Replacing the Pellicle of an MA1 sextant

8 10 2016

This post is preceded by  “A Countinho-Pattern Bubble sextant’; “How to Refill C Plath Bubble Artificial Horizon”; “The SOLD KM2 Bubble Sextant”; “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.”


Figure 1: MA 1 light path diagram.

Figure 1 shows the light path diagram for the MA 1 aeronautical sextant click on it to enlarge). Its near relative, the MA 2 uses a spirit level as a horizontal reference, but the MA 1 uses a mirror whose surface is maintained horizontal by a hanging weight damped in fluid. Light from a bulb passes through a condenser lens system, through an orange-coloured reticle, through a thin transparent membrane or pellicle, through an objective lens and then is reflected off the mirror. On its return path it is reflected off the under surface of the pellicle into the eyepiece.

Rays from the observed object pass via the index prism and an objective lens system, are deflected through 90 degrees by a pentaprism and then pass via another reticle into the eye piece in such a way that the black lines of the object reticle, the orange lines of the mirror reticle and the observed object may be in view together. The object is kept in coincidence with the orange reticle, as far as possible in the centre of the field of view, as indicated by the object reticle.

The pellicle was probably made from flexible collodion, a substance based on nitro-cellulose that forms an exceedingly thin film, but which suffers from the disadvantage of being very delicate and prone to distortion and wrinkling if it becomes damp. Forty-odd years after manufacture, the dry nitrogen with which the instrument was filled has often been replaced by air and the dessicator has become saturated with water, so that the image of the orange reticle becomes distorted by wrinkles. While very thin transparent plastic films are now available in the form of food wrap, it seems to me to be much simpler to replace the pellicle by a microscope cover slip. Its thickness is about 0.14 mm and so light is reflected off both surfaces of the glass to give a double image of the horizontal reticle line. This is not necessarily a disadvantage, as it is easier to place an image of a star between lines than it is to superimpose the image upon a single line, though motion of the aircraft is likely to make this a moot point.

There is a method using food wrap and I include it as an appendix.

The first step is to remove the dessicator by withdrawing two countersunk screws circled in white in Figure 2. This makes it slightly easier to remove the left side of the instrument together with the shades mechanism and also allows the dessicant to be refreshed if the granules are pink or white. The end of the dessicator unscrews and the granules can then be tipped out onto a shallow tray and baked in an oven at 120  Celsius for 20 minutes or until they have regained a blue colour.


Figure 2: Remove dessicator.

Then the four corner screws are removed as shown in Figure 3. Remove the screws completely before attempting to lift off the side cover. Then lift the rear of the cover a few millimetres and slide the cover backwards for a few millimetres, to avoid fouling the shades mechanism on the frame.


Figure 3: Remove left cover.

This exposes the pellicle on its frame. Removal of two screws allows it to be lifted out for inspection, holding it by its edges (Figure 4).


Figure 4: Remove pellicle frame.

If the pellicle appears to be flat and undamaged, dust particles may be removed by gently blowing dry air on to it or by very light brushing  with a soft camel hair brush. If you are ham-handed it is better to leave dust in place rather than risk damage to an intact pellicle. If, however, the pellicle is broken or wrinkled, remove its remains with a finger nail and then clean the front machined surface with ether solvent. It is then a simple matter to place three tiny drops of super glue on the frame and, using tweezers, to glue a microscope cover slip in place (Figure 5). A nicely worded request for a small handful of cover slips is unlikely to be refused at your local medical laboratory. They are perfectly clean as they come from the maker, so try to keep them that way.


Figure 5: Replace pellicle with cover slip.

Figure 6 shows the eyepiece view after replacing the pellicle with a cover slip.


Figure 6: View through eyepiece, using cover slip as semi-reflective mirror.


Using food film as a pellicle replacement is slightly more difficult:

1)  Obtain a circular embroidery frame such as may be had for a very few dollars and stretch a piece of food film (Clingwrap, Gladwrap etc) over it, adjusting the frame until the film is perfectly flat and wrinkle free. Make sure that it is perfectly clean and free from finger marks.

