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    Re: Sextant Error and Whole Horizon Mirrors
    From: Aubrey O?Callaghan
    Date: 2002 Jul 10, 08:18 -0400

    One of my sextants is a Cetus Russian sexant.
    The magnifying telescope has all the bells and whistles referred to by
    Bruce, inverting image and parallel wires.
    However it has a split mirror.
    At 21:03 09/07/02, you wrote:
    >The only way I know of for sure to overcome the problem you are talking about
    >is to have one of the old-fashioned sextants or circles with an inverting
    >'scope. I've never been so fortunate as to use one but, besides being more
    >powerful and having a wider view, the inverting 'scopes had parallel wires in
    >the field of view so it was easy to keep the objects in the middle while you
    >made contact.
    >The problem you are describing is the problem of keeping the plane of the
    >observation parallel to the plane of the instrument. It's not much of a
    >problem for small angles but gets bad with wider angles. If you have a copy
    >of my "Tables for Clearing" look under "Odds and Ends" for a discussion of
    >this, and a table showing the effect in different situations.
    >It's also what I'm talking about in the last paragraph of that July 7th
    >posting. It seems your experience and conclusions are about the same as mine,
    >though my sextant isn't whole horizon. I hope other members will experiment

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