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    Re: [Fwd: [Fwd: Davis Sextants]]
    From: Mike A. LeButt
    Date: 1999 Jan 25, 7:17 PM

    I have not subjected my Davis Mk 25 to any scientific tests but, as stated
    previously, from an exactly known location I have taken many sights over
    the years with intercepts of 0-1 nm and dozens under 3 nm (the requirement
    for Power Squadron exactly-known-position sights).  I have had none of the
    maintenance or adjustment problems mentioned by others (Davis Mk 25 circa
    1983).
    
    At 17:08 1/25/99 -0800, John Brenneise wrote:
    >Gentlemen,
    >	it seems to me that some proper QUANTITATIVE analysis is in
    >order here.
    >Has anyone subjected any of the sextants to which this thread of
    >discussion refers to a test
    >of  thermal expansion issues?  Has anyone tested to see if a laser beam,
    >directed through the
    >sight tube while the index arm is modulated, sweeps out a straight line
    >against a wall?  Have any
    >other quantitative measurements been made?  If not, how can we avoid
    >misunderstanding each
    >other with regard to precision issues?
    >
    >There also is a practical issue of the mass of the sextant when trying
    >to take a sight from the
    >deck of a pitching and rolling boat.  The greater inertia of a metal
    >sextant will help to remove
    >some jitter from the process.
    >
    >In practice, what precision do the MK15 et. al. deliver?  With my Astra
    >IIIB and an artificial
    >horizon consisting of plate of oil placed upon the south facing deck (of
    >my condominium), I was
    >able to get a fix that landed within four nautical miles of that
    >reported by my hand held GPS receiver.
    >(FYI, when using an artificial horizon, the Dip corrections for Hs are
    >not appropriate, although the
    >corrections for refraction still apply.)
    >
    >Incidentally, the US Sailing test only requires a fix precision of ten
    >nautical miles to receive
    >credit for a proper sight reduction.
    >
    >John
    >
    >
    >> -----Original Message-----
    >> From-	carl [SMTP:dashmanc{at}idt.net]
    >> Sent:	Monday, January 25, 1999 2:43 PM
    >> To:	navigation{at}roninhouse.com
    >> Subject:	[Fwd: [Fwd: [Nml] Davis Sextants]]
    >>
    >> 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{at}jax-inter.net).
    >> >
    >> > Rick
    >> > S/V One With The Wind, Baba 35
    >>
    >>  << Message: [Fwd: [Nml] Davis Sextants] >>
    >
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