The SOLD KM2 Bubble Sextant

4 11 2013

This post is preceded by  “C Plath Bubble Horizon Attachment”;“A gummed up AN5851-1 averager”, “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  ”Refilling Mark V/AN5851 bubble  chambers” ,  ”Overhaul of MkV/An5851 bubble chamber” ,  ”AN5851-1 : jammed shades carrousel” ,  ”A Byrd sextant restored” ,  ”Update on Byrd Aircraft Sextant”, “A nautical sextant bubble horizon” and “Sealing A10 vapour pressure bubble chambers.”

In 1939, the SOLD-Libellen-oktant (SOLD level-octant), made by C Plath, went into service with the Luftwaffe (German Air Force). I have not been able to discover where the “SOLD” came from, but it is a fair bet that it is an acronym which includes the words “Sextant” and “Libelle” (Level). While it was used extensively by Luftwaffe reconnaissance aircraft especially, it is less well known that Naval  versions appeared,  with better protection from salt water spray and a self-contained power source in the form of a rechargeable NIFE battery.  The aircraft versions were powered via a flying lead. While the results of observations made with a bubble sextant at sea can be expected to be rather inaccurate, in some circumstances, on war patrol at night for example, or in poor weather when no horizon is visible, even a poor result can be better than none at all. Since this is a blog about the nautical sextant, it is with the nautical version that this post is concerned, specifically a KM2 (Kriegsmarine 2) of 1944. It differs otherwise only in minor details from the Luftwaffe version.

Please note that all illustrations may be enlarged by simply left- clicking on them. Return to the text by clicking on the back arrow.

Optical lay-out

Figure 1 shows the light path.  It is perhaps slightly easier to understand the path for night time observations of a star. The star is located by looking upwards through the partially reflecting mirror 5.  Light from the bulb 1 is reflected off mirrors 7 and 6, through a circular bubble level 2, off the reflective face of a pentaprism  3 into an objective lens 4, whose focal plane is at the bubble. The lens renders the rays parallel, so that the bubble then appears to be at infinity as it is observed reflected off the mirror 5. While keeping the star in view, the mirror 5 is  rotated to bring the image of the star and the image of the bubble together, when the angular altitude of the star above the horizontal may be read off a scale attached (indirectly) to the mirror 5. The radius of curvature of the bubble cell and the focal length of the objective lens are chosen to be the same, so that once the star and bubble images are coincident, they tend to move together when the sextant body is disturbed. For daytime observations of the sun, it is the bubble that is viewed through the mirror while the sun’s reflection off mirror 5 is observed indirectly. For this type of indirect observation, the mirror 7 is removed and a frosted window substituted, so that diffused daylight illuminates the bubble chamber, though daylight viewing with the bubble chamber illuminated by the lamp is possible too.

Figure 1: Light path.

Figure 1: Light path.

Averaging observations

Pitching and rolling of a ship imparts accelerations to the bubble chamber which for practical purposes may be considered as being random, and the bubble is seldom at rest. With early bubble sextants, a handful of observations were taken of the body and the altitudes and times averaged. However, random errors are reduced as the square root of the number of observations, so that, for example, to reduce the average error to a quarter, 16 observations have to be made and to reduce them to a fifth, 25 have to be made.

The next step was to provide some sort of drum on which marks could be made by the observer when he considered the bubble and body to be in coincidence, but it was soon pointed out that the bubble might well be considered to be at rest when it was in fact the subject of a large, but constant acceleration. The next step was to cause the marker to operate automatically at intervals of about a second, leaving the observer to maintain coincidence and to choose the median at the end of the observation period. A variety of clockwork averagers  giving a reading of the mean of the regular observations followed. The SOLD sextant, however, continuously  integrates rather than averages the observations. There is, however, probably very little practical difference in the results obtained by a British Mark IXA instrument, which averages 60 observations over 120 seconds and the SOLD which continuously integrates observation over a period of 40 to 200 seconds. The former is, however, more compact and of a better ergonomic design.

General arrangement

Figure 2: General arrangement, left side.

Figure 2: General arrangement, left side.

The optical parts, the battery and the integrator are sandwiched between two aluminium alloy plates joined together by pillars at intervals. Figure 2 shows the parts related to the outside of the left hand plate. At the front is a rectangular timing unit with its winding knob and starting lever. A shaft with a pinion on its end passes through the left hand plate to the integrator unit. A little below it and to the front is the daylight window which slides into a socket between the plates. This window can be withdrawn and a 45 degree mirror 7, Fig.1, substituted for night-time illumination of the bubble chamber. A slide allows a red filter to be placed in front of the bulb so that the bubble field appears to be red and night vision is preserved. The level unit control and air chamber lie outside the plat,e while the bubble chamber itself is held rigidly in the optical path between the plates. Provision is made to remove the level unit without having to dismantle the whole instrument. The left hand handle, behind the bubble control, has within it a rheostat to control the brightness of the bubble lighting, and below and to the rear of this can be seen part of the battery housing which lies between the plates. A slide to receive a 2-power telescope is to the rear of the top of the handle and above this, mounted between the plates, is a bracket containing two filters or shades to reduce the brightness of the sun or moon before their rays reach the observation mirror (5, Fig 1).

