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    Re: sextant precision.
    From: George Huxtable
    Date: 2005 Jun 23, 09:51 +0100

    >Bill wrote-
    >I am coming around to your way of thinking on sextant mirrors.
    How gratifying it is to find a sinner who sees the light by thinking about
    it and convincing himself...
    >I have done some overlay drawings with the pivot point in the center and at
    >the lower end of the index mirror. As you stated, things seem to work out
    >fine in either scenario.  In the case of wanting the "point" to be at the
    >axis of the index mirror, this is a design consideration.  When I located
    >the pivot point at the bottom end and placed the mirror at an angle to
    >observe a high body, the line of sight from the scope to the horizon mirror
    >to the index mirror was such that it missed the index mirror entirely.  So
    >it is optimal to have the "point" at or near the axis so one can see the
    >index mirror while still using the center of the horizon mirror.
    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.
    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.
    Bill ends-
    "Thanks for serving as a sounding board."
    That's a pleasure. I often find, when I've got my own ideas wrong, that
    it's the very act of trying to convince  a somewhat-sceptical somebody-else
    that's the trigger for the light to dawn. That's why I value the Nav-l list
    so highly; sloppy reasoning gets jumped on very quickly.
    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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