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    Bubble sextant trials
    From: Alexandre Eremenko
    Date: 2007 Feb 14, 09:19 -0800

    Today I made my first observations with my new MkIXa
    bubble sextant (purchased on E-bay for $41).
    
    Position 40d27'.2N, 86d55'.8W Sun altitude, refraction -1.5
    1. Without the clockwork averager:
    GMT:              Reading:          Error:
    16:30:40          32d30'             -1.0
    16:32:05          32d38'              0.0
    16:33:00          32d42'.5          -0.1
    16:34:05          32d49'              1.0
    16:34:50          32d52'              0.3
    16:35:32          32d54.5           -0.7
    16:36:30          32d58.5           -1.3
    Average error = -0'.26, sigma=0'.8
    
    2. Now with the clockwork averager:
    Time at the end of observation: 16:39:10, reading: 22d08'.5.
    Assuming the average GMT=16:38:10 I obtain the error of 0'.7.
    
    Remarks. The manual says to time the beginning and the end
    of an observation. However I do not see how to time the
    beginning with sufficient precision: one needs time to record it,
    then to align the images before you trig the clockwork.
    Instead, I measured the time that clockwork works: it is 1m57sec,
    so in my observations I time the end and then subtract one minute
    to get the average GMT of the observation.
    
    Further remarks. The averager seems useless for observations from
    a stable platform. Besides, it is very heavy, and my hands get tired
    in the two minutes needed for an observation. So Ken's idea to cut
    the averager off seems reasonable for backyard observations.
    I wonder if the averager will help on a small boat.
    
    I noticed that something strange happens with the bubble:
    as I observe, small bubbles come from 10 oclock direction and
    join my main bubble. So it grows:-)
    Do I have to rotate the bubble wheel anticlockwise (relaxing the
    tension
    of the membrane) after the size of the bubble is adjusted?
    
    The observation I posted above was my 3-d trial. In the first two I
    was
    lerning how to keep the bubble and the Sun in a good position, how
    to read the scale and what is the right size of the bubble.
    It seems that smaller bubble gives better results.
    
    Will try some stars if weather permits.
    
    Alex.
    
    
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