Figure 3: General arrangement, right side.

Figure 3: General arrangement, right side.

Figure 3 shows a view of the right hand side of the instrument. At the front is a rheostat used to vary the brightness of illumination of the scales. It is marked “Hell” (light) and “Dunkel” (dark). Behind this is the main drum, the periphery of which is divided into three lots of ten degrees and a vernier (not normally used) allows readings to one minute. There is a mechanism within the hum of the drum which limits it to two and two-thirds turns, so that altitudes up to 80 degrees may be measured. As the drum rotates, a linkage to the observation mirror causes it also to rotate. Once approximate coincidence between the bubble and body images has been obtained, the clutch above the drum, is engaged, linking the drum to the integrator mechanism. The values of further movements of the drum to maintain coincidence are then integrated. to give a mean value at the end of the observation period. Above and to the rear of the drum is a housing for the illuminating system of the scales. At the end of the observing period, the integrator lighting comes on and when the scale lamp switch button is depressed it goes out and the scales lighting comes on.

Figure 4: General arrangement  top view.

Figure 4: General arrangement top view.

Figure 4 shows some parts already described. There is an additional view of the shades bracket with the shades in the operating position. The main switch can be seen at the top of the handle with the on position (“Ein”) marked. The various scales, illustrated more clearly in Figure 5,  are shown, together with the window through which the integrator reading is viewed. The observing mirror is seen for the first time. Figure 5 shows a view of the top as if from a body at an altitude of about 30 degrees.

Figure 5: View as from a body at 30 degrees altitude.

Figure 5: View as from a body at 30 degrees altitude.

Figure 6 shows a view seen as if taking an indirect observation at about 30 degrees. The image of the bubble field can been seen in the objective lens, through the semi-reflective observation mirror.

Figure 6: View of indirect observation.

Figure 6: View of indirect observation.

Observation mirror controlling mechanism

The inner surface of the drum, shown face-on in Figure 3, has a spiral groove machined into it (Figure 7).

Figure 7: Drum spiral.

Figure 7: Drum spiral.

The drum rotates about a fixed conical axis (Figure 8) and a follower, seen on the right-hand side of the figure, engages in the spiral groove of the drum. A lever attaches the follower to the axis of the observing mirror, so that as the drum is rotated, the follower moves out along the spiral, thus rotating the mirror about its axis. A long helical spring prevents backlash by keeping the follower against the inner wall of the spiral groove.

Figure 8: Controlling mechanism

Figure 8: Controlling mechanism

Integrator controlling mechanism

When the clutch-engage lever (Figure 3)  is depressed a spring presses a conical peg into one of the holes drilled radially into the periphery of the drum at each whole number of degrees. This then links the drum to a radial lever arm that rotates about the drum axis, seen at about 1 o’clock in Figure 8. The teeth of a  sector attached to the lever and rotating about the same axis engage with a rack, one end of which carries a pin which engages with a fork attached to the integrator roller axis. Thus, as the drum is rotated back and forth to maintain coincidence between the observed body and the bubble, the integrator axis is also rotated. The spring-loaded forked device seen at about 5 o’clock in Figure 8 prevents backlash in both directions via a pin at the lower right of the sector. Figure 9  shows the mechanism in operation as if the drum had been rotated forwards. Rotation backwards would engage the other jaw of the fork.

Figure 9: Anti-backlash mechanism in operation

Figure 9: Anti-backlash mechanism in operation

Integrator mechanism

On the inner end of the integrator roller axis is a roller which is vertical when the radial lever arm is in the mid-way position with the clutch engaged (Figure 10). The axis runs in ball bearings while the roller itself rotates about plain conical bearings. A long horizontal spindle is pressed against the roller by a spring mechanism on the right hand plate. When the timer is started, its clockwork motor pinion engages with a rack attached to the carriage that carries the spindle and moves it from right to left. As long as the roller remains at right angles to the spindle, it simply rolls along it, but as soon as the roller is tilted, the resultant forces cause the spindle to rotate one way or the other, depending on which way the roller is tilted. The rate of spindle rotation is proportional to the amount of tilt.

Figure 10: Integrator mechanism.

Figure 10: Integrator mechanism.

