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    Re: Advice concerning sextants
    From: Gary LaPook
    Date: 2010 Feb 23, 19:49 -0800

    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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