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Here is a link to John Karl's article in Ocean Voyager:


http://www.oceannavigator.com/ME2/dirmod.asp?sid=&nm=&type=Publishing&mod=Publications%3A%3AArticle&mid=8F3A7027421841978F18BE895F87F791&tier=4&id=5CCB2C3BEE60466293198B45815C66E8

gl


John Karl wrote:
>
> This is an old, complicated, and interesting topic.
>
> And yes, Jeremy, you're right you see a whole-horizon effect with a
> traditional half-silvered horizon mirror. I know of only two places
> that address this. One is the Ocean Voyager summer 2008 issue that I
> had mentioned in an earlier post. The other is my "CN in the GPS Age"
> book.
>
> And Jeremy, if you see a whole horizon with the trad mirror, I can see
> from here (N44.065, W88.607) that you're using a prism scope. Prism
> scopes have an internal focal plane that accomplishes this. The Navy
> Mark II scope also has an internal focal plane and likewise gives the
> whole-horizon view with its trad mirror. And BTW, it has the widest
> field of view of any sextant i've seen (I've demonstrated this to
> Ken). In fact it's the best view I've see in any sextant, although a
> bit dim for stars (and the twilight horizon) because of the scope's
> small aperture.
>
> And Ken may be right in principle, but it seems to be insignificant
> because viewing the transmission and reflection from one side, then
> looking at the same transmission and reflection from the other side, I
> can discern no difference in the whole horizon mirror that I have.
> It's symmetric as far as I can observe. So I conclude that, as a
> practical matter, the whole-horizon mirror transmits approximately
> half the intensity over twice the area of the trad mirror, giving the
> same image brightness (and likewise for the reflected image).
>
> The more important point is contrasts in dim twilight. The
> whole-horizon's superposition of the two low-contrast images produces
> a combined image of even lower contrast. This effect is observable
> only because the eye is very sensitive to contrasts in dim light.
>
> I agree with Jeremy. My favorite combination is the trad mirror with
> the prizm scope.
>
> JK
>
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