The Nautical Sextant

Recently, I was lucky enough to come across a very rare and unusual sextant made by Freiberger Prazisionsmechanik in 1958 at the height of the Cold War. The seller had been unable to fathom out how it worked. It came to me in the battered original box, which was devoid of all traces of glue and which appeared to have been sitting at some time in a pool of water. You will have to imagine what the outside looked like when I received it, but this is what it looks like now:

There are some familiar parts  like an index mirror that rotates when the release catch is released and swung, but the slow motion knob didn’t work, there is no scale or micrometer to be seen and on looking through the lower eyepiece, there was nothing to be seen. The seller had thrown in a telescope, but on examination it appeared to have been no closer to Freiberg than M’bai and showed signs of having possibly been turned using a blunt nail as a tool. The wonder was not that it worked poorly but that it worked at all. The only piece of the original left after restoring was the conical casting and the objective lens. Everything else had to be replaced by newly machined parts.

Even before receiving it, I had formed an idea about how it might work, as I had seen a picture of it on page 69 of Peter Ifland’s Taking the Stars. On dismantling it (none too simple a task), this is the view of the interior that greeted me:

The back of the instrument had been sealed to the frame by some sticky waxy material but it had failed to prevent moisture penetrating. After further dismantling and cleaning, it looked like this:

Light enters through the translucent white window and passes through a glass scale attached to what I will call the index arm though it carries no index. It is then diverted through 90 degrees by a small prism into the objective lens of a low-powered microscope so that the scale can be viewed by means of the eyepiece. The trouble was that even when I had cleaned the scale, I still could see nothing through the eyepiece. I dismantled the latter and found a graticule that was opaque with dirt and once I had cleaned that, I could then see that the graticule contained a linear scale divided into sixty parts. A view of the main scale still eluded me until  I examined the prism – after a fashion – as I could not release it from its mount to which it was solidly attached by corrosion and, probably, shellac. At least I was able to see that the hypotenuse face had at one time been silvered and painted over, but the paint and silver had corroded. Now if the hypotenuse face had been left plain, the prism would have worked just as well, provided it remained clean. If the silver corrodes or the face receives a dirty thumb print, the totally internally reflected image is degraded, in this case, completely. Restorers of some American bubble sextants will have faced the same problem. I could not directly access the hypotenuse face, but I was able to patiently scrape off the paint and silvering with a sliver of brass shim stock, aided with slips of cigarette paper soaked in alcohol. It was then just a matter of adjusting the focus of the objective lens until an image of the scale came into focus at the same time as the graticule.

The main scale is divided into degrees and the eyepiece graticule is used to subdivide the reading into minutes. The glass scale was obviously de-centred, as the degree markers descended across the graticule as the degrees reading got higher. Happily, the instrument contains within itself the means of re-centring the scale. It can be released by loosening three screws that hold a substantial clip and heating the whole part up slowly to melt the shellac that is supposed to hold it where it is set. The scale can then be recentred by trial and error until every degree marker intersects the graticule to the same extent, and then fixed with something like Araldite.  This is the view through the restored optical system:

The scale is reading 36 degrees 08 minutes. Peter Ifland says it can be read to 6 seconds, or one tenth of a minute. I think this is rather optimistic. It can be very rapidly read to the nearest minute and perhaps even to half a minute and, since the image is apparently at infinity, the eye does not need to refocus when changing rapidly from telescope to microscope eyepiece.

Glass scales were introduced into theodolites by the Swiss Heinrich Wild in the early nineteen twenties and it seems to have taken the rest of the world another quarter of a century to catch up. They are made by photographically reducing very large and accurately divided masters on to glass and etching the divisions . They are typically accurate to a  second, depending on the size of the scale. The one used in the Skalen (scale) sextant is of about 80 mm radius, roughly twice that of a T2 Universal theodolite.

In poor light, the scale can be illuminated by a fitting that dovetails onto the bracket below the lighting window (see below). Presumably, the original had as a power source something like a torch handle that sat in the user’s pocket, but it was absent from my instrument. In  daylight, it would benefit from a mirror to divert light from a bright part of the sky into the window, along the lines used in the popular Wild T2 theodolite. I plan to work on this idea (now successfully completed on 27 March 09. See photo below).

Another feature of the Skalen sextant, at a time when nearly all sextants used tapered bearings for the index arm, is the very substantial plain parallel bearing, very similar, if not identical, to those found in the Freibergen Trommel (drum) sextants and the Russian SNO-T. Properly made and fitted, this should never wear out. The general workmanship is of a very high standard.

Dimensions:

End of eyepiece to front of horizon shades 260 mm; Top of handle to bottom of release catch 220 mm; Thickness of frame 30 mm; Height overall from back of handle to top of horizon shades 160 mm. Weight 1.6 kg; Index mirror 52 x 42mm; Horizon mirror half-silvered, 56 mm diameter; Telescope 3.5 x 40 Galilean.

Box dimensions 300 x 290 x 190

Here is the instrument alongside a Tamaya 632D for comparison:

It is not clear why this sextant never became popular. The principle is sound, the instrument is very rigid and weighs about the same as other sextants at  about 1.6 kg, it is easy to use and the slow motion device, once cleaned and lubricated, works well, without slippage or backlash. On the other hand, none of its features of difference give it an overwhelming advantage over conventional designs. Do let me have your thoughts on the matter.

I give brief details of this sextant in The Nautical Sextant.