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Subject: Hand bearing compass
From: George Huxtable (george@XXX.XXX)
Date: Tue Mar 25 2003 - 18:49:55 EST
Phil Guerra asked for recommendations for a hand bearing compass.
Rodney Myrvaagnes recommended the French "hockey-puck" design, and I agree.
Originally produced by the French firm Morin, the same design became the
"Mini-compas" and the "Opti-compass"
This is why I like it.
With most bearing compasses, you have to align your eye, the object on the
horizon you are looking at, and a couple of marks on the compass. When
that's done, you can take the reading of the compass.
The clever thing about the Morin design is that this alignment becomes
unnecessary, due to the way the optics works. The lens and 45-degree
mirror, through which the scale is viewed, is designed so as to make that
scale appear to be focussed at infinity. This allows those of us who don't
need glasses for distant vision to do without them when using the compass.
But more than that. When you look horizontally through the little lens, you
can see about 10 degrees of the compass scale, and just above that lens,
you look directly at the horizon. Then, for any seamark that can be seen
just above the scale, its bearing is the reading on the scale that's
visible directly below it. If your arm moves, and as a result the seamark
moves with respect to the little window on the compass, the reading on the
scale moves too, and the mark and its bearing-reading stay locked together.
So this compass doesn't have those sighting marks to align it, as other
compasses do. It doesn't need aligning at all, except sufficiently to put
the mark being observed somewhere over the window, 10 deg wide. It's quite
a revelation when you try it.
(In a way, it's a similar effect to the way a sextant works. In a sextant,
you align the image of an object in the sky with the horizon. As long as
both can be seen in a mirror, that's good enough for the alignment to be
adjusted. No part of the sextant itself has to be aligned. It's a good
principle.)
Of course, this compass isn't immune from the effects of the swirl of the
liquid or the card, in rough conditions. It's just like other compasses
that way.
An important matter is illumination for night bearings. In the short term,
the best illumination uses a Tritium "Betalight", which is powered by
radioactive decay. At the start, it gives a lovely green glow, behind the
part of the scale that's in view. However (from memory, so I could be out)
Tritium has a half-life of something like 4 years, so while the compass
itself remains working, its light can get very dim over time. In my own
case, this has coincided with a certain dimming of my own vision, to make
matters worse..
Although the betalight comes in a replaceable glass capsule, I have found
that no firm wants to know about replacing it with a new one, and disposing
of the old one.
There's another type of lighting, powered by stored energy during daylight,
like some luminous watches: but this is very short-term, and the
fluorescence has largely faded before dawn, though it can be boosted by
shining a bright torch into its eye.
I have instead changed the lighting for a red LED (a light-emitting diode),
powered by a small battery cell from a calculator. Many cells are magnetic,
which you have to be careful about.
Rodney added-
>The Autohelm flux-gate has its virtues as well. It is also light, etc,
>and stores 9 readings for recall. It must be held level to work
>correctly. As long as you are conscious of this, the flat top is easy
>to keep parallel to the horizon if you can see one.
The Autohelm fluxgate compass, without gimbals, is very, very, sensitive to
being tilted, which I regard as a serious drawback. Not too difficult in
calm weather in daylight, perhaps, but imagine yourself out on a wild
night, trying to keep the compass parallel to an unseen horizon, when
you're not at all sure which way is up. To make matters worse, the
instrument stores a series of bearings without allowing the observer to see
what those readings are. It's a flawed design. But it may well be possible
to design a better compass employing a fluxgate.
George Huxtable.
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contact George Huxtable by email at george@XXX.XXX by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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