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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 24, 08:10 -0600

    George Huxtable wrote:
    > I think it's accepted (isn't it?) that from the clear, unsilvered part
    > of the horizon mirror some fraction of the sunlight incident on it
    > from the index mirror will be reflected by the glass surface into the
    > telescope. And in roughly equal amounts from the front surface and the
    > back.
    Full agreement.
    > The argument put forward, which I wish to dispel, is that this
    > would create two separate light beams, displaced vertically by some
    > amount that depends on the thickness of the horizon glass, [and
    > therefore to two separate images of the Sun as seen through the
    > telescope]. The only part of the argument that I argue with is the
    > final phrase, that I have put within square brackets.
    This is where we differ, and also whether the reflections from the glass-
    air surfaces play any role in normal sextant sights.
    I'm afraid I'm rushed right now but I promise to consider your message
    more fully later on. Two things right now: Last night I was observing
    reflections from the glass part of the horizon mirror using my eye with the
    scope removed. I had to do this in a dim room to see the reflections
    clearly, but with my eye located in the normal line of sight (i.e. from
    where they eyepiece would be if the telescope was there) I always saw a
    double reflection. These were not reflections from the index mirror as I
    was unable to see those in the dark room. Also, I tried looking through
    the scope at a white wall with a bright ceiling lamp being brought down
    from the index mirror onto the wall (this time with the scope). Normally, I
    see a full-horizon view of the lamp. When I block the horizon mirror (but
    not the glass) with a piece of paper I see no lamp at all. There was the
    faintest hint of a highlight, but no detail whatsoever. If I block the horizon
    glass with paper (leaving the mirror open), then I see a full horizon image
    of the lamp.
    One more thing, regarding my anecdote about the porch light going on
    behind me. I repeated it last night and found that from where I was
    standing, the light itself was reflected in the horizon glass, my face was
    partly reflected in the horizon shades below the glass. When the light
    went on I immediately lowered the sexant and looked into the horizon
    glass from above the telescope. I probably did the same thing when this
    happened by accident a few days ago and this last image was the one I
    remembered. You are correct that this anecdote is irrelevant.
    Ken Muldrew.

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