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Subject: bubble sextant test results
From: Paul Hirose (paulhirose@XXX.XXX)
Date: Thu Jan 02 2003 - 16:22:05 EST
Over the past few days I've made about 30 sun observations with an
early 1940s Link A-12 bubble sextant. Comparing my results with the
USNO online sight reduction page, standard deviation was 1.8 minutes.
The instrument appears to have about +12 minutes index error.
Cosmetically it's in beautiful condition, so I don't know how it got
that far off. But at least the error seems consistent.
I just took single shots, although the A-12's averager is in working
condition. It's a simple affair which marks a pencil line on the
plastic altitude setting drum (which is of large diameter) each time
you press a thumb trigger. At the end of the observation period you
set the knob so the marks are evenly distributed either side of the
pencil lead, then read the altitude.
Of course you must use the mid time of the observation period to go
with that average altitude. The bother of doing that is the main
reason I didn't use the averager. I don't think it would have improved
my results much anyway.
The bubble on my A-12 seems a little big, about three Sun diameters. I
had the feeling a smaller bubble would have made centering easier.
Along with my A-12 observations, I also took about 20 with a Kollsman
periscopic bubble sextant I recently bought on eBay. Standard
deviation for these was 1.6 minutes. One bad shot 5.2 minutes off
spoiled my numbers. May have been a blunder reading the altitude.
Index error is -1.6 minutes based on all the observations.
The Kollsman is rather heavy at 7.4 pounds, and was designed to be
installed in a mount in the aircraft ceiling. However, with upturned
palms at about chin level you can support the sextant while
manuipulating its controls with reasonable ease.
Field of view through the 2x telescope is generous. In fact it has an
actual field of view much wider than the non-magnifying A-12. Stars
look pin sharp in it.
The clockwork averager on the Kollsman is quite a box of tricks. It
continuously integrates the altitude for up to two minutes, after
which a shutter blocks the telescope. Unlike earlier fixed-time
averagers, it can be stopped manually too. The day my Kollsman arrived
the sun was going in and out of clouds, and several of my practice
runs had to stopped early that way. A fixed-time averager would have
been frustrating in these conditions.
One delightful feature is a dial on the averager which shows half the
duration of the averaging run. That makes it easy to determine the mid
time of the observation period: you simply deduct the "half time" from
your watch reading at the end of the observation.
Using an automatic averager is quite a novel experience, so much that
I haven't tried the Kollsman with single shots yet.
At first I found it tedious to hold on target for the full two
minutes. But after some practice I can now do three observations in a
row without trouble. The sustained concentration maintaining a good
sight picture is a much a factor as the physical effort of supporting
the sextant.
Though I'm well short of his skill, I agree with Dr. Kolbe that visual
acuity has little to do with operating a bubble sextant. It's more a
matter of learning the moves to simultaneously steer the bubble and
the body, and developing the eye to judge that two independently
wobbling objects are in good average alignment. My results should be
typical of what a beginner can expect, hand-holding the instrument on
land.
I'm keen to try the Kollsman on stars but it needs 28 V (AC or DC) to
run the bubble illuminator. Maybe I'll wire up a couple 12 V lantern
batteries in series. Anyone know what its bubble looks like at night?
The A-12 bubble has a poor night lighting arrangement in my opinion.
You see the it as a dark circle against a lighted background, so in a
light polluted sky the stars can be hard to see.
But either sextant works well for Sun shots. Physically the A-12 is
much lighter and was well designed for use in the hand. The Kollsman
is easier to read (no vernier), has variable bubble size, and probably
more accuracy potential. And it definitely has a high "coolness
coefficient" in use, the averager ticking loudly and ominously like a
time bomb as you track the body.
The same guy in Canada who sold me the Kollsman periscopic sextant has
another one up on eBay now, and if it's in as good a condition as mine
the price is right. I also saw two British Mark IXs up for auction.
One thing I've noticed though - eBay bubble sextant sellers are almost
always utterly clueless about these instruments so you must ask
questions early, thoroughly explain what you want looked at, and avoid
any technical words.
I've bought 20+ items on eBay and haven't been burned yet. A couple
were below expectations, but they didn't cost much either.
Bubble sextants are a lot of fun since they let you do real celestial
at home without the inconvenience and limitations of a separate
artificial horizon. Just grab watch and sextant, go out in your yard,
and shoot the sun!
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