2) After cleaning old pellicle off the frame, place a smear of two-part epoxy adhesive (e.g. Araldite) around the frame, just outside the flat machined area, avoiding placing adhesive on the flat surface itself.

3) Set the frame down on the centre of the film, adding a weight of a few hundred grams to hold it in place until the adhesive dries. Then trim off excess film with sharp nail scissors or the like.


29 12 2013

This alphabetical list of posts may help you to find what you want. When you have found a post of interest, enter the part of interest as a search term in the search box.

A10 vapour pressure bubble chambers, Sealing

Admiralty pattern vernier sextant

Admiralty pattern micrometer sextant

AN 5851-1 bubble sextant averager., Gummed-up

AN 5851-1. Jammed shades carrousel

Battered Observator sextant, A

 Battery Handle Structure, C Plath

 Box Sextant, A

Broken legs

 Bubble Horizon Attachment, C Plath

Bubble illumination of Mk V and AN 5851 bubble sextants

Bubble Sextant Restoration Manual, A12

 Bubble Sextant Restoration Manuals, A10 and Mark IX series

bubble sextants, Aircraft

Bubble sextant, Hughes Marine 

 Bubble Horizon, A Nautical Sextant

 Byrd Aircraft Sextant, Update on

Byrd Sextant Restored, A

Carl Plath’s Earliest Sextant

C Plath Sextant Lives Again

 C. Plath Drei Kreis sextant, Restoring a

 C. Plath Vernier Sextant, A Fine

C Plath Yachting Sextant

C18 sextant named J Watkins

C19 sextant restoration

C Plath Sun Compass

Carl Plath micrometer sextant

 Carl Plath Sextants, Eighty Years of

circular sextant mirrors, Making

Compass, a C Plath sun

Compass, an improvised sun

Damaged Rising Piece, A

 Dip Meter, A Russian Naval

 Dip Meter, An Improvised

Distance Meter, A Stuart

Distance Meter, Fleuriais’ Marine

Drowned Husun Three Circle Sextant, A

Ebony quadrant, restoring a

Errors, Backlash and Micrometer

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

 Filotecnica Salmoiraghi of Milan, A Fine Sextant by

Freiberger Drum Sextant lighting unit

Freiberger Drum Sextant (Trommelsextant)

Freiberger Skalen Sextant

Freiberger Yacht sextant

A French Hydrographic Sextant

Half-size Sextant by Hughes and Son, A

Half-size Sextant by Lebvre-Poulin, A

Heath and Company’s best vernier sextant

Heath Curve-bar sextant compared with Plath

Heath Vernier Sextant Restored

Hughes Marine Bubble Sextant

Hungarian sextant via Bulgaria, An

Hydrographical sextant, a French

Ilon Industries Mark III sextant

Jesse Ramsden and his Dividing Engine

 Keystone Sextant Case, Making a

 LEDs 1: miniature screw bases., Adapting to

LEDs 2: Plath bubble horizon unit, Adapting to

Lefebvre-Poulin, a Half sized sextant by

left-handed sextant, Unusual

Legs, broken

Mark V / AN5851 sextant bubble chambers, Refilling

 Markk V/ AN5851 sextant bubble chamber, Overhaul of

mirrors, How flat are sextant ?

 monocular mounting, Making a sextant

Observator Classic sextant, restoring a

Observator Mark 4 sextant

 Prismatic Monocular, Making a

 Quadrant Restored, An Early C19 Ebony

Quadrant Restored, an old wooden          June 2018

 Scale Lighting Systems, Later Tamaya

Sextant ‘scopes for myopes

Sextant Mirrors, New  for Old

Sextant Calibrator, A

Sextant Frame, Evolution of the

sextant shades, Polarising

Sextant, 210 years on,

Shackman sextant and a link to Jesse Ramsden

 Shades-adjusting Tool, Making a

Simex Sextant(s

Skalensextant, Inside the

SNO-T Mirror Bracket Repair

 SOLD KM2 Bubble Sextant

Sounding Sextants 1

Sounding sextants 2

Sounding sextants 3

Spanish Vernier Sextant, A Late

Spencer, Browning and Co sextant

Sun compass, a C Plath

Sun compass, an improvised

switch overhaul, Tamaya

Tamaya Collimation Blunder

The Case of the Broken Screw

Troughton and Simms Surveying Sextant

Turn-of-the-century French Sextant

US Maritime Commission Sextant, A

USN BuShips Mark II sextant: some design oddities

USSR SNO-M sextant, The

USSR SNO-T sextant, The

 Vernier Sextant, British Admiralty

Watkins, J, A C18 sextant named

Which lubricant?

