5 August 2011
I have now completed a manual on the structure, overhaul, repair, maintenance and use of the SNO-T sextant, with notes on the Freiberger Trommelsextant (drum sextant) where it differs significantly from the SNO-T. There are over 60 pages of detailed colour photographs and diagrams that will take you step by step through the overhaul of this fine instrument, down to the last nut, bolt and washer. It is available as a download of 5MB for US$19.50 or, for an extra US$4.00, on a CDROM posted to you anywhere in the world. To purchase, please send the correct amount via PayPal, with a note to remind me what it’s for, and use nzengineernz@gmail.com as the payee. You receive it as a pdf file attachment. Here are a couple of illustrations from the manual to whet your appetite:
Figure 1 : Index shades mounting.
Figure 2 : Step of dis-assembling micrometer mechanism.
Now, for a brief account of the SNO-T sextant, please read on.
The Navigational Sextant with Illumination, Tropicalised succeeded the SNO-M sextant in about 1976. While the SNO-M was a clone of the CPlath WW II sextant, the SNO-T was a modified copy of an early version of the Freiberger Prazisionsmechanik Trommelsextant, with the addition of a self-illuminating magnifier. The radius of the Trommelsextant is about 170 mm, as its worm has a pitch of 1.5 mm, while the SNO-T’s radius is 160 mm, the frame is more strongly braced and the worm is of 1.4 mm pitch. The final point of difference is that the Trommelsextant is a sextant, while the SNO-T is a quintant, reading up to 140 degrees. A Freiberger employee told me the SNO-T was made in Leningrad (now St Petersburg), though the closeness of design makes it highly probable that the conception was German.
The Freiberger Trommelsextant was developed commercially in the 1950s and by the 80s was selling with a 3.5 X 40 telescope and a modified micrometer release catch position. The shades were also mounted on to the edge of the frame rather than on the face as in the earlier instrument and the SNO-T.
The design of the SNO-T departed from the traditional in several respects. The most immediately noticeable is in the micrometer assembly, which is totally enclosed and closely embraces the rack. The release catch rotates the micrometer screw away from the rack by means of an eccentrically mounted bearing assembly and the pressure exerted on the rack by the worm can be adjusted to give silky-smooth rotation. The rack is machined directly into the frame. The very robust index arm lies behind the frame (i.e. on the right hand side) and so the handle has to be mounted on a sub-frame that bridges the index arm. Another radical departure from traditional design is in the form of the index arm bearing, which is a very substantial parallel bronze journal running directly in a hardened seat machined directly in the frame. The frame itself is of aluminium alloy. The edges are very substantial and braced by an elegant web on the face. A photoluminscent micrometer drum magnifier was provided, allowing easy estimation to tenths of a minute.
The 1976 specification claimed an instrumental accuracy of +/- 6 seconds with no more than 6 seconds backlash. A later specification modified these to 12 seconds.
The sextant was supplied with two telescopes of exceptional quality, probably by Zeiss. The inverting telescope, a 6 X 30, was identical to that supplied with the SNO-M, while the Galilean star ‘scope was a 4 X 40. There was also a kit of extras, comprising a spare (front-surface) index mirror, a cleaning brush, an oil bottle, two screwdrivers, two sighting vanes, a pin wrench to adjust the micrometer drum and a mirror adjusting wrench. The instrument itself weighed 1.5 kg. This very high quality sextant, possibly the best ever made, was let down by being contained in a grey-painted pinewood box and restrained in poorly engineered wooden pockets which were prone to disintegrate in transport. However, when the paint is stripped off and the wood stained and polished the result is very attractive. Currently selling for around US$400, this represents to my mind a substantial undervaluing.
Many more details of the construction of this and other sextants may be found in my book, The Nautical Sextant .
Figure 3: SNO-T sextant, left hand face
Figure 4 : SNO-T sextant, right hand face
1st July 2012
Robert Lawrence has kindly brought to my attention a sextant sold in 1995 by Sewill of Liverpool, a long-established firm of instrument makers. Although it is named Cetus Primus, it is plainly an SNO-T with some modifications. Figure 5 shows a finely-cut vernier for the micrometer that matches the style of the instrument, though anyone with a good quality dividing head could cut his own. While the magnifier could then be dispensed with, it has on its underside some photoluminescent paint that, once charged by exposure to a 40 watt lamp for a few minutes, gives about 40 minutes-worth of scale illumination.
