NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: [Fwd: [Fwd: Davis Sextants]]
From: Mike A. LeButt
Date: 1999 Jan 25, 7:17 PM
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@idt.net] >> Sent: Monday, January 25, 1999 2:43 PM >> To: navigation@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@jax-inter.net). >> > >> > Rick >> > S/V One With The Wind, Baba 35 >> >> << Message: [Fwd: [Nml] Davis Sextants] >> > >=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= >=-= TO UNSUBSCRIBE, send this message to majordomo@roninhouse.com: =-= >=-= navigation =-= >=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= > > =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= =-= TO UNSUBSCRIBE, send this message to majordomo@roninhouse.com: =-= =-= navigation =-= =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-=