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    Re: averaging devices on sextants
    From: Paul Hirose
    Date: 2004 Oct 7, 16:43 -0700

    Alexandre Eremenko wrote:
    >
    > I am trying to imagine how a "mechanical averaging"
    > device could possibly work, on what principle.
    
    The U.S. military A-12 bubble sextant (early 1940s) has a thumb lever
    which puts a pencil mark on the edge of the plastic altitude setting
    knob. You push the lever periodically during the observation run. When
    the run is complete, turn the knob so the pencil is at the midpoint of
    the marks. Then read the "average" on the vernier altitude scale.
    
    Three identical knobs are provided with the sextant so you can use
    them in quick sequence for a 3-star fix.
    
    The AN5851-1 (also called the A-14) sextant of WW2 is much more
    sophisticated. A clockwork mechanism in the base accumulates the
    average on a counter (like an odometer) during a 2-minute run. The
    manual explains the averager's operation in detail. I don't have one
    of these sextants so I haven't paid close attention to what the manual
    says, but I can send it (5 MB PDF) if you want.
    
    One disadvantage of this averager is that the length of the
    observation is fixed at 2 minutes. The mechanism automatically stops
    and drops a shutter across the field of view. If a cloud covers the
    celestial body before that, the average will be bad.
    
    In the A-15 (same sextant with a different averager) that limitation
    is removed. A ball and disk integrator keeps a continuous running
    average.
    
    The principle of the integrator is simple. Imagine a tennis ball with
    a pencil through its center. Sharpen both ends of the pencil and hold
    it between left and right index fingers so the assembly can spin
    freely. With the pencil horizontal, let the ball touch the center of a
    rotating phonograph turntable. It doesn't rotate. If you shift it left
    of center, it rotates one direction. Right of center, the ball rotates
    the opposite direction.
    
    In the sextant averager, winding the clockwork drive positions the
    ball at the center of the disk. Also, a pointer which records ball
    rotation is reset so it points to a zero mark.
    
    When you press the start trigger, the disk begins turning at constant
    speed. At the same time, a clutch connects the altitude knob to the
    ball, such that knob movement above or below the starting altitude
    moves the ball to one side or the other of the disk center.
    
    When you stop the averager, the disk stops turning. The altitude knob
    connects to a different part of the averager, so now it directly
    rotates the ball. Turn the knob to return the ball rotation pointer to
    the zero mark. This removes any accumulated ball rotation, and also
    sets the sextant to the average altitude of the observation.
    
    A dial on the averager displays half the duration of the observation.
    You add that value to the start time to obtain the mid time of the
    observing run.
    
    Periscopic bubble sextants had the same kind of continuously
    integrating mechanical averager. I've also seen them with digital
    electronic averagers. Those averagers were probably retrofitted years
    after the sextants were manufactured.
    
    
    There are plenty of bubble sextants available cheap on eBay. But the
    sellers are generally clueless on operating the instruments. If you
    want the averager tested, you'll have to walk the person through the
    process!
    
    
    

       
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