Freiberger Yacht sextant

24 06 2011

This post is preceded by “Inside the Freiberger Skalensextant” and “The Freiberger Skalensextant.”

Freiberger Präzisionsmechanik of the then German Democratic Republic, produced three sextants over the years: the uncommon Skalensextant; their main line, the Trommelsextant, aimed at professional navigators; and in the late 1970s they introduced their Yacht sextant. The latter was aimed at “Western” yachtsmen in competition with Plath and Tamaya at a price of around US$600, when a full-sized sized sextant by these makers could scarcely be had new for under US$1000. It had few features in common with its big brother, the Trommel or Drum sextant which retailed for about US$200 more, but presumably the pressure die cast aluminium alloy frame with rack cut directly into it, that they both had, was one feature that enabled Freiberger to undercut their Western rivals, or it may be that they were heavily subsidised.

Another feature which is also unfortunately in common with the big brother is that some examples have large arc errors, sometimes up to 90 seconds. As the arc is calibrated every ten degrees, it is not difficult to make a graph of the errors to help in applying corrections. Others would argue that on a pitching and rolling yacht, errors of observation are likely to swamp errors of the instrument. Many Freiberger instruments, it must be said, have the negligible errors of their competitors.

In a contemporary handbook, written in a charming Engleutsch, the Yacht sextant was “destined for use on sporting and luxury ships… with conditions and requirements for accuracy …completely maintained though weight and volume of sextant have been reduced  down to approximately half their former values. In consideration of aspects of beautiful shape and representativity, graduated arc and arc webbing have been modified that dimensional stability at extreme temperature differences and mechanical stress can be expected at the same time.”

In my description that follows, I will comment mainly on points that distinguish this sextant from the conventional, touching only briefly on parts that follow standard practice.

General arrangement (Figure 1)

The frame is certainly austerely functional and the limb has a radius of 142 mm against that of the drum sextant, which is about 170 mm, while its weight with its permanently mounted telescope is 860 G, compared to, say, the 1300 G of an alloy-framed Tamaya with a star telescope mounted. There are only three index shades and two horizon shades, quite enough for most purposes. The mirrors, which are interchangeable, both measure 59 x 29 mm. This is not the advantage that it might seems, as the horizon mirror is fully silvered, that is to say, there is no plain glass half, the horizon being viewed directly rather than through a thickness of plain glass. The edge can be fully protected with paint, whereas the junction between the silvered and unsilvered parts of the traditional mirror was always a weak point for sea water to attack the silvering. However, each half of a Galilean telescope “sees” only its own field of view: if half of the objective lens is covered over, half of the field of view is lost. With the traditional arrangement of mirrors, some of the light from the index mirror is reflected off the front surface of the plain part of the horizon mirror into the telescope, so that the images of the body and of the horizon to a large extent overlap. In this instrument, with the fully silvered mirror and a Galilean telescope, there is overlap of about one sun diameter (about half a degree) as opposed to about two and a half diameters for a full glass, and this may make for difficulties for  an inexperienced or occasional observer.

Most modern sextants seem to have Galilean telescopes with about x 4 power with an objective of 40 mm, while the Yacht sextant has one of x 2.4 power, with an objective of 22 mm, so the field of view is about the same. Early yacht sextants had ordinary, second surface mirrors. One that I have seen from 1985 has first surface mirrors which give a slightly brighter image, imperceptible to the eye (which can just detect a doubling in brightness), but at times of poor contrast between the sky and the horizon, even small differences in brightness can make for a useful increase in contrast.

Figure 1 : Front view of Yacht sextant.

The arc is a  sliver of screen-printed aluminium secured to the frame with glue and two pins which, rather surprisingly are made of steel. Predictably, in my example which had seen a lot of sea use, they had rusted and the glue had also given way. The telescope is a 2.4 x 22mm Galilean that focusses by rotating the objective lens mounting rather than the eyepiece and is  less prone to go out of focus as the eye presses against the scope. The eyepiece is provided with a deep flexible rubber pad, a perhaps necessary adjunct for a lightweight telescope used on board a pitching and rolling yacht.

