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    Re: Pivot Line: was sextant precision.
    From: George Huxtable
    Date: 2005 Jun 23, 21:30 +0100

    George wrote-
    >>As a general rule the mid-point of the reflecting plane of the index
    >>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
    >>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
    >>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
    >>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 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?
    Answer from George.
    No, the pivot line I referred to is the axis of the index arm. What I was
    trying to say was that the index mirror, in its housing, has to be fixed to
    the index arm in such a way that its angle changes corresponding to the
    angle-changes  of that arm as measured on the arc. But that's all, really.
    It may be convenient, but it isn't essential, for the axis of the index arm
    to pass through the reflecting surface, and to bisect the area, of the
    index mirror, though it usually does. Perhaps an earlier incarnation of the
    SNO sextant had a rear-silvered mirror, in the conventional position, with
    the pivot axis of the index arm passing through its reflecting surface.
    Later perhaps (I'm just speculating) this was replaced by a front-silvered
    mirror in the same frame, so now the reflecting surface was displaced a few
    mm further forward. The sextant would then work just as well, but as Alex
    found (the hard way) one of the conventional methods of  setting
    perpendicularity of the index mirror didn't work any more.
    >There are so many conversations on this forum that would be much clearer if
    >we could send the occassional diagram with our comments!
    I agree, though I understand the reasons for the rule. We should gradually
    get better at drawing, and interpreting, word-pictures, out of necessity.
    When I worked in a lab., there was a blackboard and chalk in EVERY room
    (especially the tea-room; an important feature in British science), and
    when I retired I found it very hard even to think (and impossible to argue)
    without a stick of chalk in my hand. I find it difficult to put those
    chalkings into words now, but we all do our best.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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