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    Re: How does the AstraIIIb split mirror work?
    From: Fred Hebard
    Date: 2004 Apr 23, 11:05 -0400

    On Apr 23, 2004, at 10:29 AM, Noyce, Bill wrote:
    > Fred Hebard wrote:
    >> There would be a shift in position of the object if it were
    >> reflecting off the unsilvered front of the mirror in one half
    >> of the image and off the silvered back in the other half.
    > Only if the object is very nearby, or if the front & back faces
    > of the mirror aren't parallel to each other.
    > Assuming the faces are parallel, the angular displacement would
    > only be the parallax from shifting position of the reflecting
    > point by a fraction of an inch.  For anything more than a few
    > feet away, this is unnoticeable.  And besides, the back of the
    > plain glass provides some reflection too, as does the front glass
    > of the rear-silvered mirror.
    >> Occasionally, I do see that, but most of the time, only the
    >> single image is apparent.
    I'm certainly not an expert in optics, or any other branch of physics,
    but I recall fairly strongly that mirrors can be handled by simple
    plane geometry, and that the angle of reflection equals the angle of
    incidence.  The parallax difference here would seem to operate because
    the viewer is only a few inches away from the mirror.  The simple
    diagram I drew seems to indicate this to me.  I will try to transmit an
    image if that would be helpful in clarifying this.

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