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    Re: Instumental error?
    From: Alexandre Eremenko
    Date: 2005 Apr 20, 00:55 -0500

    Fred,
    > I wonder if you could clarify how the standard deviation,
    > average error
    > and maximal span were computed.
    
    I measure some distance several times, say 7 times in 10 minutes.
    A star-to star distance can be considered constant for this period.
    The average is the arithmetic average.
    (x_1+x_2+x_3+x_4+x_5+x_6+x_7)/7 in this case.
    Here x_1...x_7 are my sextant readings.
    The standard deviation.
    I subtract the average from each x_j, square the difference
    add the results, divide by 6 and extract the
    square root. (Actually the calculator does it all for me).
    Maximal span. I take the maximal value of x_j and subtract
    the minimum value.
    
    For the Lunars (where the distance changes more quickly,
    I feed all 7 distances to Fred's program,
    and look at the errors this program produces.
    Then I do the above manupulations on these errors.
    
    > I would prefer that the standard deviations be given
    > to an extra digit.
    
    The purpose was to detect and estimate the systematic error.
    It is clearly much larger than the standard deviation.
    
    > the development of
    > proficiency is an important component in minimizing error.
    
    Sure. I was shooting distances since October 2004
    almost every evening when weather permitted.
    Trying to develop this proficiency.
    But when I made almost simultaneous shots of the same distance
    with two sextants (The other was Bill's new Astra) and obtained
    a correct result with the Astra, and +0.4 error with my sextant,
    I finally concluded that there is arc error.
    
    > I was very accurate and precise in altitude shots (with a
    > Cassens + Plath, not the Husun),
    
    My Sun shots with art horizon were always much better...
    until I realized that I compare the final result, not the sextant
    reading,
    so I divided my error by 2:-) With actual sextant reading the
    systematic error is about the same.
    
    > The curves can take
    > surprising twists.  I regret I don't have one to share
    
    You can see many on the web. Sometimes the pictures
    are good enough to read the certificate.
    
    > described calibrating an Astra IIIB using equipment to which he had
    > access.  He mentioned that he fit the "observations" using Fourier
    
    I know a very clever method of calibrating your sextant
    "at home" that is without stars, but you need a theodolite
    or another instrument more precise than the sextant,
    and with precise level. Unfortunately I don't have a theodolite.
    
    Can describe the method to the list if anyone is interested.
    (I recently read about it in one XIX century paper on
    the Internet).
    
    Alex.
    
    
    

       
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