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    [Fwd: Davis Sextants]
    From: Carl D
    Date: 1999 Jan 25, 6:26 AM

    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!
    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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