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    Re: The "big" sextant manufactures
    From: John Karl
    Date: 2007 Oct 26, 09:29 -0700

    It's quite understandable that the small mirrors on the Navy Mark II
    might lead to assuming it has a narrow field of view.  But this is not
    the case.  The Mark II has a 9d FOV compared to most other sextants
    that have something like a 4d FOV.  And the shades on many sextants
    reduce their FOV even further.
    The FOV has nothing to do with the objective lens diameter.  Rather
    it's a question of practical optical engineering, which I know nothing
    about.  The Mark II scope itself has a 9d FOV which is not obstructed
    at all by the small mirrors.  So the sextant as a whole has the same
    9d FOV.  The explanation for this is the "apex" of the FOV cone lies
    near the objective lens, which is very close to the mirrors because
    the scope is so long.  So its FOV is unobstructed by the mirrors.
    Thus it's not true that other sextants, such as the Mark III or the
    Plath Navistar, are easier to use at sea.  The exact opposite is true,
    with the Mark II having more than twice the FOV of any sextant I have
    seen. (Some prism scopes do have a FOV approaching 6d.)  For those
    that haven't experienced the whole-horizon wide-FOV using a
    traditional horizon mirror, it makes taking a sight so much more
    enjoyable and esthetically beautiful.
    Nonetheless, I'm not necessarily recommending the Mark II over other
    sextants.  Its small aperture is a disadvantage,  making stars and the
    twilight horizons noticeably dimmer than sextants with much larger
    Simple ray tracing (as I discuss in my book) shows why a lens using a
    focal plane can image the entire field even though half of the
    objective is blocked off.  This analysis is confirmed with all scopes
    that I have seen that use an internal focal plane.
    But using the same simple ray tracing, I can't exactly see why the
    Galilean scope (no eternal focal plane) doesn't display the whole
    field also.  I think it's related to vignetting, depth of field, and
    practical optical engineering, not something explained by simple ray
    tracing.  Nonetheless, experimentally, every Galilean scope I've seen
    shows only half the FOV when the other half is blocked off at the
    So I understand why prism scopes show the entire field, but not why
    Galilean scopes don't.  In my book I explain the former and ignore the
    latter -- you don't expect an author to confess his ignorance, do you?
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