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    Re: The "big" sextant manufactures
    From: Greg Rudzinski
    Date: 2007 Nov 08, 15:38 -0800

    Extraneous reflected light on the Navy Mark 3 can be knocked out by a
    3/8 inch piece of electrical tape placed at the base of the horizon
    mirror. Field of view issues can be reduced if a sextant is preset to
    the approximate altitude. Any navigation calculator will make short
    work of the presetting task.
    
    Greg Rudzinski
    
    On Oct 25, 11:21 am, John Karl  wrote:
    > 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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