Figure 2 : Rear view of Yacht sextant

The rear view (Figure 2) shows that Freiberger retained the same arrangement for the index arm as in the Trommelsextant (and the Soviet SNO-T) siting it behind the frame out of harm’s way. To avoid having to have a bridging piece, the black anodised alloy handle is attached to the frame only at the top, by three screws. The release catch at the lower end of the index arm swings the worm out of engagement in the plane of the rack and, as will be seen, has a much simpler (and undoubtedly cheaper) arrangement than in its larger brother.

Index arm bearing

Figure 3 : A cornucopia of screws.

Removal of the handle reveals a sextet of screw heads beneath. Two pass through the bearing (strictly speaking, the journal) and attach the index mirror mounting to the deep flange on the other side. Four attach the index arm to the journal (journal : the part of a shaft that is enclosed by a bearing). Removing these allows the journal to be withdrawn. The fit is necessarily a very close one and care has to be taken that it does not capsize and jam. Figure 4 shows the bearing dismantled and it is seen that the large parallel bronze journal runs directly in the frame. There is no provision for taking up wear as none is to be expected in a slow-moving part with such large bearing surfaces.

Figure 4 : Index arm bearing details.

Worm and release catch

The Trommelsextant swung the worm out of engagement with the rack by having a relatively complex eccentric bearing that rotated the worm in its bearing out of the plane of the rack. Elegant though the method was, it must have been very expensive to produce. In the Yacht sextant, Freiberger have managed to give a large degree of protection to the worm by enclosing a substantial bronze swing arm with integral worm shaft bearing in a U-shaped alloy casting, that is attached to the index arm via a tongue and three screws (Figure 5). The shaft, immediately on emerging from its bearing, is enclosed by the micrometer drum and the knob for rotating it, so that a common consequence of dropping the sexant, a bent worm shaft, is avoided.

Figure 5 : Worm enclosure

The release catch is a cranked metal stamping. When it  is operated, its end bears on a pin that passes through an oval hole in the enclosure casting into the swing arm, and presses the swing arm down agains a leaf spring that lies between the swing arm and the bottom of the casting. Upon detaching the casting from the index arm, some more details are revealed (Figure 6).

Figure 6 : Swing arm from above.

The bronze swing arm rotates about a pin for an axis (seen in Figure 5) and has a long and substantial bearing for the worm shaft. End float of the latter is prevented by trapping the worm between an L-shaped phosphor-bronze  pre-load spring and a PTFE washer. The aluminium alloy drum, divided into minutes and attached to the knob by two screws, was heavily corroded and pitted in my example, and I was obliged to make a new one.  The knob is retained on the worm shaft by a grub screw and a pin nut. Figure 7 perhaps allows a better appreciation of the internal workings of the release catch and swing arm.

Figure 7 : Side view of worm mounting.


The index shades are mounted directly into the frame, as shown in Figure 8.

Figure 8 : Index shades mounting.

They are mounted on a screw that has a longitudinal key way machined in it. The screw passes through a narrow tongue of the frame, through the shades and separating washers, through a Belleville washer (a washer having the characteristics of a short, stiff spring) and finally into the body of the frame. The separating washers have an integral tongue that fits into the keyway, so that forces from the rotation of one shade cannot be transmitted to the next. The Belleville washer provides enough friction for the shades to stay in the positions in which they are put. It might be thought that the friction could be varied by tightening the screw, but a transverse steel (!) taper pin prevents it from rotating. Driving it out places at risk the slender tongue of the frame which would pose great problems to replace it, if broken off.

