RE: [Fwd: [Fwd: [Nml] Davis Sextants]]
Subject: RE: [Fwd: [Fwd: [Nml] Davis Sextants]]
captainmike7@XXX.XXX
Date: Mon Jan 25 1999 - 21:17:04 EST
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@XXX.XXX]
>> Sent: Monday, January 25, 1999 2:43 PM
>> To: navigation@XXX.XXX
>> 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@XXX.XXX
>> >
>> > Rick
>> > S/V One With The Wind, Baba 35
>>
>> << Message: [Fwd: [Nml] Davis Sextants] >>
>
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