shade, I can't safely look >at the sun at all. So how do I compare two measured angles, observed with >and without that shade? What do I look at, to do that job? Others must have >met that same problem. Suggestions, please." And Frank responded- >Yes, I know what you're talking about. What you need is something brighter >than the Moon and fainter than the Sun. That's a big range: around 14 >magnitudes or a factor of 400,000 in luminosity, so lots >or possibilities... I have >used the setting sun which is faint enough to look at comfortably with a >medium shade and bright enough to see through a very dense shade --there >is *some* >altitude near the horizon where this should be possible. I've also used >metal rooftops that are reflecting almost direct sunlight. You get an IC >using >the medium shade (whose shade error you've already determined with the Moon >perhaps) and then you get an independent IC with the dense shade. A little >arithmetic yields the shade error of the dense shade. It's the large factor between the brightness of the sun and the brightness of terrestrial objects that's the reason why the sun shade has to be so very dense, and that's why there's a problem. Yes, it should be possible to deduce a shade error by building it up from measurements of the errors of less-dense shades, but that also builds up the errors in those multiple measurements. So, if it's possible, I would prefer to measure the error of that darkest sun shade (which is the most important one to know about) in a single measurement. But is that possible? Perhaps I could watch a fading setting-sun until it got dim enough that I could tolerate looking at it through the sextant's telescope with no shade at all (a procedure which carries risk and would need to be done with some care). And then, if I swung that dense shade into place, would the sun then be so dim that I couldn't see it at all? I suspect so, but can't be sure until I've tried it. Does any textbook address this problem more thoroughly than Lecky does? Lecky points out that in the best instruments the shades are arranged so that they can be rotated through 180 degrees (top to bottom, that is), and taking the mean of observations made with those alternative positions will null out shade error (and the difference would determine that error). Do any modern sextants have their shades constructed that way? Perhaps the best way for me to assess the error of that darkest shade might be to detach it from its normal mounting (which is rather easy) and instead cobble it back into roughly the same spot using insulting tape, in such a way that it can be inverted, top to bottom. If that shade is indeed found to be prismatic, it could then be oriented to such an angle that it gave rise only to a small sideways displacement of the image, and didn't affect sextant readings. George. =============================================================== Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222, or from within UK 01865 820222. Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.