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    Re: Sextant Errors
    From: Bruce Stark
    Date: 2002 Jul 7, 16:13 EDT

    Robert! Glad to see you're on the LIst!
    
    Janice and I have been gone for a several weeks, and I have a lot of catching
    up to do.
    
    Regarding side error, I'd say it's important to have the index mirror as near
    perpendicular as you can get it, using the usual procedure given in all the
    manuals. Then, if you want to see a star and its reflection side-by-side you
    can get that when you adjust the horizon mirror. Good idea if you want to
    check index error before or after a round of star altitudes—and have perfect
    eyesight. I'm no optician, but don't believe such a small side-error could
    have any practical effect.
    
    Those of us who see stars as squirming blobs should stick to the method
    Maskelyne, Moore, Bowditch, etc. preferred for finding index error. That is,
    taking a series of measurement of the sun's diameter on and off the arc.
    
    Those who have good eyesight should be getting better results from their
    lunars than I am, and if I miss GMT by more than a minute of time with a
    sun-lunar it's reason to look for a blunder. A while back I worked the six
    sun-lunars Lewis took at the Three Forks of the Missouri and at Camp
    Fortunate in 1804. Lewis did better than I generally do, and he was sick and
    had a lot on his mind.
    
    I'm thinking maybe some of the lunarians on the list haven't given enough
    thought to getting both mirrors perpendicular to the frame of the sextant.
    You hardly ever have to make this adjustment, but should check it now and
    then to make sure it's perfect. Index error, on the other hand, is best left
    as it is. But measure and record it just about every time you take a lunar.
    
    Another possibility is confusion about how the instrument error shown on the
    maker's certificate inside the sextant box should be applied. Some believe
    this error should be corrected for by applying it with the OPPOSITE sign to
    that shown on the certificate. But, with sextants I've seen, the certificate
    shows CORRECTIONS, to be added or subtracted according to the + or - sign
    given.
    
    Finally, I seem to get much better results, especially with the longer
    distances, if the telescope is centered on the part of the horizon mirror
    where the clear and silvered parts meet, and contact is made there, next to
    the silvered part. I THINK this helps keep the observation parallel to the
    frame of the instrument, but perhaps am kidding myself. Feedback would be
    appreciated.
    
    Bruce
    
    
    

       
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