Figure 5 : Sewill Cetus Primus micrometer vernier
Figure 6 shows the horizon mirror of the same instrument. It has the so-called “full view” mirror that allows a full view of the horizon when observing the body. The inventors claimed that it reflects most of the light at the blue-yellow end of the spectrum (stars, sun, moon) and transmits most light at the red-orange end (twilit horizon). An experienced user said that it made easy sights easier and difficult sights harder.
Fitted to the mirror is a Davis prism attachment. This is in essence a narrow prism with an angle of about 6 degrees that allows a view of the horizon at one side of the mirror, off to one side of the rest of the horizon image. When the attachment is properly adjusted, if the two horizon images are brought into line, the frame of the sextant will be vertical and there will be no need to rock the sextant when taking sights. This would be of particular use when taking high altitude sights
Figure 6 : Sewill Cetus Primus “clear-view” mirror with Davis prism attachment.
Postscript, October 2013 The horizon shades of every SNO-T I have seen are mounted upside-down. See under “Blunders” category.
The Nautical Sextant 
Hello Bill, thank you for publishing this interesting blog, and thank you for all the information contained in your book which I received a couple of days ago. In my youth I was a Captain of the Merchant Marine and, as such, have gathered a lot of experience in the proper use of sextants. At that time, astronomical navigation is all we could use to find the way… Our chronometers and sextants were always sacred pieces of equipment.
Some days ago I started to collect sextants (do’nt ask me why…). The first one to reach me has been a SNO-T, manufactured 1986, obviously never used (no signs of tinkering…). Please, let me make some comments on this sextant.
It shows a considerable backlash (almost 2′) which might be due to hardened grease, etc. From a design point of view, I consider the pre-load spring solution to minimize backlash better than this adjusting screw, the point of which can be easily damaged. A further design issue: I wonder why the worm hat such an enormous length. Because it is a cylindrical one – not obloid – in theory just the flank of one thread bears the load, the other ones do not touch the rack. I find the magnifier-illuminator absolutly useles and bothering. You do not need an illuminator (well, I never used one…). Further, I find that the release catch is not especially ergonomical. Quite unusual.
I realize that, for reasons unknown to me, you are a real fan of this type of sextant. Today, if I had to choose, I would prefer a traditional English one.
Thank you, Albert, for your comments.
I suppose I am a fan of the SNO-T because of its fine engineering and superb optics.
It does have a helical pre-load spring and the adjusting screw is to allow you to adjust the amount of pre-load. If you have 2 minutes of backlash, it is almost certainly because someone has tinkered with the small adjusting screw on the back of the micrometer casting. If you screw it in and out, you will find that you can go from considerable backlash, discernable when you move the index arm back and forth with your fingers, almost to locked immobility. By careful adjustment, there should be a position where everything moves smoothly. The only backlash then will be in end float of the worm shaft, which was specified as less than 6 seconds.
I am not at all sure that you are correct in writing ” in theory just the flank of one thread bears the load, the other ones do not touch the rack.” This may be true of pinions with low tooth counts, but the rack of a sextant effectively has a tooth count of 720 and W.O Davis writes in his “Gears for Small Mechanisms” that “…the tooth form is always finished by the two or three pitches (of the hob) which are located over the centre line.” We may conclude from this that at least two or three threads of the worm are in contact with the rack, and if you look carefully at the SNO-T’s worm, you will see that polishing extends over about six threads.
It is true that there is no need for all the threads beyond this, but they may add a little stiffness to the shaft as well as placing well out of engagement the minor distortions to the thread that occur when the thread-cutting tool enters and leaves the cut.
It is a simple matter to remove the magnifier (just one screw), and if the peg that is left behind offends, it can be amputatd with a hacksaw. I have no experience of this, but I am told that navigators of small vessels like submarines and trawlers, whose horizon is relatively close, can make acceptable star shots in total darkness on a clear night with appropriate optics (e.g. 6 x 30 or 7 x 50 prismatic monocular), provided their eyes are fully dark adapted. The SNO-T self-illuminated magnifier does not need batteries, but I would question the need for magnification, as it is easy to estimate parts of a minute without it.
The release catch I agree is placed unconventionally. The Freiberger Trommelsextant, whose design the SNO-T closely follows (it may have been vice versa), placed the release catch next to the drum. I cannot say that I have noticed any diffference in the ease of use.
