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    Re: The "big" sextant manufactures
    From: W F Jones
    Date: 2007 Oct 25, 22:13 -0400

    You have referenced both the MK II and MK III sextants in your post.  I have never
    examined or used a MK II but vaguely recall years ago discussions regarding the
    improvements the USN demanded in the MK III version.  One significant difference
    was the MK III mirrors are much larger making measurements at sea significantly
    easier.  Sometime after WWII, the MK II was widely available in the surplus market
    at very low prices.  The same cannot be said for the MK III since very few MK III
    legally made it to the market place.  There was a commercial version of the MK III
    but I've not seen one and understand very few were sold.  Someone from Scientific
    Instruments posted earlier this summer that they were selling the old inventory and I
    assume the collimator used to certify the MK III sextant.
    It seems unlikely to the me that anyone would choose the MK II if a MK III was
    available.  I legally own a MK III and it has proven itself to be accurate and reliable.  I
    usually use a ring sight and leave the telescope in the case.
    Frank J.
    Rochester, NY
    From:               John Karl 
    To:                 NavList 
    Subject:            [NavList 3592] Re: The "big" sextant manufactures
    Date sent:          Thu, 25 Oct 2007 12:21:48 -0700
    Send reply to:      NavList@googlegroups.com
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    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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