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    Re: The "big" sextant manufactures
    From: George Huxtable
    Date: 2007 Oct 25, 22:14 +0100

    John Karl wrote-
    | 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.
    I was somewhat intrigued to read this, about the field of view that remained
    unobstructed even though half the objective was obscured, that applied to a
    terrestrial scope and not to a Galilean. John Karl has had some interesting
    things to say about sextant scopes, in his book "Celestial Navigation in the
    GPS age", that I haven't read elsewhere, and they provide fuel for thought.
    [I hope to report back more on that book when I have had time to study it a
    bit further, unless someone else jumps in first.]
    In his book, John writes on page 105  "Sextants use three types of
    telescopes; the prism, the terrestrial, and the Galilean telescope". He
    omits a fourth category, that of the inverting telescope. In my own Vernier
    sextant, from the early 20th century, the higher-gain telescope, with x6 and
    x12 eyepieces, is in that category; the other is x3, Galilean.. But it's not
    only "historical" sextants that have such inverting scopes: I think Alex may
    confirm that his modern Russian SNO is fitted similarly. An inverting scope
    certainly takes a bit of getting used to, but once upon a time all high-gain
    sextant scopes were made that way, and navigators managed to adapt to their
    star-groups being upside down, and the sea being above the sky. So that
    category deserves a mention.
    I have compared 5 different scopes that I happen to have at home, from the
    viewpoint of how they behave when a card is drawn across to obscure an
    increasing part of the view, a few inches in front of the objective, to test
    John's contention above, about which he gives more detail in his book.
    The inverting scope from my old Vernier sextant, used with the higher-gain
    (x12) eyepiece, behaves just as John describes. As you move a card across
    the view, the brightness progessively dims, becoming dimmer from one side to
    a slight extent, but you can't really see an image of the edge of the card.
    I have compared that with two Galilean scopes with a gain of x3, one from a
    plastic sextant, the other from my old Vernier sextant, and the two behave
    similarly, and very differently from the x12. Just as John describes, the
    card cuts off the image from one side, and you see a slightly fuzzy image of
    the edge of the card, across the field of view.
    And I have compared also two x6 telescopes, the inverting one from my
    Vernier sextant with its x6 eyepiece, and a prismatic monocular, not from a
    sextant at all. These two behave similarly, and in a way that is
    intermediate between the x12 and the x3, with the card-edge apparent, but
    much less sharp than with the x3.
    What do I conclude from that evidence? It seems to me that this
    field-of-view shadowing effect may not be related to the TYPE of scope, as
    John implies, but simply to its magnification. John attributes it, instead,
    to the presence or absence of an intermediate focal plane, between different
    telescope types; being absent in the Galilean case. My observations don't
    actually disprove that contention, but leave me unconvinced by it. I would
    like some more evidence, or closer reasoning than John has provided, please.
    John ends with the somewhat pejorative comment- "Most sextants have cheap
    Galilean scopes that don't behave this way." Maybe a Galilean scope is
    cheaper than others, but in many circumstances it's the appropriate one to
    use in low-gain applications, in a small craft at sea, when anything
    stronger than x3 is just an embarrassment.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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