Mending a 1975 SNO-T sextant.

28 10 2020

A little while ago I bought a SNO-T sextant on a local auction site, not because I did not have one (I have two), but because the price was too attractive to miss. I found there were two major and one minor problems with the instrument: one hinge had torn from the the case, one shade was a “foreigner” taken from an SNO-M sextant and the star or Galilean telescope was out of alignment. Some may not think the damage to the case to be important, but a sextant is an instrument of precision and if it is not contained safely in its case it may suffer unnoticed damage. Figure 1 shows that a cure could not be effected with filler or “builder’s bog”. I had to saw and chisel away the damage down to fresh wood and glue in a piece of close grained wood .

Figure 1: Case damage.

Figure 2 shows the result, with screw holes marked out and pilot holes for screws drilled.

Figure 2: Wood inserted

As a later photograph will show, the designs of the shades for the SNO-T and the SNO-M are very different, so I elected to make a new bracket and use the glass from the SNO-M. I began by marking out a piece of aluminium alloy plate and boring a hole to make a seat for the glass.. Rather than swaging the glass into place, I planned to use modern two-part epoxy glue.

Figure 3: Boring the shade bracket blank.

The next step was to saw the outline of the blank to shape with a piercing saw (Figure 4) and finishing by filing.

Figure 4: Sawing the bored blank to shape.

Figure 5 shows how “cheaters” are used to guide the file when making tight curves in soft metals like aluminium and brass. They are simply hardened cylinders held in place with a nut and bolt and when a gently used file begins to skate over them, the soft metal has been guided into shape.

Figure 5: “Cheating” a curve.

All that remained was to ease the glass into place with a tiny smear of glue to hold it there and paint it to match the rest of the sextant. “Hammerite” hammered grey paint gives a close match. Figure 6 shows the completed shade.

Figure 6: Completed shade on left.

The “star” telescopes of most sextants are not adjustable for collimation. The word appears to have come from the mis-copied Latin word “collineare”, to direct something in a straight line according to the Oxford Latin Dictionary. In the context of sextants, it means to direct the optical axis of the telescope parallel to the plane of the arc. Except at high altitudes, small errors are of little importance in ordinary navigation. For example at a reading of 60 degrees, a 45 minute tilt of the telescope results in an error of about 20 seconds, an amount which would normally be swamped by other errors. However, in the days of lunar distance observations and checking chronometers by celestial observations, such an error would be significant, and the higher powered inverting or Keplerian telescopes were normally provided with a means of collimation.

The easiest way to check collimation is to check that the face of the objective lens cell is at right angles to the plane of the frame of the sextant, a shown in Figure 7, where there is an obvious wedge of light showing,

Figure 7: Mis-collimated telescope

The rising piece of the SNO-T telescope is a casting that is integral with the body of the telescope and one would be ill-advised to attempted to correct the error by bending it, as aluminium castings have an unfortunate tendency to give no warning that they are about to break. Instead, it is simpler and safer to slowly and carefully file the underside of the telescope bracket, where there is usually plenty of metal, and this is what I did. Figure 8 shows that the error has been corrected.

Figure 8: Telescope collimated.

I have dealt with the rest of the overhaul in my manual on the construction, repair and maintenance of the SNO-T Sextant, which is still available for purchase (see post for 22 November 2008 for details). Figure 9 shows the instrument in its repaired case. The original colour of the case was light grey, and as it had become battered during its life since 1975, and I had some very tough gloss blue paint left over from another job, I used it on the outside, while touching up the lemon yellow of the interior and renewing the felts. The match of the yellow is not perfect, but is much better than nothing.

Figure 9: Overhauled sextant in its up-graded case.

The USSR SNO-T sextant

22 11 2008

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 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.