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Subject: [Fwd: [Nml] Davis Sextants]
From: carl (dashmanc@XXX.XXX)
Date: Mon Jan 25 1999 - 08:25:55 EST
Reasonable people can differ and, in this case, I must disagree with Rick. His
facts are accurate--the telescope is plastic and does use a form of a friction
fit--but in a spiral, not direct slide, but it is reasonably tight and easy to
adjust. I do believe the lenses are glass, not plastic. I have never noticed
a limit to the field of view--indeed, some older sextants have a far more
limited view. The mirrors are large and rectangular, and, with the full view
mirror (as opposed to half silver) gives as large a field as you could want.
It's true the telescope only gives you 3x, but that is usually more than
enough. The two major problems with the M25, are, 1) The full view mirror is
difficult to use with stars as nautical twilight approaches (as has been
noted--although it's fine for sun sights) and 2) the mirrors are very difficult
to adjust--the mounts are too light so the act of adjustments causes the mounts
to flex and the threads of the adjustment screws need to be finer to allow more
precision. Yet I cannot argue with the statement about a bad optical train.
As for the Mark 3. Here I must also disagree with Rick's interpretation. The
M3 has several serious advantages for the student. My father was a machinist
and insisted the best way to learn was with the simplest tools--you don't learn
to operate a lathe by learning how to program a computerized machine. The M3
is the cleanest and simplest way to demonstrate how a sextant works--I had no
idea what a half-silvered mirror was--it sounded like a two-way mirror and I
couldn't imagine how such a thing was constructed in the 18th century. Of
course, all it meant was a piece of glass that was clear on the left half and
mirrored on the right. The M3 doesn't even have that--you look around the left
side of the mirror to see the horizon. Optics are a "luxury" so they are
removed. The instrument is distilled to its essence and no more is needed to
learn how to shoot the sun or a star. Of course, you cannot see a higher
magnitude star--only the very bright low magnitude ones--and you need clear
skies. But you can see Betelgeuse or Rigel, and certainly Sirius without any
problems, not to mention Venus, Jupiter or Saturn (all very visible now). And,
as Rick says, it's good for a backup--a heckuva lot better than doing an
eyeball zero degree sight without a sextant--clouds always seem to be worst at
the horizon. Besides, spending $30. on a Mark 3 hurts a lot less than $450.
for the cheapest metal sextant--before you get any accessories. Used sextants
may not be a good idea for a student--older sextants have smaller mirrors and
may require repair. Used newer ones, such a C.Plaths and Tamiyas are still up
in the $1000. range at a minimum. That's hard to ask of a neophyte--once they
decide this is what they want then they can invest in a better instrument and
refine their skills.
Just my two cents!
ATB,
Carl.
Rick Emerson wrote:
> 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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attached mail follows:
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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