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    Pivot Line: was sextant precision.
    From: Robert Eno
    Date: 2005 Jun 23, 09:59 -0400

    George Huxtable wrote:
    > As a general rule the mid-point of the reflecting plane of the index
    > mirror
    > is indeed aligned with the pivot axis of the index am. But it's in no way
    > crucial that the axis should be placed exactly there, in the reflecting
    > plane or across the middle of the mirror's area.. If it wasn't, then as
    > you
    > shifted the index arm, that mirror would be displaced sideways a bit, and
    > in the end you might lose a bit of light because one edge would move in
    > and
    > shrink the width of the light-path to the horizon mirror. But it wouldn't
    > alter the sextant readings, not a jot.
    Robert responds:
    so far, so good. I got the picture.....
    George goes on...
    > Alex Eremenko has commented that in his Russian SNO sextant that is indeed
    > the case. The pivot line lies behind the front-silvered face of his index
    > mirror, not on it, as I recall. And the only disadvantage that has
    > resulted
    > is that with such a geometry, a favourite trick for checking
    > perpendicularity of the index mirror doesn't work. That trick is to look
    > at
    > at the continuity of the direct and reflected arc as seen in the index
    > mirror. It only applies if the effective reflecting plane is exactly
    > aligned with the pivot. Because, in the past, all sextants were made that
    > way, that restriction on the use of the method was never made clear. Alex
    > has to use a different method to check perpendicularity.
    Robert comments:
    You lost me here George. Can you expand on this idea a bit?  I thought I
    knew what you meant when you referred to the "pivot line" but now I am
    confused. Is the pivot line not a function of the mirror housing?
    There are so many conversations on this forum that would be much clearer if
    we could send the occassional diagram with our comments!
    Thanks George.

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