Figure 11 shows the integrator in operation with the roller tilted. At the left end of the spindle is a small drum which shows the integrated minutes of deviation of the main drum from the mid position and a disc geared to this drum indicates the total number of degrees of deviation, offset by three degrees in order to avoid subtraction when arriving at the final result. After the observation, the reading of the tens of degrees scale, the whole number of degrees shown on the main drum and the degrees shown on the integrator disc are added to the minutes on the integrator drum. The time is of course taken from the mid point of the observation. The integrator read-out is shown in close up in Figure 17, below.

Figure 11: Integrator in operation.

Figure 11: Integrator in operation.

The level unit

I have given details of the construction and principles of the level unit in the preceding post (26 June 2012) so will not repeat them here, but will describe how to remove the unit from the SOLD. Figure 12 shows the location of the locking screw that secures the unit in the instrument. It is captive in a wedge that pushes down a short arm that in turn forces the level unit against a machined seat between the plates. Unscrewing it withdraws the wedge.

Figure 12: Bubble unit removal ,1.

Figure 12: Level unit removal ,1.

Figure 13 shows the wedge fully withdrawn, when the slotted head pin just below where the wires enter the plate is pushed upwards to release the arm from the level unit.

Figure 13: Bubble unit removal, 2.

Figure 13: Level unit removal, 2.

Figure 14 shows that two conical pins on the lever arm locate the level unit accurately in position.

Figure 14: Level unit removal completed.

Figure 14: Level unit removal completed.

Lighting system

A master switch is placed at the top of the left hand handle.

a) Level unit.

When the securing stirrup between the plates under the front of the instrument is swung forwards, the cover to the bubble lamp can be swung backwards to reveal the lamp in its socket and a slide (Figure 14). The slide has a plain and a red filter, so that if required, the bubble illumination can be made in red light, to preserve night vision. The bulb is 2.4 volts with a 5 mm bayonet fitting and the ground-glass exterior is rather delicate. The rheostat in the left handle controls the lighting intensity.

Figure 15: Bubble illumination lamp and filter slide.

Figure 15: Bubble illumination lamp and filter slide.

In full daylight, a ground-glass screen is inserted into the socket on the front of the instrument and this can be seen in place in Figure 15. For night-time illumination or in poor daylight, this fitting must be removed and replaced by one having a 45 degree mirror to divert light from the bulb to the bubble chamber (Figure 16 and Figure 1, above)

Figure 16: Fittings for bubble illumination.

Figure 16: Fittings for bubble illumination.

b) Integrator

While the master switch must be on to obtain lighting anywhere on the instrument, the bubble chamber will not be lit until the timing unit has been wound fully. This resets the integrator so that the minutes drum is zeroed and the degrees disc set to 3 degrees. The bubble chamber remains illuminated until the end of the observation, when the front of the integrator carriage operates a switch that cuts current to the bubble lamp and switches on the current to the integrator lamp. This switch is seen at the left of Figure 10 and is shown in close-up in Figure 17.

Figure 17: Integrator switch and read-out.

Figure 17: Integrator switch and read-out.

Removal of a thin sheet metal cover reveals the bulb and the switch wiring (Figure 17 a). The end of the bulb is painted black to limit stray light. The integrator lighting intensity is controlled by the rheostat on the right hand plate at the front of the instrument.

Figure 17 a: Integrator lamp

Figure 17 a: Integrator lamp

c) Other scale illumination

The tens of degrees scale and the degrees drum are lit by a lamp at the rear right of the instrument. The lamp housing is shown in close up in Figure 18. Pressing the red button switches off the integrator lighting and switches on the scale lighting.

Figure 17: Scales lamp housing.

Figure 18: Scales lamp housing.

Removal of the slot-headed screw reveals the lamp fitting and change-over switch (Figure 19).

Figure 19: Scales lamp fittings.

Figure 19: Scales lamp fittings.

d) Accumulator

This was a nickel-iron accumulator of two cells with a potassium hydroxide electrolyte and although the manual in giving instructions about charging refers to allowing the cells to gas after charging, the unit provided appears to be completely sealed (Figure 20), no bad thing in an aluminium alloy instrument.  Potassium hydroxide, as well as corroding aluminium with great ease, also generates potentially explosive hydrogen gas.

Figure 20: Exterior of battery.

Figure 20: Exterior of accumulator.

The unit appears to be a rechargeable hand torch, adapted for use in the sextant, as there is a socket for a bulb in the top and current is provided by a projection at the front and the metal switch at the rear, with internal contacts in the battery compartment. The switch plays no part in the instrument other than providing a path for current flow. The exterior of the battery housing is shown in Figure 21 and Figure 22 shows its interior. When the housing is closed, the rear contacts of the accumulator and the instrument come together, and the contact at the front of the lid makes the circuit from the switch on the accumulator via a curved brass strip only when the housing is securely closed.

Figure 21: Exterior of battery housing.

Figure 21: Exterior of accumulator housing.

