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    Re: MkIXa bubble sextant performance
    From: Alexandre Eremenko
    Date: 2007 Apr 5, 08:28 -0400

    
    Jean-Philippe,
    
    >   Did you try to identify an Index error or any systematic error?
    
    I do not see how to measure the Index error directly.
    Of course, if I have a large statistics of measurements from a
    known position, I can see whether there is some systemaric bias
    and subtract it as an index correction. But so far I do not
    see any systematic bias. I mean that positive and negative errors
    seem to be equaly likely.
    
    > Did you solve
    > issue of the bubble
    > getting bigger during the observation
    
    I don't know a radical solution of this, but I reduced
    the problem.
    
    1. BEFORE making the bubble I keep the sextant for 10 minutes
    at the observation place to let the temperature of the
    sextant stabilize.
    
    2. AFTER making the bubble, and immediately before the
    observation I release the
    membrane pressure. One has to be careful after that and not
    let the bubble approach its place of origin (at 10 o'clock
    in the field of view) because it may disappear.
    
    With these precautions, the bubble stays at approximately
    stable size for 3-5 minutes, more than enough for several shots
    or for an averager observation. Anyway, even without precautions
    there are no problems with individual shots because I can
    always adjust the bubble size before a shot,
    and a very minor problem when using the averager.
    
    This bubble growth phenomenon seems to depend on the weather
    conditions but I did not investigate it carefully to make
    definite conclusions. Sometimes it does not grow, sometimes it does.
    
    Alex.
    
    
    
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