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    Re: Sextant precision
    From: Robert Gainer
    Date: 2004 Oct 2, 14:23 +0000

    George said,
    �Mostly, quintants were made for marine surveying, for measuring horizontal
    sextant angles. Not much advantage over a sextant for lunar distances, which
    as far as I know were only ever tabulated out to 120 degrees�.
    
    George,
    My instrument is indeed intended to be used for surveying and navigation. It
    looks like a standard issue Navy sextant except with a longer engraved arc.
    
    George went on to say,
    �But I wonder whether Robert's "Brandis" was actually useable over its full
    range to 162 degrees. Two things happened at large angles:
    
    1. The available slot in the vertical field-of-view would shrink until at
    some angle, at which the light was tangential to the index mirror, the
    letter-box shape of the field of view would shrink to zero vertical
    aperture�.
    
    George,
    This is true and it happens at about 140 degrees. I think the reason that
    they engraved the arc past this point is twofold. First the vernier needs
    something past the point of the reading on the arc to be able to select the
    minuets and parts from the vernier. This adds 13 degrees and 20 minuets to
    the engraved marks on the limb for this vernier. So if you wanted to read up
    to 140 degrees you need the arc engraved to at least 160 degrees and 20
    minuets.
    
    Second, if you add a penta prism to the system you can now read the arc to
    about 162 degrees and still have the extra 13 degrees and 20 minuets of
    engraving on the arc for the vernier readings. This will measure an angle of
    greater then 200 degrees in use.
    
    The Freiberger sextant that I have can also be used for surveying
    (horizontal sights) with a detachable penta prism to extend the measuring
    range to 215 degrees. This is almost double the 120 degree arc of the
    sextant. This sextant is said by the manufacturer to have less then 20
    seconds of error in the instrument.
    
    My Husun is graduated to 125 degrees just like the Freiberger but is
    supposed to have no error on any measurement until you get to 120 degrees.
    For day-to-day use that makes it as good as my Cassens & Plath which has �no
    error for practical use� according to the paperwork with the instrument.
    They clam the error is less then 9 seconds of arc.
    
    I have used a Freiberger in Africa with the penta prism option that was sold
    by the manufacturer.  It was a pain to use and it was better to just turn
    around and shoot facing the other way with out the prism attached. I never
    understood why you would need to measure more then 180 degrees under any
    circumstances.
    All the best,
    Robert Gainer
    
    _________________________________________________________________
    Get ready for school! Find articles, homework help and more in the Back to
    School Guide! http://special.msn.com/network/04backtoschool.armx
    
    
    

       
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