Dear Bill,
I found this manual captivating and an essential tool for any SNO-T owner or prospective sextant buyer. Once again, you wrote a masterpiece with detailed explanations, plenty of useful tips for overhauling/repairing delicate mechanical systems and high definition pictures with excellent lighting.
This manual is an indispensable addition to the library of any serious sextant enthousiast and the definitive work on the outstanding SNO-T instrument.
JPP
Do you know what chemical substance they use for the self-illuminating magnifier?
Thanks
Jamie
A typical photoluminescent or “phosphorescent” paint would be strontium aluminate doped with Europium (SrAl2O4;Eu). If you Google “Photoluminescent paint, you will find numerous suppliers.
I am very interested in the sextant, but are you absolutely sure they did not use radium or another dangerous radioactive substance?
Thanks you a lot for your answer
Jamie
I have no reason to believe that a radioactive substance like radium is involved, especially in light of the instructions, which read :
2.2.5 Prior to operation of sextant in the dark it is necessary to expose the illuminator. To this end it is advisable to use light sources featuring intensity of light flux equivalent to or exceeding that of electric light of 40 W at a distance of 0.5 m during 5 minutes.
This would be a pointless activity if radium had been used. I have made several airtrips in the USA carrying a SNO-T as hand luggage and no radiation counter has ever been set off (though it was swabbed on one occasion for drugs!). I hope this reassures you.
Bill
Can you advice me which filters 1-3,4 index and horizon shades to use when taking the sun and some general advice for using shades?
Thanks
James
It depends on how bright the sun is. My own practice is first to look at the sun directly through the shades (filters), to select a combination that gives comfortable viewing of the sun’s disc. Then I look at the horizon beneath the sun and do the same for the horizon shades. You can then set the instrument to zero and either look at the horizon beneath the sun and, as it were, take the horizon up to the sun, or find the sun (which may be uncomfortably bright if you find it in the left half of the field) and bring it down to the horizon. The second technique is my preferred method and it is almost essential to use it when shooting stars.
Practice makes you consistent and sometimes perfect :-).
Whaty kind of paint can be used to repair the aluminum frame with some scratches in the SNO-T?
Thanks, Ron.
I suggest Hammerite hammered finish grey paint with its shade adjusted by addition of black or white paint of the same brand. Do a trial and dry it rapidly with a hair drier to check whether it is the correct shade when dry. If it’s not, you can wipe it off with thinner before it cures fully, and try again.
do know everyone if a artifical horizon for the CHO-T Sextant is available?
I need it for exercieses a home!
The C Plath bubble horizon attachment will fit a SNO-T, if you can find one in working order, or an improvisation is possible using the bubble unit from an aircraft sextant : https://sextantbook.com/?s=bubble+horizon, or you can use a shallow dish of sump oil, treacle or food oil darkened with food colouring.
Bill
Dear Bill, I just finished your book and it is indeed indispensable, I also happened to purchase a used SNO-T, which I got familiar during my yachtmaster offshore training and prefer it over other sextants I tired. However I do not have a full set and unfortunately I am missing mirror adjustment wrench and diopters, Do you happen to know any source where these can be purchased nowadays? Thanks a lot in advance. Regards, Szabolcs
Hello Szaboics.
I am glad you found my book to be helpful.
The dioptres (what I call “vanes”) are simply two pieces of angle aluminium. The vertical part is 25 mm tall. Any two objects that are about 25 mm tall will do as a substitute, for example, two hexagon nuts. M16 nuts are 24 mm across the flats and would do quite well.
The key is more difficult. The screw heads are 2 mm across the flats. You could try B Cooke and Son in Hull, UK ( bcs@cooke.karoo.co.uk).
I make my own.
Kind regards
Hi Bill, Very interesting! I’m close to buy an SNO-T sextant that seems, by the pictures, be a complete set, but have the conventional horizon mirror. I like the full view one that I’ve seen fitted on some pictures around, including the one here. Do you know where I can even buy one?
I would like also to buy your manual, is it still available?
Best Regards,
Alessandro.
Sorry, Alessandro, I don’t know where you can buy a SNO-T with a full view mirror, except by looking regularly on e-bay, but you could retro-fit a full view mirror to a SNO-T. You just need to file a slot in the edge of the mirror with a diamond file so that it fits in the bracket.
The manual is still available for US$18.50, payable via PayPal using my new e-mail address of nzengineernz@gmail.com as the payee. I will then send you the pdf file for you to copy and, if you wish, for you to print out one copy for your personal use.
Kind regards
Bill Morris