A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Plath Sextant: Advice - Required.
From: Kieran Kelly
Date: 2004 Jan 18, 21:52 +1100
From: Kieran Kelly
Date: 2004 Jan 18, 21:52 +1100
Thank you for your comments. They were very helpful. George wrote that Cotter provides some references viz Heath, G A (not dated but about 1935) "Modern Sextants", London. Hughes A J, (1938) "The book of the sextant", Glasgow. Cotter also refers to his own paper, "The sextant and Precision Celestial Navigation", in Journal of the Instite of Navigation, vol 16 p 246, London. Thanks George I will look those up. COLLIMATION ERROR George quoted from Cotter in relation to collimation error and the old fashioned collimation rings or collars: "The collar in early sextants comprised a double brass ring constructed so as to furnish a means of adjusting the axis of the telescope to bring it parallel with the frame of the instrument..........but most modern sextants are not fitted with the means for this adjustment, the sextant maker having ensured that the axis of the collar is correctly aligned." I think Cotter is wildly optimistic here. Yes a modern sextant may have zero collimation error when it leaves the shop, as the telescope is fixed in its plane vertical to the instrument by means of the fixed sextant fork or post. Through its years of use it would need only one or two good bangs on the scope as the navigator stumbles down the hatchway or heaven forbid drops the instrument on the deck to bend the fork or post and to do so in such a way that the lack of parallelism between the instrument and the scope cannot be detected by the naked eye. Bingo! We have collimation error producing a bias every time the sextant is used. This error will not be taken out of the sight by the index adjustment either. I wonder how many mariners out there who get a consistent error may have this fault to blame. This may particularly afflict owners of second hand sextants who do not know the instrument's history. SEXTANT BINOCULARS George wrote in relation to sextant binoculars: "Such a binocular would provide a wider-angle view of what the observer could see in the straight-through path, not in the reflected path. That view wasn't limited by the size of the index mirror (as I suggested), but by the aperture of the clear part of the horizon mirror. In the way a sextant is normally held for altitude measurements, the left eye would see a wider view of the horizon (not the constellation), which would not be very useful at all." Quiet true. I have tried them out and what it does in practice is allow you to see a much broader sweep of the horizon. It is like looking through a sextant scope that has a rectangular lens not very high but very broad. It converts the view through the scope from circular to long and flat with the benefit that the silvered part of the horizon mirror can still be seen in the right hand side of the binoculars. Interestingly the fork on the binoculars screws onto the rising price so that the binoculars can be screwed in or out allowing the observer to see as much of or as little of the silvered horizon mirror as desired. George also wrote: "For star altitudes, many observers find that it's easier to locate a star by inverting the sextant, putting the star in the horizon mirror and the horizon in the index mirror. Used in that way, then the binocular could help in identifying the right star. But the binoc. would have to be suitable for using that way up. Usually, you can't use binocs upside-down because they clang the bridge of your nose. Is that the case with those on this sextant?" The way they are constructed allows for them to be used either right way up or inverted. George asked: "I presume that the binocs were "opera-glass" type instruments, rather than prismatics, at that date. Perhaps Kieran can confirm that." Yes they look like robust opera glasses with straight though the lens viewing. "Does it look like a manufacturer's fitting, or is more like an individual mariner's try-out?" Definitely a Plath fork and ring sliding into the riser, ie the unique Plath "tongue and groove" fork. Also it has special mounts holding it in the box. Again same timber and screws and style as Plath used in construction of its sexant boxes. No question it was a factory fitting. SEXTANT HORIZON MIRRORS Henry wrote: "As a matter of interest, technically speaking, the clear portion of the horizon mirror is entirely unnecessary and is an accommodation to the rectangular framework stiffening the mounting appendage. During WWII and immediately thereafter, the Japanese produced a sextant with a & x 50 monocular as a telescope and no clear portion to the horizon glass - simply a large mirror which was unframed at the outer extremity, so that there was nothing but thin air through which to view the horizon; this was really a great instrument, although some might argue that the mirror was not adequately supported." This is quiet apparent when you put the sextant binoculars on the instrument as you are looking well to the left of the horizon mirror frame demonstrating its redundancy. By the way the Plath Navistar Professional has no unsilvered part of the horizon mirror - the mirror frame encompasses just the mirror - the rest is fresh air. The small Zeiss/Freiberger Yachtsman sextant does it even better with no frame down the mirror's outside edge - a beaut little instrument. PLATH SERIAL NUMBERS Joel Jacobs wrote: "My company, Nautech, was selling 900 sextants a year, mostly in the U.S., in the 1970's so your production figures for Plath should be low." I suggested 400 a year in 1920. I could believe that with increasing automation and the fact that less of the sextants were done by hand in 1970 that they got very high production runs. Do you think my guess for about 400 hand made sextants in 1920 is still adrift? That's only a bit more than 1 per day so I agree it does sound light on. However, Joao Blasques wrote that he had a "Plath three circle sextant, serial number 6326 that was tested in the factory on September, 15, 1912" If that is the case and they were up to 10,500 by the end of 1925 then they were making 321 per year even less than I suggested, admittedly World War 1 intervened which may have slowed production considerably. SHADES Does anyone know why the shades on the old sextants were different colours i.e. dark green, light green and blue? Thanks for your help Kieran Kelly