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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@jax-inter.net).
>
> Rick
> S/V One With The Wind, Baba 35

   

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