The two horizon shades are mounted on a shouldered screw which also has a keyway and the separating washer also has a tongue that fits into the keyway to prevent it from turning  (Figure 9). The screw is driven into a mounting block hard against its shoulder and it is only this that prevents the screw from undoing. A Belleville washer between the rearmost shade and the mounting block provides friction. Close inspection of Figure 9 shows that over time the convex side of the washer wears into the soft metal of the shade, resulting in loss of friction and a very annoying tendency for the shades to flop downwards out of position. This can be countered  by means of a thin shim washer, whose internal diameter must be a little greater that the diameter of the unthreaded part of the screw, and care must be taken that the shim does not get trapped between the shoulder of the screw and the mounting block. Alternatively, the height of the Belleville washer can be increased slightly by making a depression in the end grain of a block of hard wood using a 12 mm ball bearing, resting the washer in the depression, placing the ball bearing on top and hitting it smartly with a hammer. This is likely to make the ball fly off into a remote recess of the workshop, but as it cannot now be reused for any other purpose, the loss is not a great one. When replacing the shades, they and the washers are mounted on the screw and it is started into position, with the concavity of the Belleville washer facing the mounting block. If the sextant is held with that washer facing upwards, it is then possible to wangle the hole in the washer over the shank of the screw before driving it home.

Figure 9 : Horizon shades mounting exploded.

Adjusting screws

The screws themselves are standard M2 screws with squared heads. They pass through  split collets that are threaded on the outside and which screw into bosses projecting from the back of the mirror mountings. Each collet has a tapered tip that fits into a matching female taper at the bottom of the boss, so that when the collet is tightened, it closes up around the screw, thus increasing the resistance to the screw’s rotation or locking it if required (Figure 9). There is a clearance hole through the mirror bracket for the screw

Figure 10 : Adjusting screw, collet and tool.

Though the adjusting screws for the SNO-T and the Freiberger Skalensextant are superficially similar, anyone who treats them the same is in for an unpleasant surprise; and jammed, broken or stripped screws are a common occurence. In these sextants,  the collets, which have coarse threads on the outside are not split, and the adjusting screws pass through threaded holes both in the collet and in the back of the bracket. The screws are locked by unscrewing the collet a little.  The screw can be withdrawn by rotating the collet a little to find the position at which the screw is unlocked. Any attempt to fully unscrew the collet before the screw has left the thread in the mirror mounting inevitably causes damage. Some early Trommelsextants may have the same arrangement, so it would pay to proceed with caution if backing off the nut by half a turn does not release the adjusting screw.

If you have enjoyed reading this account, you will I am sure enjoy reading my book, The Nautical Sextant, available through booksellers, from Amazon or direct from the publishers, Paradise Cay and Celestaire.

British Admiralty Vernier Sextant

23 06 2011

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” and “A fine sextant by Filotecnica Salmoiraghi.”

The Admiralty ordered sextants from Henry Hughes and Son and bought them provided that they met Admiralty specifications as determined by the National Physical Laboratory at Teddington. Pre-World War II they were given on Admiralty Form 575, but I have not been able to find a copy to discover exactly what the specifications were. Presumably they were exacting and conservative.

Figure 1 : Inspection certificate of sextant number 222**

In the late 1930’s  a batch of vernier sextants was ordered from Hughes and Son and in the early 1950’s another batch, was ordered, this time of micrometer sextants. Except for the rack and micrometer mechanism, they had many things in common: the form of the frame, the mirrors and their mountings , the shades and their mountings and their telescopes. The frame is the classical A type frame, of bronze, heavily ribbed and  the shades are conventionally mounted, but the mirrors are circular and sealed and the telescopes have coarse interupted threads that allow them to be rapidly mounted and dismounted. The sextant can be returned to its case with the index arm set in any position except at the extremes, said to be an  important practical point if a question about a reading occurs when calculating a position line (though most people would try to average several sights of the same body, when gross errors would be noticed).

A few weeks ago, I came upon an Admiralty pattern vernier sextant in good condition and enjoyed restoring it to a near-new condition, at the same time observing points of difference with other instrument types.  Figure 1 shows a general view of the instrument, with the star telescope fitted. In the background is the inverting telescope with an extra, high-power eyepiece ,  two eyepiece shades and an adjusting pick. Note the unusual mirror mountings and magnifier mounting.