Figure 22: Contacts inside battery housing.

Figure 22: Contacts inside accumulator housing.


The timer at the front left of the instrument is wound by depressing the knob (Eindrücken) and winding in the direction of the arrow up to a stop. This also returns the integrator carriage to its starting position and zeroes the integrator read-out, as well as preventing the integrator lamp from being accidentally lit. The timing period may be set to 40, 120 or 200 seconds, but only when the timer is running. It is started by depressing the trigger. The movement has a pin-pallet escapement and a monometallic balance wheel (Figure 20). The escape wheel has three set of teeth on its periphery.  A different set is moved into the path of the pallets for each timing period.

Figure 20: Timer movement

Figure 23: Timer movement


This is s short Galilean telescope of 2 power and has an objective aperture of about 28 mm, for use in indirect observations of fainter stars such as Polaris, increasing its apparent brightness by a factor of two. The recommended procedure for the inexperienced was to locate the star by direct observation and get approximate coincidence of star and bubble, before changing to indirect observation using the telescope. The telescope is focussed by rotating the objective mounting and is located in a dovetail slide on the left side of the instrument (Figure 24). It cannot be used for direct observations.

Figure 24: Telescope in place.

Figure 24: Telescope in place.


The exterior of the sheet- metal case is shown in Figure 25 and its interior in Figure 26 (These are courtesy of Alan W Heldman).

Figure 25: Exterior of case.

Figure 25: Exterior of case.

Figure 26: Interior of case.

Figure 26: Interior of case.

The two oval pockets are for accumulators, two of the large round pockets are for 110 volt blue incandescent lamps used for dropping the 110 volt DC ship-board current to one suitable for charging the accumulators, two of the large round pockets are for the ground- glass screen and mirror for bubble illumination (Figure 16), the rectangular slot is for the telescope, and four spare bulbs can be seen in place. The instruction manual is in a celluloid pocket at the front. The case also contains a charging adapter and lead, and a suspension strap, unlikely to be used aboard a ship.

The case measures 150 x 250 x 250 mm and weighs 2 kg. The instrument itself weighs 2.6 kg.

Operating instructions

The following is translation of an extract from the 1944 “Beschriebung und  Bedienungsvorschrift für den SOLD-Libellensextanten KM2”. I hope that those more expert in German than I will forgive any solecisms.

II Operating Instructions.

a)    Daytime observations

1)    Remove the instrument from the case with the left hand and insert the ground glass screen into the opening below the integrator.

2)    While holding the instrument with the top edge horizontal, find the bubble and adjust its size so it appears somewhat larger than the sun (about 1/3rd of the distance between the squared lines in the field of view). The bubble control of the level is on the left of the instrument.  Set the size of the bubble by tilting the front upwards a little so that the bubble sits over the triangle in the field of view. The triangle marks the correct place as well as giving a guide to the correct size. Return the instrument to horizontal and rotate the control back a little to the left to prevent further bubbles entering the bubble chamber.

3)    Transfer the device to the right hand, supporting it for better balance with the little finger under the level lamp housing. Wind the movement with the left hand by pressing in the winder and rewinding up to the stop. Set the running time by pressing down the starting lever and adjusting to the desired running time. Then rewind the clockwork.

4)    Point the instrument in the sun’s direction and rotate the right knob until the sun appears in the field of view. Engage the clutch when the sun and the bubble are together in the field of view. The inexperienced may find it easier to first locate the sun by the direct observation method.

5)    With a finger of the left hand, press down the clockwork starting lever and at the same time call out “Zero” to the note taker.

6)     While the clockwork is running one should try constantly to keep the bubble and the sun at the same height, i.e. to keep one alongside the other.

7)    If there is no note-taker at hand, then the time of observation is taken when the clockwork has run down. The observer then has to count off seconds from the observation time to obtain the time reading and deduct this from the reading time. The time is then recorded with the note “e” to indicate that it was the end point of the observation that was noted. The half of the time set on the clockwork should also be recorded at this time, as this will be deducted from the end time to obtain the averaged time of the observation.

b)    Night time observations.

1)    Remove the instrument from the case with the left hand and install the telescope and level lighting mirror.

2)    Remove the accumulator from the case and carefully install it in its housing. Open the lid only to 20o as there is otherwise a risk of damage to the hinge. Turn the switch in the left handle to the “Ein” (on) position.

3)    As for daytime observations (wind clockwork).

4)    The observer must make sure that the integrator lighting resistor is set to dark. The bubble field is made visible by rotating the resistor in the left-hand handle upwards with the thumb.

5)    As for (2) in daytime observations (bubble setting).

6)    View the star directly through the observation mirror and turn the right-hand hand wheel until the bubble and the star are in near alignment. Then engage the clutch.

