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    Re: How does the AstraIIIb split mirror work?
    From: Trevor Kenchington
    Date: 2004 Apr 26, 20:34 +0000

    Ken Muldrew wrote:
    > The reflection past the middle of
    > the field of view is a combined image from both the mirror and the glass
    > until the object is rotated past the point where the scope can no longer
    > get reflected light from the mirror. Then the image comes solely from
    > reflection off the two glass surfaces. The transition is imperceptable with
    > a 5x scope.
    > George Huxtable was entirely correct in this matter and I thank him for
    > sticking with it until I was able to understand the phenomenon.
    George is usually entirely correct in most matters but I'm not convinced
    that Ken is not giving him, and George himself is not claiming, a little
    too much credit this time around.
     From the above quote of Ken's message, plus an overview of the rest of
    this thread, it looks to me as though there are _two_ optical mechanisms
    at work -- their relative contributions depending on the depth of field
    of the sextant/eye optical system, the light level (e.g. Moon versus
    Sun) and maybe yet more factors. (Are some horizon mirrors given
    low-reflectivity coatings? Certainly some will have an excessive coating
    of finger grease and other contaminants that will increase the surface
    reflection from the clear half.) One mechanism is clearly George's
    suggestion of reflection from the surfaces of the glass. But that does
    not preclude Ken's mechanism of light gathered from the silvered half of
    the mirror being perceived as coming from the other half, due to the
    boundary between the two being grossly out of focus when seen through a
    telescope focused on infinity.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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