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From: Paul Hirose (no email)
Date: Thu Oct 07 2004 - 19:43:33 EDT
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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