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    Re: Centring Error Detector
    From: George Huxtable
    Date: 2007 Jan 12, 23:50 -0000

    Phil wrote-
    
    I'm looking for information about "Centring Error Detectors", machines
    to verificate and evaluate centring error of sextant.
    Here is an exampe of this kind of instrument :
    http://bib.iota.u-psud.fr/Record.htm?idlist=3&record=894512461279
    Here is an other kind :
    http://gita.grainger.uiuc.edu/IOPText/0950-7671/6/10/408/siv6i10p324.pdf
    
    =======================
    
    Frank responded-
    
     For those who
    | didn't read it, I'll give my summary...
    |
    | The purpose of the instrument(s) is to measure "arc error". This is
    a
    | combination of eccentricity (centering error), division error, and
    any other  errors
    | which are fixed over time and depend on the angle observed with the
    sextant.
    | It's the (presumably) fixed, correctable error tabulated on the
    certificates
    | pasted to the inside of the sextant case.
    |
    | The idea behind the described system is to insert a prism with a
    known  angle
    | of deflection into the light path in front of the horizon glass of
    the
    | sextant or in front of the telescope such that it occupies the part
    of the field
    | of view where the horizon side is normally visible. The simplest
    case is a 90
    | degree prism. You place it in front of the horizon side of the
    sextant's
    | telescope, and it lets that side of the telescope see straight up
    instead of
    | straight ahead. You then set the sextant arm to 90 degrees and aim
    it at some
    | object. For example you could look at the sea horizon by pointing
    the sextant
    | straight down. The mirrors let you see the horizon on the right
    side. The prism
    | simultaneously shows the horizon on the left side. Then the
    procedure to
    | measure the error is exactly the same as an ordinary index error
    sight. You line
    | up the two images and read off the error. Assuming the prism is an
    exact 90
    | degree prism, the "index correction" that has just been measured
    using the
    | prism  is actually the "arc error" at 90 degrees (arc error plus
    normal IC?). In
    | the "patented" system as described, the sextant user would be
    provided with a
    | set of prisms and holders for angles of 30, 60, 90, and (optionally)
    120
    | degrees. The prisms do not need to have exactly those angles, but
    they need  to
    | have their actual angles accurately measured before they may be
    used. By
    | making observations with each prism, a rough correction table for
    the sextant's
    | arc error could be constructed. It's a very clever idea, but if you
    had such a
    | system today, would it be any more useful than ordinary sextant
    observations
    | (or  maybe lunars) in assessing arc error?
    
    =====================
    
    Note from George.
    
    There's an extra complication here. None of the prisms referred to, in
    these two documents, are ordinary prisms. They are all PENTAPRISMS,
    the 5-sided prism that you need to get a defined angle of deflection,
    however the light comes in. So when Frank mentions a 90 degree prism,
    it's not the 90-45-45 three-sided prism that you or I would think of
    first, but a special (and presumably expensive) 5-sided job. Perhaps
    you may be able to salvage one such, for 90 degrees, from a suitable
    camera, but not the whole range of different deflection angles,
    unfortunately.
    
    The Hezzanith system, shown in the second reference, is to be used by
    an observer at sea, using an object at infinity such as horizon or
    star, but the French version appears to be designed for an indoor test
    setup for certification. It has a pair of parallel collimators, spaced
    according to the offset between the two sightlines of a sextant.
    
    Both systems use similar pentaprisms. Although the Hezzanith patent
    offers the four prisms that Frank describes, the French Fabry version
    is a bit more clever, in offering four prisms such that combining them
    in different ways, you can test all angles in 15-degree steps up to
    135 degrees.
    
    An earlier setup at Kew, for certificating sextants, (not shown in
    those websites) comprised an immense assembly of such
    collimator-pairs, all looking at a sextant mounted on its side at the
    centre of the array. Again, that was intended to tackle all angles at
    15 degree spacings, by using different combinations of angles between
    them.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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