Polarising sextant shades

18 07 2010

Various properties of light have been utilised in sextant shades or light-reducing filters. In about 1938, the property of polarisation was applied to sextant shades. Light waves are longitudinal waves, the sort one sees when waving a rope, and as they are electromagnetic waves, the electrical and magnetic waves are in planes at right angles to each other. By convention, when talking about the plane of polarisation of light, it is the electrical wave that is referred to. For practical purposes, direct sunlight can be thought of as being polarised in all possible directions.

In the late 1920’s Edwin Land invented a film that would select one plane of polarisation of light and in a perfect film, 50 percent of the light would get through. If another sheet is placed behind the first, with its plane of polarisation at 45 degrees to the first, it is found that half of the remaining light gets through. If the sheets are “crossed” at 90 degrees, in theory, but not quite in practice, no light gets through. The intensity of the light passing through is governed by the equation: Intensity out=Intensity in x 1/2 cos² θ, where θ is the angle of crossing, and since the cosine  of 90 degrees is zero, no light gets through.

Since the films are not perfect, in fact about 38 percent of the light gets through a single filter and about 0.02 percent through filters crossed at 90 degrees. When looking at the full sun through crossed filters, the sun is seen as a white disc with a bluish tinge. A pair of uncrossed filters pass about 20 percent of the light, but the human eye can just detect a halving of light intensity, so in practice the loss of light does not seem to be very great.

There are two practical realisations of polarising filters to the sextant. In the most common, one film of polaroid is applied to a fixed glass and the other to one that can rotate in the mounting that houses the fixed film. For the index shade, which has to cope with the sun, one of the films is applied to a glass that is already a moderately dark neutral grey, but both horizon glasses are  clear. Just as for ordinary shades, both index and horizon shades can be swung in and out of the light path as required. Figure 1 shows the general arrangement.

Figure 1

The other realisation has only one polarising glass each for the index and horizon shades and two detachable polarising eyepiece shades, one dark and one clear. It was probably cheaper to make, as will be seen from the details of the practical standard arrangment.

In this, the mounting for one glass nestles within the other and can rotate through limits set by two stops as shown in Figure 2, which is of an index shade.

Figure 2

The inner mounting, of the darker glass in this case, has a groove turned in its periphery to accommodate a pair of brass shoes, and these shoes are anchored by two screws that pass through the outer mounting (Figure 3), so that when in place, the inner mounting can rotate but not be withdrawn from the outer.

Figure 3

The glasses themselves are retained in the mountings by threaded brass mounting rings, visible in Figure 1. Broken polarising glasses can be replaced – for about US$35 each. A cheaper option is to replace a broken glass with plain glass to which a sheet of self-adhesive polarising film has been attached. As the sheet is of uniform thickness, prismacity does not seem to be a problem, and for those who have broken ordinary neutral density shades for which they cannot find replacement, one option is to apply a sheet of film to each side of a plain glass disc, crossed at an angle to give the desired darkness.

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Faking it. Is it a SNO-M or is it a C Plath?

14 07 2010

In my post for October 2008, I wrote about the USSR SNO-M sextant and gave my opinion that it was identical to the alumium alloy-framed C Plath sextant of WW II. This was based on examination of photographs of putative WW II Plath sextants sold on e-bay, complete with Nazi insignias and Kriegsmarine index numbers. Serial numbers placed them in 1942 and 1943 and their frames were identical to the much later SNO-M frames, with the same pattern of sprues and risers from the casting process visible on the back of the frames (Figure 1). I strongly suspect that these so-called C Plath sextants are forgeries based on changing the obviously different SNO-M index arm and engraving C Plath logos and numbers on to the ends of the limb, which are conveniently vacant for them.

Fig 1

Now it could be argued that the differences are because the backs of the frames were not finish machined because of pressures of war time production. This will not do, as there are other, more subtle differences, indicated in Figures 1 and 2, which show that even allowing for the traces of the casting process, different dies were used. The dies used for pressure casting are very expensive to make but when casting aluminium could be expected to last for up to 100,000 units. During WW II, C Plath made about 11,000 nautical sextants, so it is not very likely that duplicate dies were made or that the original die set wore out. Furthermore, I have been able to examine an indisputably C Plath sextant, whose serial number falls between those of the two probable fakes, differing by only 289 from one of them. Its frame is not the same. (April 2013) Since writing this in July, 2010, I have had the opportunity of examining an indisputably genuine Kriegsmarine sextant from late 1944. The rear of the frame had not been fully machined but other features of the frame casting were as for earlier, genuine, sextants. The official history of C Plath writes of labour shortages at this time.

Fig 2

But it is not just in the frame that there are differences. Cast into the interior of the mirror brackets are part numbers preceded by the initials “C. P.” (Figure 3), while no such numbers will be found inside SNO-M brackets; and the foot of the horizon mirror bracket is displaced upwards in the SNO-M Figure 4). Again, it will not do to argue that extra sets of dies existed, and even if they did, it is difficult to think of a reason why the horizon bracket might have been modified in the middle of a war that was slowly being lost.

Figure 3

Fig 4

The index shades seem to be identical, except that the little brass knobs have a traditional shape in the Plath and are stark cylinders in the SNO-M. Again the horizon shades seem identical, but their mounting s are not. I have circled two points of difference in Figure 5

Fig 5

The index arm of the C Plath sextant was made in two parts, joined by four screws just above the lower index arm expansion, while that of the SNO-M, which carries the serial number, is a one piece alumium stamping with an integral stiffening rib. Obviously, this must be replaced in a forgery and insignia applied(Figure 5), but there is a more subtle difference. To make the arm in two pieces requires that the upper part must be displaced downwards by the thickness of the metal. Careful examination of the upper end of the index arm will show that in the SNO-M (and the probable fakes) is sandwiched between the index mirror bracket and the disc to which the bearing shaft is attached. In the Plath instrument it is the disc that is sandwiched between the bracket and the index arm (Figure 6).

Fig 6

Fig 7

The limb and the rack of the Plath sextant are both of the same radius, while in the SNO-M, the edge of the limb has been machined away and its radius is about 1 mm less (Figure 7). This is also true of the re-incarnated “Plaths” which also all seem, like the SNO-M, to be quintants reading to 140 degrees . The genuine drum is divided to half minutes, unlike the SNO-M, which is divided only to single minutes.

Fig 8

The handle is less easy to fake. The genuine wartime Plath has the Plath logo and name moulded into it (Figure 8), whereas the SNO-M has a brass-lined hole by which it is retained in its case by means of  a spring latch. Telescopes too are difficult to replicate (though Tamaya may have done so during WW II) and I have found no evidence to suggest that Plath varied the form of the standard star telescope (Figure 9) until after WW II.

Fig 9

Fig 10

Figure 10 shows that  the famous C Plath stick man logo has feet that are too big, has bursitis in the right elbow and shows detail of the index mirror that is not visible in the genuine contemporary logo. The genuine instrument also has the letters “D. S.” stamped next to the logo, showing that it had been examined at Deutches Seewarte, the German Naval Observatory in Hamburg.

Fig 11

Finally, many of the features of the genuine article (as opposed to a re-worked SNO-M, may be seen in Figure 12, which is from the front cover of Die Woche (The Week) from April 1943.

Fig 12