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    Re: Advice concerning sextants
    From: John Karl
    Date: 2010 Feb 23, 19:33 -0800

    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.


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