Figure 1 : Admiralty Pattern sextant and accessories

Telescopes and their mounting
The sextant was issued with a 2 x 40 mm Galilean star telescope and a 5 x 25 mm inverting telescope, the latter provided with an extra eyepiece to give a magnification of about x 10. Normally, this high powered eyepiece would be provided with a pair of parallel cross hairs, so that the collimation of the telescope could be checked, but only traces of these remain. Possibly they were removed as being unnecessary, as it is not possible to adjust the collimation on this instrument.
Many Galilean telescopes are provided with an eye lens that restricts the field of view, but in this instrument (Figure 3) , the eye lens is 19 mm in diameter, so that even spectacle wearers can take advantage of  the exceptionally wide field of view that I measured as 19 degrees.  Opinion of the time seems to have been divided on the power that the star telescope should have, as, for a given diameter of objective lens, the field of view decreases as the magnification increases. Heath and Co., Hughes’s main competitor in England, noted that the diameter of the  fully dilated pupil of the eye at night was 8 mm and that this regulated the amount of light that could be received by the eye. There was no advantage, they said, in having the 20 mm diameter pencil of the low-powered telescope, as the eye could not take advantage of it. It was their view that if the emergent pencil and the pupil diameter were about the same, all the light gathered by the objective lens would enter the eye and “both the star and the horizon would appear comparatively brightly illuminated.”  Hughes’s standard star telescope was 2.5 x 30 mm as against Heath’s 3 x 30mm, so they were not too far apart. If the Admiralty wanted a 2 x 40, it was presumably felt that the wide field of view and exit pencil made quite certain that the brightest possible image of the horizon could be obtained even if some of the entering light got wasted.

Figure 3 : Galilean star telescope

In daytime, the amount of light entering the telescope is not a critical factor, and a higher magnification allows one to judge better when the image of the sun, moon or, occasionally, Venus is touching the horizon. The bodies are brighter too, so that finding them with a restricted field of view is easier. However, with a small field of view, it is easy to lose the body in bringing it down to the horizon. The inverting telescope (Figure 4)  has a measured field of view of 7.6 degrees, about the same as a 6 x 30 monocular, and bringing the body down in rough weather without losing it is much easier. For practical purposes, the high powered eyepiece would seldom be used at sea, its main purpose being for checking collimation and perhaps ocasionally when using an artificial horizon on dry land in an out-of-the-way place.

Figure 4 : Inverting telescope

As I have noted above, there is no provision for adjusting collimation. Instead, the robust triangular rising piece is manufactured so that it is square to the plane of the arc of the sextant  (Figure 5) and likely to remain that way, as wear in the area should be negligible.

Figure 5 : Checking squareness of face of telescope ring.

 Figure 6 (below) shows the internal structure of the mechanism. The socket is firmly attached from behind the frame by three screws and sits in a machined circular pocket that ensures that it is square to the frame (Figure 7). The socket is made in two parts : one with a triangular holes broached through it and a disk with a round hole for the shank of the feed screw. The two are silver-soldered together and it is impossible to see the joint.  It would not be possible to make the triangular hole without this artifice. The feed screw is held captive in the disc, between the triangular flange and the knob, which has the familiar square. This allows adjustment to remove backlash by tightening the axial securing screw.

Figure 6 : Rising piece mechanism.


Figure 7 : Attachment of rising piece mechanism to frame.

Figure 4 shows the external interupted thread on a telescope and the internal thread can be seen in the interior of the very substantial telescope ring in Figure 5 .  Alternate one-sixths of the thread are machined away, so that the telescope can be inserted with the white dots lined up and secured in place by rotating through less than one sixth of a turn. The Admiralty  may have decided on  this device by analogy with  the interupted threads of the breech closing mechanism of naval artillery, but Hughes also used it in their higher class sextants made for civilian use.
Index mirror mounting
The index mirror is sealed inside its bracket and is not adjustable. Like the telescope mounting, it was manufactured correctly in the first place and, short of breakage, would remain square to the plane of the arc throughout its life. The rear of the bracket bears the statement ” Pat. No 472814, which was accepted in 1935. Figure 8, extracted from the patent document, shows how the silvered rear of the mirror was protected from the effects of salt water by holding it against a “resilient washer” by means of a threaded ring with a lip that bears on the periphery of the mirror.