7)    Operate the clockwork trigger and carry out the observation as under 5, 6 and 7 for daytime observations.

c)    Reading the average height.

Reading is as described on page 13. The basic reading on the degree drum is added to the integrator reading.

Figure 8: Reading the Integrator.

SOLD fig 8

                                                                                                                   1     Minutes drum

                                                                                                                   2     Degrees wheel

                                                                                                                   3     Degrees wheel index

Example: Clock time, end of observation 22h 11m 38s, running time of clockwork 40 sec, index error 0’. The reading gives:

Tens scale                                                     50 o

Degrees drum                                               8o

Integrator degree wheel in 4 division       4o

Minutes drum                                                 17’

Observed altitude                                         62o 17’ (see Fig 8a)

The relevant time of this observation is determined as follows:

Half time run                                     -20s

Clock time                        22h 11m 38s

Time =       22h 11m  18s

Next calculation as usual

Because of backlash, it is possible for the degrees wheel to indicate in the area of 2 degrees while the minutes wheel is already above zero (Fig 8,b). In this case, if the minutes wheel indication is past zero, take the higher degrees figure and if below zero, the lower.

Werner Luehmann has kindly provided the following comments and photographs of his Luftwaffe version of the SOLD:

As an owner of an “aircraft SOLD sextant” I can say that the only differences between the KM 2 an the aircraft version are: (1) left handle (also the orientation of the off/on switch is perpendicular), (2) accumulator (navy) versus a 3 Volts battery (air force) or optional a flying lead with in integral resistor to reduce the on board voltage, (3) the case (wooden box for aircraft type). Also, (4) the aircraft sextant is missing the “suspension bracket” (although there was a gear available to suspend it in the aircraft). There are neither obvious other differences in parts, nor any in “sealing”.

Figure A1: Exterior of wooden case.

Figure A1: Exterior of wooden case.

Figure A2: Luftwaffe instrument in case.

Figure A2: Luftwaffe instrument in case.

Figure A3: Instructions in lid.

Figure A3: Instructions in lid.

I wish also to acknowledge the kindness of Alan W Heldman in entrusting me with his SOLD sextant, for sending me a copy of the operating manual and for providing encouragement as I struggled with the translation. I am happy to provide interested readers with the whole translation, which includes the original illustrations. “Contact me” if you would like a pdf file of the translation. I make no guarantee as to its accuracy.


Recently, Wolfgang Koeberer kindly entrusted me with his KM2 Bubble Octant, allowing me to delve a little more deeply into this interesting instrument. His was not in as good condition as that of Alan Heldman and the optical path was rather dirty.

The bubble unit was still full and intact, except that particles could be seen floating in the fluid, so my first act was to remove the bubble unit from the instrument to clean the exterior of the glasses. Apart from the floating particles, the view through the bubble unit was then clear, so I did not attempt to remove the glasses. Rather, I gained access to the interior by removing the air chamber and cycling the control until no more fluid would come out. A note of caution is needed here. When I opened the chamber, there was a faint smell of onions. Incautiously, I took a good sniff of the fluid to try to identify it and spent the next five minutes with burning nose and watering eyes. The skin on the front of my wrist, where a few drops had fallen then began to itch and burn. Plainly, if this was alcohol, the authorities had been serious about denaturing it to defeat the ingenuity of sailors in obtaining access to alcohol. On reflection, I think the fluid was probably di-methyl sulphoxide. The Soviet IMS3 sextant, a development of the SOLD, fills its chambers with alcohol, so I flushed out the unit several times with isopropanol, flushing out most of the particles in the process and removing I hope most of the vicious fluid previously present. 

On replacing the bubble unit, I found that there was no view through the instrument and there was a a rattle when it was gently shaken. On removing the bubble unit again, it became clear that the mirror labelled “6” in Figure 1 above, had detached itself from its mounting. Getting at the mounting involved some serious deconstruction of the instrument and Figure 27, below, shows its extent. I will not go into details except to remark that most people, I suspect, if they were to remove the integrator unit, might, like me, begin to despair of ever getting it back into place.However, removing the integrator unit allowed me to overhaul it an get the roller to move freely again.

Figure 27 : Access to first mirror.

Figure 27 : Access to first mirror.

With the mirror glued back on to its bracket , the bracket replaced and all the other parts necessary to be removed having been replaced, another viewing through the sextant showed that, while there had been some improvement, there were still many particles in the field of view. The mirror labelled “3” in Figure 1 I suspect is not a mirror at all but a pentaprism. At any rate, the glass surface immediately below the bubble chamber is horizontal. Cleaning it and the upper surface of the objective lens (“4” in Figure 1) brought about no improvement, so I suspect inaccessible silvered surfaces of the pentaprism, or mirror, if it is a mirror, have deteriorated. Getting at this area would have involved removing the whole left plate of the sextant and I was not prepared to take the risks involved. However, the flecks of dust or damage do not affect the function.

