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    Re: The "big" sextant manufactures
    From: John Karl
    Date: 2007 Oct 25, 12:21 -0700

    The Navy Mark III made by Scientific Instruments in Milwaukee is an a
    Plath Navistar.  As far as I know there were only two suppliers to the
    Navy; the other was M. Low of New York city.  Scientific Instruments
    made their sextants; Low bought theirs from Plath - so I've been
    told.  I've made careful measurement comparisons between the Low Mark
    III and the Navistar and have found them identical (except for the
    Mark III's pistol-grip handle).
    The Plath sextants have been a big disappointment to me.  After
    hearing about them for almost 50 yrs, I looked through one for the
    first time at the "Celestial Celebration Weekend" at the Mystic
    Maritime Museum.  I was shocked -- I thought I was looking through a
    darn toy.  The field of view was a mere fraction of the Navy Mark II
    I'd been using for years; the shades obstructed the FOV; there was
    annoying extraneous internal light; and the horizon only showed on the
    left side!  I considered it a piece of junk.
    After collecting seven popular sextant makes and models, I've
    discovered that the Mark II has a rare design that I was blissfully
    unaware of for decades.  It has a very wide VOF terrestrial scope with
    an internal focal plane.  Just like a prism scope, even with a
    traditional split-horizon mirror, they show a complete image across
    the VOF using only half the objective, essentially just like a whole-
    horizon mirror.  (You can see this in any binocs by placing a opaque
    card over half of the objective.)  Most sextants have cheap Galilean
    scopes that don't behave this way.
    The Scientific Instruments Mark III has a stated arc accuracy of 9",
    effectively the same as the Tamaya Spica 10", and, for common uses,
    not practically better than the Astra IIIB's 20".  I've not seen any
    stated accuracy claims for the Plath sextants.
    I don't consider an arc-error correction table an accuracy claim.
    After all, when the correction is made, what's the instrument's
    accuracy??  And what's the correction between the 10d tabulated
    values? ?  Incidentally, the Low Mark III arc-correction table that
    I've seen has a max arc correction of 6" between 0 and 105d.
    John Karl
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