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    Re: How does the AstraIIIb split mirror work?
    From: George Huxtable
    Date: 2004 Apr 27, 17:09 +0100

    Trevor Kenchington's contributions are always perceptive; as when he said-
    >.... I'm not convinced
    >that Ken is not giving him, and George himself is not claiming, a little
    >too much credit this time around.
    >.... 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 is quite right. The two effects must occur together, to some extent.
    In an early sextant without a telescope, observed directly by eye through a
    peep, then the greater depth-of-focus of the eye would vastly reduce any
    such apparent "spreading" of Sun light-image from the silvered part to the
    unsilvered part; wouldn't it?
    A sextant may be fitted with a mechanism (called a "rising-piece, perhaps??
    Joel would know.) that can shift its telescope bodily away from the frame,
    keeping parallel with itself.  If it could be moved far enough to shift the
    silvered part of the mirror completely out of its field of view, only
    reflection in the unsilvered glass would then remain to give a dimmed image
    of the Sun. I suspect that is precisely the purpose of such a mechanism; to
    provide a degree of controlled dimming of the Sun. What else is it for? My
    own cheap-and-cheerful plastic Ebbco has no such elaboration, of course.
    In the case of a sextant with a truncated horizon-mirror (i.e. no
    clear-glass portion to look through, just a short mirror to look past) then
    presumably such a "rising-piece?" would be able to dim this "ghost-image"
    of the Sun right down to nothing, if the telescope shifted far enough. You
    could try it for yourself by grinding-away the unsilvered part of your
    horizon mirror (though I doubt if many will...).
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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