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    Re: bubble sextant index error
    From: Frank Reed CT
    Date: 2005 Dec 14, 14:41 EST

    Paul Hirose, you wrote:
    "I have seen  collimators of that type at a survey equipment shop. Three
    were attached to a  heavy vertical I-beam to provide artificial targets
    at approximately +45, 0,  and -45 elevation. Their optical axes
    intersected just above a pillar a few  feet away, on which the theodolite
    under test was mounted.
    
    Maybe a  marine sextant could be tested with that setup. You'd have to
    hunt around for  the sweet spot where the collimator beams hit the index
    mirror and horizon  glass simultaneously."
    
    Yes, that's a setup I've seen described. You can  mount as many of these as
    you want on a large table. Where the beams cross, you  have the equivalent of
    infinitely distant sources at fixed angular intervals.  Measure those with any
    good instrument, a reliable sextant or a theodolite, and  then any other
    instrument placed there can be tested to high accuracy. This is  one good way for
    measuring arc error.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

       
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