The USSR SNO-M sextant

26 10 2008


After the close of the Second World War, the Soviet Union confiscated much of eastern Germany’s industrial capacity, including many of its workers. Included were some abandoned workshops of  the firm of Carl Plath. In recent years, two models of Russian sextants have found their way on to the market in reasonable numbers, the earlier SNO-M and the later SNO-T.  These are sometimes incorrectly represented as CHO-M and CHO-T respectively: the cyrillic letter that looks like a Roman C represents the S sound in English and the letter that looks like a Roman H represents the N sound. The letters SNO-M represent the Russian for “Navigational sextant with illumination, marine”  and the T of SNO-T represents “tropicalised”.

Carl Plath sextants of the wartime years are very collectible and sell for relatively high prices. It is not generally realised that the SNO-M is not merely a copy of the C Plath sextant but is effectively almost  identical to it. The frames are pressure die-cast in aluminium alloy. This means that molten metal is injected under pressure at several points into a closed, re-usable metal mould and the pressure maintained until the metal has set. It gives accurate, stable castings that need little or no subsequent finishing. Careful comparison of the C Plath and the SNO-M sextant frames at first convinced me that both types were cast in the same mould (Update July 2010:I no longer have this conviction. I believe I was comparing a re-worked SNO-M masquerading as a wartime Plath). The same applies to other items, like the mirror cells, shade mounts and mountings and micrometer parts. The method of securing the sextant in the case is the same: the same bakelite handle has a brass-lined hole in it through which a spring-loaded latch passes, while the legs are located in rubber-lined pockets.

The two parts of the SNO-M sextant which do show some points of difference are the index arm and the telescope. The index arm in the Plath was usually a flat strip of metal attached to the index arm expansion by four screws, whereas in the SNO-M it is an aluminium pressing with a median reinforcing rib. The telescope is almost cetainly by Zeiss, as Zeiss-Jena fell within the Russian sphere. An inverting telescope, it magnifies x 6 and the objective lens is 30 mm in diameter. Focussing is by means of a friction mounted draw tube and there are four cross wires that form a square to help in collimation and to ensure that observations are made in the centre of the field of view. All the lenses are bloomed to reduce stray reflections and light loss. Blooming was a war time German military secret. The wartime C Plath had a x 3 “star” telescope in a variety of external forms(Update July 2010: I now believe that the form was constant throught the war).

Collectors should be wary of putative Plaths that turn up with a x6 ‘scope identical to that of the SNO-M. It is possible these latter are forgeries, as it is relatively easy to replace an index arm and stamp logos and serial numbers on it and the limb. However, there is quite a lot of work involved in repainting a sextant and making the dies for logos etc.  The difference in selling prices may have made it worthwhile.

The SNO-M has an additional piece of equipment, an illuminated magnifier. This is mounted on a pillar attached to the index arm expansion by a single countersunk screw from below. The walls of the magnifier tubes are painted with photoluminescent paint, which, having been “charged” by exposure to light for about five minutes, re-emits it for about forty minutes. In addition, the graduations on the limb and micrometer drum are also filled with the same type of  paint, so that they can easily be read in total darkness. There is no micrometer vernier, but the magnifier allows easy estimation to sixths of a minute, the absolute limit of precision with a x 6 telescope.

In effect, if you buy a SNO-M, you are buying a C Plath with a few improvements. It has metaphorically come from the same mould and may have been produced with the same machinery, possibly even by the same workmen. For more details, buy my book The  Nautical Sextant . Many of the detail photos are of the SNO-M as representing C Plath and Tamaya practice.




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