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    From: Alexandre Eremenko
    Date: 2004 Sep 29, 15:30 -0500

    Dear Fred,
    Thank you for your prompt and interesting reply!
    
    > Accuracy is how close you
    > are to the target; precision is the repeatability of your shots.
    
    I am sort of satisfied with both when I measure the Sun alt
    and star-to-itself distance.
    
    With star-to-star distances I have very bad PRECISION
    (much worse than I expected) and apparently bad accuracy as well
    (assuming that the  Fred-Chauvenet method of refraction correction
    has high accuracy).
    
    > An inaccurate but precise sextant might give measurements
    > that are all
    > off by 1.0' of arc, but consistently off, all measuring between 0.9'
    > and 1.1' in error.
    
    This can be easily corrected by determining and adding the
    "instrument error".
    
    > An imprecise but accurate sextant might give
    > measurements that averaged being off by 0.0' of arc,
    > but the individual
    > measurements might range from -1.0' to +1.0' in error.
    
    The existence of such sextant is somewhat more surprising for me:-)
    You cannot increase accuracy indefinitely by averaging.
    
    Thank you for stating this distinction so clearly.
    
    > There may be some truth to the statement that
    > the XIX century sextants
    > were more accurate & precise than modern sextants.
    
    Is there any hard evidence? Has anyone really compared?
    I would be VERY interested to do this. (But where to get a good
    XIX century sextant in perfect condition, is a separate question).
    
    > the larger Hughes sextants often had no detectable
    > errors at any angle
    
    I think one of the Russian sellers I know is selling a
    Hughes 1945 "Gothic" (top of the line)
    in good condition now, almost for
    the price
    of an SNO-T. I am very tempted to buy it, especially after what
    you just said, but I am afraid because who really knows whether
    it is in good condition:-)
    
    > I think it
    > important that you become proficient with using the sextant, by
    > whatever means, before attempting to calibrate it with star to star
    > distances.
    
    I am a total novice. Never held a sextant in my hands until
    2 weeks ago when I bought this SNO-T.
    
    > I think an error of 0.2' of arc is about the
    > best that can be obtained.
    
    > A few times I have gotten very accurate and precise,
    > to under 0.1' of
    > arc.  More typically, my standard deviations
    > lately range from 0.1' to
    > 0.3'.
    
    The results you report are of the same order of magnitude
    that I obtain. Which sextant did you use to obtain these results?
    The only obstacle to improving my results for the Sun
    seems to be the water motion in the art horizon because
    of the slightest vibration. My balcony floor vibrates when
    a dog is passing by on the ground under my balcony:-)
    But this can be fixed of course.
    
    But what about your older attempts with star-to-star distances?
    Were you successfull in determining instrument error?
    
    It seems to me that star-to-star distances should be MORE precise
    than Sun's altitude.
    
    Alex.
    
    
    

       
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