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    Re: How does the AstraIIIb split mirror work?
    From: George Huxtable
    Date: 2004 Apr 23, 01:05 +0100

    Jim Thompson asked-
    >How does the body appear across the entire circular view of the telescopes
    >when using a split mirror?
    I don't know about the Astra IIIb, but here, I hope, is a general answer to
    Jim's question.
    The reflection from the silvering is getting on for 100%. But even an
    unsilvered glass surface reflects light to some extent, just as you can see
    in a window-pane. Light can be reflected in this way from both surfaces of
    the unsilvered part of the horizon mirror. So you still see an image of the
    Sun in that part of the glass, but a significantly dimmer one than in the
    Some old sextants would take advantage of this, by arranging to reduce the
    brightness of the ray from the Sun, by moving the telescope exactly
    parallel with itself so as to look only at the image in the unsilvered
    part. Indeed, this is the only purpose I can see in having that unsilvered
    area of glass at all. Otherwise, in a split-horizon sextant the horizon
    mirror could just as well be truncated at the edge of the silvered part.
    On the BBC reenactment, in the Endeavour replica, of Cook's passage North
    of Australia, the navigators, who were observing lunars for longitude, had
    modern sextants, both split-horizon type and whole-horizon type. They
    commented, when trying to locate the Moon high up in a bright daytime sky,
    that it was very difficult in a whole-horizon sextant, but the greater
    contrast of a split-horizon instrument allowed the Moon to be picked up
    much more readily.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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