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    Re: How does the AstraIIIb split mirror work?
    From: Jim Thompson
    Date: 2004 Apr 25, 19:46 -0300

    > -----Original Message-----
    > > From: Jim Thompson:
    > > I repeated the experiment, this time blocking the horizon mirror's
    > > clear window from the front.  Not surprisingly I guess, for all 3
    > > sighting devices (sight tube, 4.5x30 and 6x40 telescopes) I only saw
    > > the index mirror's image in the right half of the field of view, and
    > > it stopped sharply at the center of the field of view where the
    > > vertical edge of the mirror ended.
    > From: Ken Muldrew
    > This is surprising. When I block my horizon mirror's clear window from
    > the front (leaving the mirrored half unblocked), I see the index mirror's
    > image across the entire field of view with a 5x36 scope.
    > The paper that I'm using to block the glass is lying right
    > overtop of the glass, between the scope and the glass. When
    > I do this with a sight tube I can clearly see the paper and
    > the mirror/glass split, but when I do it with a 5x scope, the
    > paper is too dim to notice and I see a full view of the index
    > reflection across the whole field of view. I did this in partial
    > darkness, so perhaps if there was a bright light shining on
    > the paper, it would reflect enough light to interfere with the
    > image. I see the effect you described in your initial post of
    > this thread even if there is no glass surface for the image
    > to reflect off of.
    I just repeated this on the setting sun, 6x40 scope.  The sun was about 8
    degrees above the horizon. I brought the sun's image down to the horizon and
    then blocked the clear window from the front (telescope side).  Holding the
    sextant vertically, the sun's reflected image disappeared at the center when
    I turned my head and sextant horizontally to swing the image across the FOV.
    The sun's reflected image disappeared before center on the right if I tipped
    the sextant from vertical toward the right, but persisted almost all the way
    across to the left if I tipped the sextant from vertical toward the left.
    So as others have pointed out, verticality is critical.
    I'll try again with low light.

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