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Author
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Topic: What style of sextant horizon mirror do you recommend, traditional or full view?
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David Burch
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posted February 08, 2006 03:28 PM
For most applications in celestial and coastal navigation, we recommend the traditional half-silvered mirror over the optional one called by various names such as "full-view" or "whole-horizon." Here is the story:
If you have never taken a sight before and are presented with a sun in midday with a dark blue sea and light blue sky, and you were asked to compare the two types of sextants, you might indeed choose the full-view style. It will at this first use of a sextant in these ideal conditions seem easier. And indeed it is this reaction that has led many new users into choosing this option.
What you will soon learn, however, is that this is indeed a very easy sight, and regardless of what sextant you have in your hand, you will in a few minutes of practice be doing it just fine with a traditional mirror. With this standard type of sextant (used since 1750's) you do have to coordinate keeping the sextant pointed at the object as you move around some and rotate (rock) the instrument. With the full-view model, you have broader leeway here and this is easier.
On the other hand, for other sights, things are completely different. The full-view mirror works by splitting the light spectrum in half according to color, by means of special optical coatings on the glass — invented by Davis Instruments (or at least first applied to sextants by them), the folks who make plastic sextants, and later copied by other manufacturers. The surface reflects the bluish half and transmits the yellowish half. The net effect is you see at the same time light passing through it and light reflected from it — but only roughly half of the light intensity in each case. Hence the problem. For faint stars, you are losing half the light so the stars are more difficult to see.
But that is not the main problem. The main problem comes in when viewing anything that is about the same color as the sky. A daytime moon in a "white" sky, for example, can sometimes not be taken at all with that style of mirror. Also when the sea and sky are nearly the same color — which is fairly often — then it is very difficult with this model to check the index correction.
Another drawback shows up when you use the sextant for coastal piloting, either with vertical sextant angles or horizontal angles, such as the famous 3-body fix, which is such an accurate means of piloting it is usually called "sextant surveying." In each of these sextant piloting measurements, you are looking at land overlapping land images where they differ only in the shade of color. These sights are significantly more difficult with the full-view type of mirror than with a traditional split mirror.
In a nutshell, "full-view" mirrors make the easy sights easier and the hard sights harder. We do not recommend them as an option for metal sextants.
As it turns out, the top of the line Davis plastic sextant (Mark 25) comes with them as standard equipment — Davis Instruments did, after all, invent this full-view technology, though it is now offered as an option by most sextant manufacturers. If the plastic sextant is intended primarily as a back up and will thus most likely be limited to sun sights in clear skies, then it does not really matter which one you use, and this one might even have some virtue if the back up will be done by someone out of practice on the sight taking.
For completeness we should mention this exception as well. Very high sights (Hs >85°) are difficult because, with the sun essentially overhead, it is difficult to keep the sextant pointed toward the sun's direction — it's very figuratively like deciding which way is south at the North Pole. They are definitely doable, but it takes special techniques in both the sight taking and of course in the analysis. You cannot use conventional sight reduction methods for near-overhead sights. Well, for these rare sights, a full-view type of mirror makes them a bit easier than a split-view mirror. That said, we still do not change our recommendation.
From: Starpath, Seattle, WA
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