None of the electrical system worked, but this was relatively easy to solve by accessing all the electrical switch and bulb contacts and cleaning them. I also soldered in a battery carrier for 2 AA cells that fits comfortably into the original battery compartment.

The daylight window and night lighting mirror (Figure 16) were absent. The window was simple to make, but the mirror required more thought and experimentation. The original had an elliptical mirror glued to the end of an aluminium fitting. After a brief trial.I found trimming mirrors to an elliptical shape is not an easy job, so instead I cut off the end of a piece of 32 mm diameter alloy bar at 45 degrees in the band saw and then filed and emery papered the cut surface to a mirror finish, ending with some metal polish. I then turned it down for 60 mm to a snug fit in the end of the sextant and parted off at 63 mm to leave a flange of full diameter (Figure 28). A screw with a head 4 x 3 mm in dimensions fits into a slot in the sextant to ensure correct orientation. It works very well. If there is any tendency for the fitting to slide out in use, the head of the screw can be expanded by opening up the slot with a stout screw driver.

Figure 28 : Night lighting  mirror dimensions.

Figure 28 : Night lighting mirror dimensions.

As a final task, I fitted the sextant with a borrowed x 2 1/2 telescope and took observations of the sun near the meridian for a total time of 15 minutes in order to determine the index error of 16 minutes “off the arc”. I made no effort to correct this as the index mirror was very well seated after more than 70 years and I had no wish to break it.

As this web site was started as an encouragement to readers to buy my book “The Nautical Sextant” I hope this post will act as further encouragement to potential readers; and I would also like to draw attention to my book “The Mariner’s Chronometer” More details may be found at

Eighty Years of Carl Plath Sextants

13 11 2012

See also my posts for 18 December 2010 and 6 December 2012

In this post I will be surveying some C Plath sextants in my possession. I have given detailed coverage already to the Plath Dreikreis or three-circle sextant in January 2010 and to a 1953 ladder pattern sextant in December 2011, so I will be only summarizing details of those instruments. I also looked at Plath’s standard wartime sextant of WWII in the context of detecting fakes in my post of 14 July 2010  and I will give a general overview of that instrument. The last production instrument was Plath’s Navistar Professional and as I have not previously given any cover to that, I will take a detailed look at it. The National Maritime Museum (NMM) at Greenwich has a few Plath nautical sextants  and the excellent photographs of the instruments allows one to follow the evolution of the micrometer sextant. Following this link will take you to their sextant collection:!csearch;authority=subject-90227;collectionReference=subject-90227;makerFacetLetter=p;makerReference=agent-17323;start=0  ; and I will give the reference numbers to allow more rapid retrieval. Most of the photographs in this post will stand enlarging to 200 percent by right clicking on the figures. Use the back arrow to return.

Carl Christian Plath took over the business of David Filby in 1862. Philby was an instrument maker, but just as with clock and chronometer makers, he did not actually make the sextants he sold. However, Plath bought a dividing engine from Repsold in about 1865 and his own  first sextants probably date from  shortly after that date. As with most early instruments, few of them survive. It seems that they were regarded simply as tools of the trade by their users and most often unsentimentally discarded when their owners retired. No doubt, some were lost at sea and others found their way to attics and basements, only to be thrown out  as unidentifiable junk after the owner’s death. The earliest Plath sextant that I have dates from about 1909.

Figure 1: Front of Dreikreis vernier sextant

The main interest of the Dreikreis (three circle) sextant, apart from its relative rarity, is in the ribbed three circle frame, also adopted by Heath and Co and Hughes and Son, the two leading makers in Britain. That shown in Figure 1 is a vernier instrument. The sliding block arrangement (Figure 2) is rather more complex than usual and is covered in more detail in my post for 24 January 2010. Around about 1907, Plath began to advertise a micrometer sextant whose mechanism  scarcely changed for the remainder of the time that sextants were manufactured. A three circle micrometer sextant by Plath is illustrated in the NMM on line collection, reference NAV1130, and the photographs have sufficient detail to allow one to compare the micrometer mechanism with later ones.

Figure 2: Tangent screw mechanism of Dreikreis vernier sextant

Plath were still producing vernier instruments in 1920, the date of the sextants shown in Figure 3, but by then was using a fine adjustment system that was on its way to being a micrometer, in that the “endless tangent screw” operated on a fine rack cut into the back of the limb, as shown in Figure 4. More details will be found in my post of 4 December 2011. This layout was patented by Heath and Co in 1910 and required only the addition of a micrometer drum and the adoption of a suitable pitch for the rack, which was what Heath and Co did, but Plath had already placed the rack of their micrometer sextant on the edge of the limb and this was the pattern adopted by most other makers.