Worm with wrong thread angle?

Worm Turns, A

Simex Sextant(s)

16 06 2012

In about 1963, Captain Svend Simonsen, a merchant seaman officer who had retired from the sea, is said to have tired of selling shoes instead, so he set up a navigation school, at first teaching students in their own homes and then by correspondence. By 1971, it was a thriving business, trading in Santa Barbara, California, as the Coast Navigation School. Captain Simonsen arranged with Tamaya of Tokyo to provide him with sextants named “Simex”, for him to on-sell to his students and others. There were, it seems, several models for sale, with various telescope, shade, mirror and frame options, reflecting the options that Tamaya offered under its own name, hence the plural in the title of this post. Recently there came into my hands a sextant named “Simex Mariner”. It had plainly been dropped, as a leg and the telescope stud had been broken off, but it had been a high end sextant, as it has a bronze frame, polarising shades, scale illumination and a micrometer vernier reading to 0.1 minute. Figures 1 and 2 show it after overhauling and  checking, and replacing broken parts.

Figure 1 : Front (LHS) of Simex Mariner sextant

Figure 2 : Rear (RHS) of Simex Mariner sextant.

The bronze frame is of a standard 162 mm radius with a black crackle finish. The index mirror measures 32 x 48 mm and the round horizon mirror is 50 mm diameter. A  contemporary German sextant has mirrors of 41 x 56 mm and 56 mm diameter respectively. The design of the latter’s micrometer mechanism is almost identical to that of the Simex, following the design principles devised by C Plath. While most post-1950 sextants either omit  a micrometer vernier altogether or divide it to 0.2 minutes, the Simex vernier is divided to 0.1 minutes (Figure 3). Given the uncertainties of the dip of the horizon and that the absolute limit of the sextant’s measurement accuracy may be no better than 12 seconds (0.2 minutes), the vernier is probably an unecessary extra. Possibly it was added to attract traditionalists.

The scale lighting system follows Tamaya’s earliest practice and does its job perfectly adequately. It is easy to change the bulb and batteries. The latter lie diagonally across the width of the shaped hardwood handle and the handle itself is canted at 20 degrees to the vertical to give a slightly more comfortable grip. The switch mechanism is simple and easy to access by removing two woodscrews, unlike in many later battery handles, particularly those by C Plath, where is is nearly impossible to remove the switch when it is in need of servicing.

Figure 3 : Detail of micrometer and illumination.

The shades use crossed polarising filters to give continuously variable darkening of the view. The idea was first mooted in a patent of about 1938 and some versions of the US Navy Mark II sextant were fitted with polarising shades, but the material seemed to fade after a few years. In my Simex, the index shade worked perfectly well, but one of the filtersof the horizon shade had lost all its polarising properties. Access is easy, by unscrewing the retaining ring shown below in Figure 4. The material is sandwiched between two sheets of plain glass, and by soaking the filter in acetone overnight it is usually possible to slide the two apart and clean off the remnants of the polaroid material. As mine had faded uniformly, I simply stuck a layer of self-adhesive polaroid film to the outside of the glass, a solution that has worked well.

My Simex also has an astigmatiser added to the index shade. This is simply a weak cylindrical lens whose axis is horizontal when swung into place. Its purpose is to draw out images of stars or the sun into horizontal lines and it probably finds its main use in combination with a bubble horizon though, at least with stars, it may be helpful to remove tilt of the sextant by lining up the extended image of the star with the horizon.

Figure 4 : Astigmatiser and polarising shade.

