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    Re: How does the AstraIIIb split mirror work?
    From: George Huxtable
    Date: 2004 Apr 23, 16:30 +0100

    In reply to my comments about a split-horizon mirror, as follows-
    >> 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.
    Ken Meldrew replied-
    >This is hard to believe. Surely the front surface reflections would play
    >havoc with the image if they were significant.
    Why so, Ken? As long as the front and back surfaces are plane-parallel then
    reflections in them will be precisely aligned with each other, and quite
    >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.
    I ask Ken to explain how Jim could possibly see an image of the Sun that
    way, looking AROUND the horizon mirror. Any such extraneous light-path
    around the horizon mirror would give him a view of the horizon only.
    So my explanation still stands, even if Ken Meldrew finds it hard to believe.
    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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