Figure 3: Plath vernier sextant with “endless tangent screw”.

Figure 4: Rack of sextant shown in Fig 3

A micrometer sextant with a ladder pattern frame of smaller radius dating from 1917 is in the NMM collection, reference number NAV1250 and this design of sextant remained standard until about 1942 when, on a war footing, an instrument that was easier to produce and used less strategic metals was standardised. However, as the magazine cover in Figure 5 shows, many earlier instruments were still in use on the Germany Navy or Kriegsmarine by December 1943. Note that the aperture of the telescope has also increased from 28 to 40 mm, more than doubling the light grasp, with a corresponding increase in the size of the mirrors. This increase in light grasp is important in conditions where contrast of the horizon is poor. When Plath resumed sextant production in the early 1950s it was this pattern that they used, with a bronze frame, rather than the technologically superior wartime sextant which behaved equally as well. Figure 6 gives a closer look at a 1953 sextant of the  type in use in Figure 5. The colour of the drum is different and is divided to whole mnutes rather than half-minutes, but otherwise the two sextants are the same

Fig 5: “Die Kriegsmarine” magazine, December 1943.

Figure 6: Standard ladder pattern sextant.

Figure 7 shows the micrometer mechanism in close-up. The worm is conical so that the end of the shaft that bears the drum is further away from the frame, allowing the use of a larger drum than would have been the case had a cylindrical worm been used. End play in the worm shaft bearings is taken up by the axial preload spring that bears on the rear end of the shaft. A radial pre-load spring keeps the worm in engagement with the rack and a keeper on each end of the expanded lower end of the index arm prevents the mechanism from lifting away from the front of the limb. The worm in its bearings  is attached to a swing arm that rotates about a bearing. When the release catch is operated, the worm swings out of engagement with the rack in the plane of the sextant frame, allowing the index arm to be swung rapidly to any desired position. When the catch is released, the worm swings back into engagement with the rack under the influence of the radial pre-load spring, ready for fine adjustments to be made via the worm. As will be seen, this mechanism was later simplified, probably to reduce production costs, before a final burst of simplification in the Navistar Professional model.

Figure 7: Standard Plath micrometer mechanism

While the standard sextant had a bronze frame with bronze mirror brackets and weighed with its telescope a hefty 1.9 kg (4.19 lbs), the wartime standard sextant had an aluminium alloy triangulated frame (Figure 8) with alloy brackets weighing a mere 1.2 kg (2.65 lbs). Once the very expensive moulds for the frame and brackets had been made, many thousands of the parts could be turned out rapidly by the pressure die-casting process; and the serial numbers increased by about 4,500 between mid 1942 and the end of the war in mid 1945. While aluminium alloy frames were perceived to be inferior to bronze ones by mariners, in fact they are rather more rigid and stable and at least as corrosion resistant. Figure 9 shows the instrument in use during wartime as illustrated by the magazine Die Woche for 14th April 1943. The 0.7 kg reduction in weight makes using this model of sextant a pleasure and it has not been my experience that a lighter sextant is more difficult to use when there is a strong wind.

Figure 8: WWII Kriegsmarine standard sextant

Figure 9: Sextant in use during WWII (1943).

As noted above, when Plath resumed sextant manufacture in the early 1950s, they produced their standard pattern bronze-framed instrument, but by 1975, while retaining the ladder pattern, it was slightly better designed with sharp corners eliminated. I give a general view of the front of this “Navistar” sextant in Figure 10 of this post. A battery handle with scale illumination via light guides had been added and the handle was canted at a  more ergonomic angle. The mirror brackets and shade frames were now of aluminium alloy and the classical tapered index arm bearing was replaced by a plain parallel one of white metal running directly in the frame. Its weight with batteries remained about the same at 1.9 kg. The telescope in older instruments can be dismantled for cleaning or drying out, but in the newer instruments, the objective lens was glued into place in the telescope body and the eye lens was glued into the plastic eyepiece, making any servicing rather problematic. Figure 11 of my post of  18 December 2010  shows how problematic it can be.

Figure 10: Front view of early Plath Navistar sextant

The shades were retained by a non-standard nut requiring a special tool (Figure 11 of this post) to tighten it if the shade friction was insufficient. Such a tool cannot be bought, so any one wishing to dis-assemble the shades for cleaning and greasing had to make one of their own. There was no question of simply sending the shades assembly to the makers for adjustment, as the bracket was now part of the frame (Figure 12), rather than being attached with screws as previously, so the whole instrument had to be sent.. It appeared as though a sextant too expensive to throw away had been made for a throw-away age.

Figure 11: Non-standard pin wrench to adjust shades friction.

Figure 12: Rear view of sextant in Figure 10

Figure 13 gives details of the micrometer, from which it can be seen that its manufacture had been considereably simplified without, one hopes, sacrificing quality. While the release catch itself was an alloy die casting, the fixed part of the release catch was an extension of the plastic of the light guide and was easily broken off.

