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Subject: [Nml] Davis Sextants
From: Rick Emerson (rick@XXX.XXX)
Date: Sun Jan 24 1999 - 12:05:39 EST
With all due respect, in my experience, Davis sextants are over-rated.
The problem isn't the use of plastic for the frame (although I do
think it does warrant some consideration). The problems are in the
optical train. First off, the telescopes I've seen are far too small,
giving a limited field of view seen through plastic lenses with less
than optimal contrast. The focus mechanism relies on a friction fit
between two sliding tubes which leads to shifting focus. The mirrors
are small, further limiting the field of view.
Since the object of the exercise is to make visial observations, using
a recalcitrant optical train is, as I said to a friend, a little like
trying to iceskate on sand.
As to the suggestion that a Mk 3 is a good training sextant, I simply
can't agree. A simple sight tube makes a challenging job even more
difficult. The index arm, again, makes the job harder. Now, if it's
a Mk3 or nothing, I'll gladly take the Mk3 - the one sitting in my
basement right now will be in our offshore "ditch bag". But as a
learning tool, the Mk3 is, to my mind, simply inadequate.
In any training setting, it's important that students get positive
feedback from their efforts. If the tools are baulky, hard to use,
and produce uncertain results, the student is less likely to have a
positive experience.
I realize there's a problem with investing in the cost of even a
relatively inexpensive sextant such as the Mk15 or Mk25 but some
learning experiences are simply more expensive than others.
Rick
S/V One With The Wind, Baba 35
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