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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 27, 10:50 -0600

    On 27 Apr 2004 at 17:09, George Huxtable wrote:
    > 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.
    I don't think George argued against the mechanism that I proposed,
    but he argued (correctly) against my criticisms of the reflection
    from glass mechanism that he had proposed.
    > >.... 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?
    Yes, this is correct. If you cover the clear glass part of the
    horizon mirror and use a sight tube, then the reflected body does not
    appear to move past the edge of the mirror. With a 5x telescope, the
    image can be moved almost all the way to the edge of the field of
    view (with the glass covered), but the glass must be uncovered in
    order to move the image all the way to the edge of the field. Both
    mechanisms are clearly at work and (with a reflection of the moon, at
    least) it is difficult (impossible?) to tell how much each mechanism
    contributes to the final image.
    > 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 can move my scope out, but not enough to take the mirror out of the
    field of view (If I had cut the mount longer I could do this, but
    it's now too late). However, with the scope moved out, the point
    where the reflection of the body vanishes (with the glass part of the
    horizon mirror covered) moves toward the center of the field of view.
    > 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?
    It could be just an easy way to change telescopes without losing the
    line-of-sight alignment, but allowing some flexibility for large and
    small objective lenses (a large lens would be centered on the horizon
    mirror when it was farther out from the sextant frame).
    Ken Muldrew.

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