Figure 13: Plath Navistar micrometer construction

These instruments were no longer supplied in mahogany cases like pre-war sextants, but in heavy, moulded, black bakelite cases with various retention systems that did not always survive heavy handling, though the cases themselves are almost indestructable (Figures 14 and 15).

Figure 14 : Case of Navistar sextant

Figure 15: Retaining latch and pocket of Navistar sextant

In 1977, Plath introduced their Navistar professional sextant, just in time for it to be made obsolete withn a few years by the advent of the Global Positioning System. The frame was a hefty triangle of aluminium alloy 14 mm (0.55 in) thick. While this undoubtedly made it cheaper to manufacture, it had the unfortunate corollary that there was nothing by means of which one could pick it up, except for the handle, so it was provided with a light moulded plastic case in which it sat handle uppermost. This led to a further difficulty: because it was not provided with legs, this is the only place that it can be put down and then only with the index set at – 6 degrees. Figure 16 shows a front  view of the instrument and Figure 17 shows it in its case. What appear to be stubby legs on the rear of the limb serve only to act as stops for the index arm.

Figure 16: Front view of Navistar Professional sextant.

Figure 17: Navistar Professional in its case, rear view.

Extensive use was made of high-impact moulded plastic, in the handle, the telescope body, the shades mountings. the lower end of the index arm and release catch and the micrometer drum.  The latter was a mere 15 mm (0.6 ins) in diameter with figures of a size to tax aging long-sighted eyes (Figure 18). The same slightly inadequate drum was used in the Plath Navistar “Traditional” in which the frame took the form shown in Figure 10. The shades, two each for the horizon and index mirrors, were mounted so that they rotated around their mirrors (Figure 19). They are easy to use, but if the sextant is placed carelessly on a flat surface face downwards it is the shades that will suffer. The two mirrors were identical so only one size was needed for replacement, but because the horizon is viewed directly rather than through glass as in a traditional full glass horizon mirror, overlap of images is rather limited. The mirrors were first surface with overcoating.

Figure 18: Micrometer drum and arc.

Figure 19: Horizon and index shades.

The index arm journal took the form of a bronze bush surrounding a boss extending from the bottom of the index mirror bracket and the bronze-coated journal ran directly in the frame, being secured by means of a C-ring and a nylon washer (Figure 20).

Figure 20: Index arm bearing.

The micrometer mechanism was even simpler than that shown in Figure 12. There was no longer a swing arm. The worm shaft bearing rotated about a boss formed on its base and an arm extended down wards. A spring at the end of the arm abutted against one wall of the plastic enclosure for the mechanism while a U-shaped wire link attached the arm to  the release catch, which formed another wall of the enclosure made springy by slotting the base of the wall. Axial preload for the worm was provided by a spring clip (Figure  21).

Figure 21: Micrometer mechanism of Navistar professional.

The scales were illuminated by a system of light guides within the black plastic covering to the lower end of the index arm. The source of the light was simplicity itself, being simply a wire bulb-holder that also embraced the battery. An orange cap when depressed pushed the positive end of the battery against the central contact of the bulb and thus compeleted the circuit (Figure 22). A ridge on the cap latched against an internal cut-out to prevent the whole falling out, but  slots cut in the cap to allow the ridge to spring into the cut-out had sharp corners which in my specimen have already led to a crack forming, and I anticipate having to machine a brass  replacement some time in the future. Meanwhile, I have drilled a hole at the end of the crack to reduce stress concentration.

Figure 22: Lighting system.

The 4 x 40 Galilean telescope and its integral bracket were of high-impact plastic and there was no provision for servicing the internal surfaces of the lenses as both are cemented into place. An interesting telescope could be supplied which has a front filter that allows normal viewing of extended sources but which acts as an astigmatiser for point sources (Figure 23).  Almost needless to say, this filter cannot be removed for servicing of the lens beneath.

Figure 23: Astigmatising telescope.

The sextant is quite easy to use and is no doubt very rigid for its weight of 1600 G (3.5 lb), but the placing of the handle at 120 degrees means that when it is turned to view the micrometer drum it is out of balance. The telescope has a fairly limited field of view and the micrometer is difficult to read. Instrumental accuracy was given as better than 20 seconds. I doubt that this sextant found much favour with traditonalists, but by the time my specimen was sold (1988) sextants were already falling into disuse.

If you enjoy reading about navigational instruments and technology of the sea, you will probably enjoy reading the book which gave rise to this web site, The Nautical Sextant as well as my more recent The Mariner’s Chronometer, both of which are available via the amazon web sites  in North America and Europe. The Nautical Sextant is also available from the joint publishers, Paracay and Celestaire.

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