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    Re: [Fwd: [Fwd: Davis Sextants]]
    From: John Brenneise
    Date: 1999 Jan 25, 6:08 PM

    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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