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    Re: Centring Error Detector
    From: Alexandre Eremenko
    Date: 2007 Jan 12, 10:19 -0500

    
    Phil,
    
    > - if exist a book describing precisely
    > the use of this  instrument,
    
    It exists and I even have a photocopy of it.
    It's the book on Hezzanith sextants.
    It is listed among the sextant accessories,
    and there is a clear picture of it.
    
    The centring error detector of Hezzanith
    is just a set of few prisms with a simple
    arrangement to attach them to the front
    of a telescope.
    
    One prizm for each angle, 30, 60, 90, 120.
    The arrangement is so extremelly simple that I
    even thought of making one.
    The question is whether one can have a prizm
    made to specifications (and to high precision
    of 0.1' say). But even if one cannot, the prisms
    with approximate angles can be used, you can just
    measure the angles of the prisms in advance
    with a sextant which is known to have no arc error.
    
    The principle of work was described by Frank, but
    let me repeat: it is very simple. One prism turns the
    incoming ray from a star, say by 30 degrees,
    and sends it to the telescope. Another ray from the same
    star goes throuygh the normal way (reflected in the
    sextant mirrors) and also to the telescope.
    When you see the two images coincide, the sextant should show
    30 degrees+arc error at 30 degrees. You need several prisms,
    one for each angle.
    
    Alex.
    
    
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