The Nautical Sextant

Previous posts in this category cover:  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” , “A fine sextant by Filotecnica Salmoiraghi” and “A British Admiralty Vernier Sextant.”

 A few weeks ago on e-bay, I bought a sextant that was said to be Bulgarian. The seller was Malaysian and I seemed to have been the only bidder for this interesting-looking instrument so that my finances stretched to its purchase. The pleasant and communicative seller initially sold it without a case, as it was broken (Figure 1), but I contacted him to ask him to keep the case and send it with the sextant, which he very happily did. When it arrived, the brass placard on the ruined pinewood box read “Gamma Budapest”, but there was a stencilled Cyrillic word on the top which, if it were Russian, would transliterate as something like shtuomluskhii, which does not appear in my rather small Russian-English dictionary. The Hungarian language uses Roman letters and its nearest neighbour with a sea coast, onto the Black Sea, and that also uses Cyrillic letters, is Bulgaria. Hungary itself is totally landlocked, but it has not always been so. However, the Treaty of Trianon following the First World War removed huge fragments from the Kingdom of Hungary, and gave them to Austria, Czechoslovakia, Romania, Bulgaria and the former Yugoslavia. Hungary had the misfortune to be on the losing side in the Second World War too and lived under the heavy hand of the Soviet Union for many years after.

Figure 1 : Base of ruined case

Description

As a guess, I would say that the sextant was made in the years following WWII, as the shades and micrometer mechanism are identical to those of C Plath sextants of the time, many parts of which seem to have reached Britain and the USSR as plunder and reparations. The frame, however, is of a most unusual form, cast in aluminium alloy with the rack cut directly into the frame (Figure 2). The lower case Greek letter gamma (γ) forms part of the frame and there is of course a star (though not a navigational one), gamma sextans to complete the allusion in the name. The frame is covered in a thick and tough coat of black gloss paint, so it is not possible to judge whether the frame was die cast. Very few must have been made , so that it would be hard to justify the cost of the die. More likely, it was sand cast as the front of the frame (except for the arc, which has been machined) is not flat.

Figure 2 : Sextant as received

Cutting the rack directly into the frame is not a problem in itself, but I was taken aback to see that the worm was made of steel rather than the more usual brass used on bronze racks or hard bronze used on aluminium racks. The threads bore a light patina of rust (Figure 3). Almost as undesirable, the brackets for the shades are cast as one with the frame (Figure 4), so that if part should get broken, and shades are very vulnerable to damage, it is practically impossible to make good the damage. The mounting of the shades in the brackets is standard for Plath sextants of the time, on a cylindrical pin prevented from rotating by a crossed taper pin and adjusted by closing up the cheeks of the bracket with a screw let into the end of the pin   A refinement is missing : that of a key way in the pin and keys in the washers that separate the shades, so that rotation of one shade is not transmitted to an adjacent one.

Figure 3 : Steel worm.

Figure 4 : Shades mountings.

The mirror brackets show signs of internal machining so that they are either die castings or  have been cast separately and machined afterwards. Of interest to lovers of detail are the clips for holding the mirrors in place and the method of locking the adjusting screws . The clips (Figure 5), which bear conical points, are fastened to bosses on the front of the brackets, whereas practically all other makers secure them to the edges, where there is little metal and a high risk of stripping threads. The adjusting screws (Figure 6) bear directly opposite the points, passing through a threaded hole in a separate brass piece and then through a clearance hole in the back of the mirror bracket. The brass piece is secured to the back of the mirror bracket and is split so that a pinch screw can close it up and lock the adjusting screw. This is a very practical and effective arrangement though of course it adds to the cost of the instrument.

Figure 5 : Mirror clips.

Figure 6 : Mirror adjusting acrews.

The index arm and its bearing are conventional. The taper of the bearing is typical of C Plath practice, rather steeper than in English sextants. and, again as in C Plath sextants, the index arm expansion that bears the micrometer mechanism is a separate piece, attached to the arm by four screws. Like war time Plaths and early US BuShips Mark II sextants, there is no micrometer vernier and the drum is divided to half minutes. The Galilean (star) telescope is 3 x 40 mm aperture and has  binocular-type eyepiece focussing.

Restoration

Apart from making two new pieces for the floor of the case, and reassembling it, there was relatively little for me to do in the way of restoration once I had taken the instrument apart and cleaned all its parts. I could not, however, leave a rusty steel worm in place and so I made and fitted a new one of brass (Figure7) .  I have given  an account of making a worm in a separate post (6 July, 2009, A worm turns). The micrometer drum had weathered to a dark nicotine brown, but careful cleaning and rubbing with 1000 grit emery paper converted it to a much more legible light orange colour. The telescope rising piece was also, surprisingly, made of steel, but I felt that it could be left, as it is a non-critical component. A thorough cleaning of the lenses of the ‘scope brought about a pleasing increase in clarity of view.

Figure 7 : Two worms and their shafts.

Calibration

The instrument quality is generally rather good, so I was disappointed to find that it is the most inaccurate sextant that I have calibrated so far, though, paradoxically, it is quite precise. The reading of the sextant give values that are between 14 and 104 arcseconds (0.2 to 1.7 minutes) in error, but when the errors are plotted on a graph, the graph is very close to a straight line, so that the errors can be allowed for to give  readings that are very close to the correct ones. I will be giving more details of this and its probable cause in a separate post in the Chasing tenths of an arcminute category, but Figure 8 shows the table and graph of errors with a line of best fit plotted on the graph. Unfortunately, the errors are non-correctable, but an accurate estimate of the reading may be had by applying a correction from the graph.

Figure 8 : Calibration table and graph of errors.

The final photograph, Figure 9,  shows the completed sextant in its repaired and re-varnished case.

Figure 9 : Completed restoration.

István Benkó has kindly sent me the following information about the makers of the sextant: Gamma Optikai M?vek was a Hungarian camera maker in Budapest. It was founded as Gamma Finommechanikai és Optikai M?vek Rt. (Gamma Works for Precision Mechanics and Optics Ltd.) in 1939. Its most famous cameras are the Pajta’s in 1955 and the technically advanced SLR named Duflex designed by Jen? Dulovits in 1947.