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Subject: [Fwd: [Fwd: [Nml] Davis Sextants]]
From: carl (dashmanc@XXX.XXX)
Date: Mon Jan 25 1999 - 17:42:44 EST
Rick makes some good arguments and some points that I can and cannot agree with. As
for the issue of quality variations: I only have experience with my Davis Mark 25 and
3 and I have not seen the problems Rick describes, only the ones I have described. As
for the telescope: unless you are engaged in shooting marginally visible objects
(granted that can happen) the optics on my Davis don't lose much. It is true that the
larger the objective, the more light you should be able to collect--but only if the
magnification is kept constant. Check with binoculars--a 7x50 will collect far more
light than a 10x50 (and of course more than a 7x40). Again, for tough visibility you
need a higher quality instrument.
I agree that the whole horizon vs half is a matter of preference--again I have no
experience with other Davis sextants, but my whole horizon gives as large a field of
view as my Tamiya, albeit a half horizon. Perhaps the half version in a Davis is too
small--I don't know.
As for shooting the sun on a clear day--magnification is nice, but unnecessary. Would
it help a beginner? Well, my opinion is maybe and for that I can't justify the added
cost of even an Astro (though maybe a Mark 15).
As for the lathe analogy: here I believe Rick has missed my point. The items he omits
are necessary to the operation of a lathe (specifically, a metal lathe, not a wood
one, as my dad was a machinist) and to overcome them would require even a skilled
machinist to go to extraordinary lengths are are akin to omitting the sight tube, sun
filters and vernier marks on the index arm. Yes, it's that extreme. It's nice to
have auto-feed, auto-cleaning, and computer control on a lathe--it certainly speeds
both setup and production and are pay for themselves, but they are not needed.
Rick has clearly been better at finding good, used and new sextants at bargain prices
than I have and I will defer to him on that.
ATB,
Carl.
Rick Emerson wrote:
> It appears that there are different production runs of Davis sextants
> in circulation. In citing the sliding tube telescope, I'm referring
> to a Mk15 I owned (and since have sold). Again, I base my opinions on
> both that Mk15 and other instruments inspected elsewhere. As to the
> lenses' construction, it's quite possible the lenses are glass. That,
> however, does not ensure a high quality (either in focus, color, or
> contrast). Again, it's been my experience the optical train is less
> than optimal.
>
> As to the question of whole horizon versus half silvered mirror, this
> is a matter of personal preference. My argument is that the index
> mirror - horizon glass combination in Davis sextants is simply too
> small to be adequate. (For the record, I prefer and own sextants with
> traditional half-silvered mirrors but have used both.)
>
> One of the tricks of getting a good sight is setting the lower limb of
> the sun's disk right on the horizon. The point of a telescope is to
> magnify the image, ensuring the best "kiss". Shooting through a sight
> tube makes this task harder than it needs to be.
>
> As to star sights, a large objective helps to "scoop up" light in
> difficult conditions; without the lenses, again the student is left to
> make do with the unaided "MkI eyeball".
>
>
> To use the lather analogy, my argument is that the Mk3 is the
> equivalent of a lathe with no tool rest, no gearing for adjusting
> rotation speed, and the most basic of chucks. A good machinist can
> rise above these limitations but a student, faced with the same
> equipment, must work under a double load: learning to use any lathe
> and learning to work around the limitations of a lathe without the
> benefit of experience needed to do so. I argue that a student should
> spend his or her time on learning the business of celestial navigation
> and not how to cope with a piece of equipment's quirks.
>
> As to the cost of used Plaths, I've seen used Plaths in good repair
> offered for $750 in stores and seen auctions for them, on eBay, close
> in the $500-$700 range. I have also seen them go at much higher
> prices but my point is that $1000 is perhaps a bit high as a typical
> floor. As to the size, over time, of Plath's optics, filters, and
> mirrors, my '61 Plath has the same size as the current models.