Figure 8 : Extract from patent on sealing mirrors

Horizon mirror mounting
 This is altogether more complex, since light has to pass through it. Figure 9 shows the mounting exploded. In the left half of the figure the mirror can be seen. A disc of optical glass is cemented to the back of the mirror to protect the silvering and the edges of the mirror and glass where they join is bevelled, so that there is a thin wedge of cement to prevent infiltration of water. There are four dots of silver on the rear of the mirror (Figure 10), presumably to ensure that the cement layer was of uniform thickness , to prevent any prismatic effect. The sandwich is contained in a cell and retained there by a threaded ring and washer.

Figure 9 : Horizon mirror mounting exploded.

Figure 10 : Rear of horizon mirror sandwich.

The right half of the photograph shows how the cell is attached flexibly to the bracket, by means of two slightly curved leaf springs, attached at their ends to the cell and in their middle to the bracket.  The usual two adjusting screws bear on the cell to adjust out side and index error, and are provided with protective screw-on covers. 
The bracket bears the caption “Patent number 30340/34”, but this is in fact a patent application number, one higher than the application number for the index mirror patent. I have not been able to trace the patent itself and assume that one was never granted, possibly because some important aspect had already been anticipated.
Magnifier mounting
Many manufacturers contented themselves with mounting a magnifier at the end of a swinging arm, centred about two thirds of the way down the index arm, with the centre of the lens approximately over the junction of the arc and vernier scales. Obviously, at each end of the vernier scale, the lens centre would be a little out of line with this junction.  When a Ramsden type of compound magnifier was used, this was of little consequence, as the Ramsden provides a relatively flat field, but many cheaper instruments were provided with a simple plano-convex lens, which suffers from quite severe off-axis distortion.
While Hughes provided a Ramsden type magnifier in their top-of-the line pre-WWII vernier sextants, in this Admiralty pattern sextant, there is only a simple magnifier, but it is carried in a substantial mounting that allows the lens to follow the curve of the scales (Figure 11).

Figure 11 : Magnifier mounting

 A curved bed is mounted on two pillars and a short dovetail slide with keeper underneath carries the magnifying lens mounting. An elegant little brass knob completes the assembly.
Tangent screw and clamp
 The tangent screw mechanism is one that was (and is) commonly used in theodolites for obtaining slow motion. I touched on it briefly in my description of a Troughton and Simms sextant and I repeat the description here (see Figures 12 and 13).
A block that can slide in guides on the back of the lower end of the index arm can be clamped to the limb. A tongue or lug projects from the sliding block and is sandwiched between the end of the tangent screw and an opposing spring, both of which are contained in a tubular frame that is secured to the lower end of the index arm. When the clamp is released, the index arm is free to move over the arc. When it is secured to the limb by the clamp, the tangent screw can be used to make fine adjustments, and the spring inside the spring box provides motion in the opposite direction. It also takes care of backlash, which can be an annoyance in a vernier instrument, even though it does not affect the accuracy of the reading.  Other makers were content to leave the ends of the dovetail slide open, but in this instrument they are closed off with end pieces.

Figure 12 : Tangent screw mechanism in situ.


Figure 13 : Tangent screw mechanism exploded.

 Finally, Figure 14 shows the instrument in its refurbished substantial mahogany case. Like all Hughes sextant cases, the handle is on the right hand side, so that the case is not set down on its hinges, and the recessed hook latches face to the left, so that gravity keeps them closed when the case is carried by the handle. The handle is of the type commonly found in military chests and is usually let into the wood, but that detail has been omitted in this case. A “belt and braces” approach to securing the sextant has been adopted, with a pocket and boxwood retaining latch for the handle, while three felt covered pads in the lid hold the instrument secured when the lid is closed and latched. The corners have box-comb joints and top and bottom are, as in most sextant cases, glued and screwed on with counter sunk brass screws.

Figure 14 : Sextant in case

 If you have enjoyed reading this account, you will, I am sure, enjoy reading my book The Nautical Sextant, available through booksellers, from Amazon or direct from the publishers, Paradise Cay and Celestaire. Intending buyers in Australia and New Zealand may find it interesting to Contact me, as I am able to offer them a discount on the published price.