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    Re: accurate sextant
    From: Alexandre Eremenko
    Date: 2008 Feb 22, 10:38 -0500

    
    George,
    
    > What struck me was its rather strange
    > (to me) construction, in that the
    > index arm swings on the "wrong" side of the frame;
    
    This is not exactly so. The arm moves INSIDE the frame.
    If you look carefully you see that the frame has
    a complicated shape which allows the arm to move
    inside. The handle is attached to the
    "other part of the frame", behind
    the arm. Of course you can call the part of the
    frame behind the arm a bridge, but it is an integral
    part of the frame. The purpose of this
    arrangement is to protect the arm.
    
    Freiberger has similar construction of the frame.
    In general Freiberger looks very similar, except
    it does not have the inverting scope and does not
    have the scale magnifier/illumination device.
    
    > I ask those familiar with this
    > instrument whether they see any
    > advantage in the straight-through, inverting,
    > 6x telescope, compared with an
    > equivalent, non-inverting, prismatic ocular
    > of similar power and
    > light-grasp? Presumably,
    > the prismatic would be a bit heavier; are there
    > other differences?
    
    It is very much lighter, and lets more light through.
    In this telescope, the light passes through only two lenses.
    All astronomical telescopes-refractors are made this way,
    and the reason is to minimize the loss of light.
    So the designers of this telescope surely had the
    same reason.
    In addition, this is the only "modern" sextant telescope
    that has wires. Wires are handy for many adjustments,
    and make it easier to hold the object in the center of
    the field.
    
    In the old days (until approx. 1940-s) many sextants had
    inverting scopes with wires. However the SNO scope has
    a substantial advantage in comparison with these old
    scopes: much larger field of view, and much larger
    diameter of the objective lens. Again this lets more
    light through.
    
    Alex.
    
    
    
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