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    Re: Sextant optics
    From: Paul Hirose
    Date: 2004 May 5, 13:50 -0700

    George Huxtable wrote:
    > 3. Only that part of the exit pencil that can pass through the pupil is
    > collected by the eye to form an image. Any excess light goes to waste.
    That's true. However, the wasted portion of the exit pupil gives your
    eye latitude to move off-center while remaining fully illuminated.
    That should be beneficial on a pitching deck.
    With an oversized exit pupil, I believe the portion of objective lens
    that's active depends on your eye's position. If it's exactly centered
    in the exit pupil, the center of the objective is being used. Say your
    pupil size is 5 mm and the scope is 4x40. Then the center 20 mm of the
    objective is active. If you move your eye off-center in the exit
    pupil, the active area of the objective moves too. This could be used
    to vary the relative illumination of the horizon and body. That's what
    intuition tells me, anyway. I own only bubble sextants, so I can't
    test the theory.
    There are times when an undersized exit pupil can help. Some astronomy
    buffs report sharper stars with 10x50 binoculars compared to 7x50. The
    smaller exit pupil avoids the outer area of the eye's lens, where
    aberrations tend to be worst.
    Also, as we age our maximum pupil opening tends to decrease. Many
    people in middle age can attain only 5 mm in darkness, though in their
    youth they could get 7 mm.
    Some years ago I measured my maximum pupil size. The procedure
    involves poking a pair of pinholes in a piece of metal foil and
    holding it to your eye while viewing a suitable target at night. It
    must be bright enough to be visible, but dim enough to keep your pupil
    wide open. By trial and error you find the widest pinhole spacing that
    fits your pupil.
    I think the test can also be done by determining the widest opaque
    strip you can see around on both sides. Same principle as the
    pinholes, really.

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