There is quite a variety of ways of attaching shades to their mounting brackets and the method may appear mysterious to someone seeking to overhaul or tighten a shade that has become annoyingly loose so that it does not stay where it is put. Figure 5 shows the horizon shade in its mounting and Figure 6 shows it exploded. At a casual glance, the two grub screws may be overlooked and the heads of the mounting pin or adjusting screw mutilated in futile attempts to turn them. Each grub screw must first be backed out, using a well-fitting 1.5 mm screwdriver to do so, as if the slot is destroyed, the overhauler will be faced with an even greater problem.

Figure 5 : Horizon shade and bracket.


Figure 6 : Horizon shade and bracket, exploded view.

Mirror mountings, index arm structure and index arm bearing are all conventional.


C Plath Sextant Lives Again

20 03 2011

The preceding posts cover : C Plath Micrometer Sextant; A Damaged Rising Piece”, “SNO-T Mirror Bracket Repair”,  “A Worm Turns”, “The case of the broken screw”, and “Worm with wrong thread angle?

Following the end of the Second World War, in November 1948 the venerable firm of Carl Plath was dismantled and its machinery distributed as war reparations, but by the autumn of 1950 it was able again to exhibit sextants at the Paris Shipping Salon. By the time Theodor Plath celebrated his 85th birthday in November 1953, the firm of C Plath was making between 1200 and 1500 sextants a year. About a month ago I received a C Plath sextant made in that year and have been spending some of my leisure time in restoring it.

Although the seller described it as being “in good condition”, this was far from the case, as there was widespread corrosion of screw heads, the mirrors had decayed, the index arm was jammed solid by verdigris and the release catch could not be operated, as the micrometer swing arm was seized solid. To add to the sextant’s woes, it had broken away from its moorings in its black bakelite case and bent the micrometer shaft as well as breaking off part of the plastic micrometer thimble. When buying second hand sextants, I routinely urge the sender to use plenty of packing inside the case to guard against such accidents, but on this occasion, my request had fallen upon deaf ears. As will be seen later, there was a hidden problem that became apparent only when I calibrated the instrument after restoring it.

Plath’s 1953 instrument showed little difference from instruments 20 years older. The bronze ladder frame of 162 mm radius had a conventionally placed rack and the design of the micrometer mechanism had not changed since it was first invented by the firm in about 1907. Though other makers made variations on the theme, the design was very sound and was copied, slavishly by Tamaya, and with minor modifications in attempts to have points of difference, by other makers.

In common with Tamaya, C Plath early realised the importance of light grasp in the optics and had large mirrors with telescope apertures to suit. My instrument came with a 6 x 30 prismatic monocular which gives an erect, bright image with a large field of view. Submariners of the US Navy in the Pacific Theatre in WW II had noted that by using such a monocular with their sextants, it was often possible to take accurate star sights in darkness, provided the observer’s eyes were fully dark adapted. Presumably, the experience of U-boat navigators was much the same and noted by C Plath, or there may have been liaison with Tamaya during the war. The latter firm supplied some instruments with a 7 x 50 monocular

My instrument was supplied with an astigmatiser in place of one of the index shades (Figure 1). This is a cylindrical lens that draws out the image of a star into a line. The axis of the cylinder is arranged so that the line is horizontal when the sextant is held with the frame vertical. According to Dutton, this can be of use when observing bright stars or planets with a dim horizon, though it probably comes into its own mainly when used with a bubble horizon.

Figure 1 : Astigmatising shade

As usual, my first task was to remove all the main fittings from the frame: mirrors, shades, telescope mounting bracket and handle. The structure of these fittings was conventional, with the exception of the handle, which was fixed rigidly to the frame at the top but at the bottom made contact with it only via a rubber bush and spring washer (Figure 1). I assume someone had the idea of mounting the handle kinematically in this way to avoid redundancy of support and the introduction of additional strains to the frame if the handle should expand at different rates to the frame. It was not copied by others.

Figure 2 : Kinematic handle mounting

With the parts that stick out removed, I could then swing the index arm out of engagement with the rack, and remove the index arm with micrometer mechanism and bearing journal. Strictly speaking, the shaft enclosed by a bearing is the journal, while the enclosure is the bearing, but the whole assembly is often referred to as the bearing. At this point, the journal parted company with the thick brass disc on which the index mirror sits. It had been silver soldered in place during manufacture, but corrosion had made its way into the joint and the battering the instrument had received in transit was probably the last straw.

