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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 22, 19:55 -0600

    > Jim Thompson asked-
    > >How does the body appear across the entire circular view of the
    > >telescopes when using a split mirror?
    George Huxtable answered:
    > 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 silvering.
    This is hard to believe. Surely the front surface reflections would play
    havoc with the image if they were significant. Isn't the phenomenon that
    Jim's talking about simply due to the fact that he's using a telescope?
    Sincd the telescope is focussed at infinity and the horizon mirror is right
    in front of it, the lens will gather light from off-parallel rays coming from
    behind the obstruction. These can then be refracted back into the field of
    view. Although this part of the image will be dimmer, it's still there. For
    example, if I stick my finger in front of my telescope (or binoculars,
    closing one eye so that it's a monocular), I can see the whole scene as if
    I'm looking through my finger. The part of the image that should be
    blocked is just dimmed, not extinguished. The greater the magnifying
    power of the telescope, the less the depth of field, so the further removed
    the horizon mirror is from the focus. With a sight tube (maximum depth of
    field), the mirror presents a more-or-less complete blockage because the
    eye can keep it in relatively good focus while still looking at infinity. With
    a 6x telescope, it will look like the mirror isn't there.
    Ken Muldrew.
    > 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.
    > George.
    > ================================================================
    > 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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