>
> In general, new Japanese sextants (both Tamyas and other makes) are
> not attractive because of the yen / dollar exchange rate, not because
> of design or construction. Even Celestaire, who sells Tamayas, says
> this. This may or may not carry over to used Japanese sextants. It's
> up to the buyer to decide if the price is acceptable
>
> Regarding the issue of repairs, basic misalignment is easy to
> identify. Problems with the index arm bearing are equally easy to
> identify; either the arm moves smoothly or not. The arc's thread and
> screw can be inspected with ease.
>
> I grant that a used sextant may have subtle errors which render it
> unfit for use in land surveying but a used sextant, more than suitable
> for small boat navigation, can be located at a good price.
>
> Finally, I'm not sure what accessories need to be ordered with an
> Astra. The Celestaire bubble sight, in my experience, is not
> reliable. The sextant comes with oil, spare springs, and tools for
> more maintenance than most sextants will ever need (insert here a rant
> about more sextants being damaged by "maintenance" than use at sea).
> While Celestaire is the importer, Defender lists the Astra for $420
> and I paid even less through St. Brendan's Isle, a cruising chandler
> (sbi@XXX.XXX
>
> Rick
> S/V One With The Wind, Baba 35
attached mail follows:
dashmanc@XXX.XXX writes:
> 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.
It appears that there are different production runs of Davis sextants
in circulation. In citing the sliding tube telescope, I'm referring
to a Mk15 I owned (and since have sold). Again, I base my opinions on
both that Mk15 and other instruments inspected elsewhere. As to the
lenses' construction, it's quite possible the lenses are glass. That,
however, does not ensure a high quality (either in focus, color, or
contrast). Again, it's been my experience the optical train is less
than optimal.
As to the question of whole horizon versus half silvered mirror, this
is a matter of personal preference. My argument is that the index
mirror - horizon glass combination in Davis sextants is simply too
small to be adequate. (For the record, I prefer and own sextants with
traditional half-silvered mirrors but have used both.)
> 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,
One of the tricks of getting a good sight is setting the lower limb of
the sun's disk right on the horizon. The point of a telescope is to
magnify the image, ensuring the best "kiss". Shooting through a sight
tube makes this task harder than it needs to be.
As to star sights, a large objective helps to "scoop up" light in
difficult conditions; without the lenses, again the student is left to
make do with the unaided "MkI eyeball".
> 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.
To use the lather analogy, my argument is that the Mk3 is the
equivalent of a lathe with no tool rest, no gearing for adjusting
rotation speed, and the most basic of chucks. A good machinist can
rise above these limitations but a student, faced with the same
equipment, must work under a double load: learning to use any lathe
and learning to work around the limitations of a lathe without the
benefit of experience needed to do so. I argue that a student should
spend his or her time on learning the business of celestial navigation
and not how to cope with a piece of equipment's quirks.
As to the cost of used Plaths, I've seen used Plaths in good repair
offered for $750 in stores and seen auctions for them, on eBay, close
in the $500-$700 range. I have also seen them go at much higher
prices but my point is that $1000 is perhaps a bit high as a typical
floor. As to the size, over time, of Plath's optics, filters, and
mirrors, my '61 Plath has the same size as the current models.
In general, new Japanese sextants (both Tamyas and other makes) are
not attractive because of the yen / dollar exchange rate, not because
of design or construction. Even Celestaire, who sells Tamayas, says
this. This may or may not carry over to used Japanese sextants. It's
up to the buyer to decide if the price is acceptable
Regarding the issue of repairs, basic misalignment is easy to
identify. Problems with the index arm bearing are equally easy to
identify; either the arm moves smoothly or not. The arc's thread and
screw can be inspected with ease.
I grant that a used sextant may have subtle errors which render it
unfit for use in land surveying but a used sextant, more than suitable
for small boat navigation, can be located at a good price.
Finally, I'm not sure what accessories need to be ordered with an
Astra. The Celestaire bubble sight, in my experience, is not
reliable. The sextant comes with oil, spare springs, and tools for
more maintenance than most sextants will ever need (insert here a rant
about more sextants being damaged by "maintenance" than use at sea).
While Celestaire is the importer, Defender lists the Astra for $420
and I paid even less through St. Brendan's Isle, a cruising chandler
(sbi@XXX.XXX
Rick
S/V One With The Wind, Baba 35
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