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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 25, 20:00 -0600

    Jim Thompson wrote:
    > 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.
    Hi Jim,
    I tried this (bringing the sun down to a neighbor's roof, though) and I
    think the whole issue is now clear to me. With my 5x36 scope centered
    on the horizon mirror (I pull the eyepiece back as far as I can so that the
    mirror is plain, if not well focussed, and then move the scope back and
    forth on its mount until the split line is in the center of the field of view;
    the scope is then re-focussed to infinity), and the glass part totally
    blocked between the scope and the horizon glass, I can rotate the
    sextant so that the reflected sun moves all the way across the field to the
    right but when rotating the sextant so that the reflected sun moves to the
    left, the image of the sun is blocked close to, but not quite at, the left
    edge of the field of view. If I move the telescope out from the sextant (I'm
    using a Tamaya sextant, so the scope mount should be the same as the
    Astra) so that the split would appear on the right side of the field of view
    (closest to the sextant body) if I was able to see it, the sun vanishes at
    the center of the field of view when the sextant is rotated (but again, the
    sun can be rotated to the right all the way to the edge of the field). If I
    move the telescope in closer to the sextant body, then I can rotate the
    sun from one side of the field of view to the other.
    When the telescope is centered, and the sextant is rotated so that half
    the sun vanishes, removal of the paper does not affect the image
    whatsoever. I think it is clear that in this sextant, at least, reflection from
    the glass surface plays no role at all in normal sighting with the
    instrument. I think these reflections are so much dimmer than the rays of
    light that enter the telescope from the edge of the field of view (reflected
    by the mirror, not the glass) that the mirror-reflections are completely
    responsible for the image sweeping past the apparent edge of the mirror.
    The illusion is due to the reduced depth of field with a higher power
    That being said, I am now convinced that I was completely wrong about
    the double reflection from the glass portion of the horizon mirror. Only
    nearby objects give a double reflection; celestial bodies should give a
    single reflection, as George and others have been patiently explaining to
    me. I did an experiment last night that convinced me otherwise, but there
    must be artifact that I haven't appreciated. I accept that the mirror will
    give a single image, I just think that it is far too dim to be noticed.
    Ken Muldrew.

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