Before proceeding with the micrometer mechanism I thought it best to fix this problem. To a large extent, interference fits and hard soldering have been replaced in industry by the use of anaerobic industrial adhesives. These are usually based on cyanoacrylates with an inhibitor that prevents polymerisation by water vapour in the presence of oxygen. In the absence of oxygen, the adhesive sets hard and strong. While it was possible to remove all the verdigris from inside the disc and from the journal, the fit was a little loose, so I took advantage of the gap-filling properties of Loctite 680 high strength retaining compound. The disc was attached to the index arm with three screws. Once these had been removed, a little wangling separated the parts. While it is not absolutely essential that the journal should be square to the disc, it does make life easier, so I used a lathe set up as a makeshift jig to maintain the parts square to each other while the adhesive cured (Figure 3). The disc is held squarely in the chuck, while a tail centre is brought up to the centre hole in the end of the journal. As an aside, Loctite seems to cure exceptionally quickly when in contact with brass, sometimes within seconds, so it is useful to have a dry run to check that assembly can be finished without the adhesive curing  while the parts are still in a partially assembled condition.

Figure 3 : Journal repair

As mentioned above, the design of Plath’s micrometer mechanism remained unchanged for many years, but as the twentieth century drew to a close, manufacture moved in the direction of greater simplicity (see previous post in this section). Figure 4 shows the general arrangement, together with the internal structure of the front bearing, which has to accomodate axial as well as radial loads.

Figure 4 : Micrometer mechanism

The worm shaft rotates within two bearings carried on a swing arm chasis which itself swings about a bearing, so that the worm can be swung out of engagement with the rack against the pressure of a leaf spring, which normally holds the worm firmly engaged with the rack. A collar on the worm shaft is held against a thrust face in the front bearing by an axial pre-load spring that presses against a ball bearing let into the rear end of the shaft. These two springs take up all clearances (apart from a thin film of oil or grease) so that there is no lost motion when the worm engages the rack or rotates.

The front bearing with its thrust face is made in two parts that are held together with four screws and located by two dowel pins visible at top left and bottom right of the lower half-bearing. The hole down the centre of the bearing would have been bored with the two parts assembled together and then split for assembly. Machining the recess for the thrust collar I would rate as rather a difficult boring operation, and this probably accounts for its later abandonment as labour and other costs rose.

After dis-assembling the mechanism down to the last screw and cleaning off all the verdigris, dried oil, grease and dirt, and putting it together again, it quickly became apparent that all was not well. Rotation of the micrometer shaft was very stiff and the resistance varied. The damaged micrometer drum wobbled as it rotated and it was very clear that the micrometer shaft was bent. Figure 5, in which the rear end of the shaft is held in an accurate collet in a lathe, shows that there was a “run out” or eccentricity of the shaft just beyond the front bearing of 0.2 mm, while none was apparent in the rear half of the front bearing.

Figure 5 : Runout of bent micrometer shaft.

While engineers routinely straighten bent shafts using large hydraulic presses, straightening is not really an easy option for parts of small precision mechanisms, though one might attempt it in desperation. If one is well-equipped, the simplest option is to renew the whole worm shaft. For this a lathe with a taper turning attachment  is needed together with the ability to cut a thread of 1.4 mm pitch, a non-standard thread. I have touched on taper thread cutting in a previous post (A Worm Turns, 6 July 09). The other main challenge is to form the collar between two cylindrical bearing surfaces. The cutting tool, shaped like a broad parting tool, necessarily takes a comparitively broad cut on a slender shaft, with a risk of chattering that will leave a poor finish. Figure 6 shows the bearing surface being formed with the shaft held between centres and with solder wire wrapped around it to help reduce the tendency to chatter.

Figure 6 : Forming second half-bearing

The next task I tackled was to make a new micrometer drum and thimble. I started by straight knurling a length of 26 mm aluminium alloy bar and them Loctited (if such a verb exists) a larger collar on to it. Once the Loctite had cured, I turned the collar down to size and then, while everything was still nice and rigid, scribed sixty divisions. Figure 7 shows this in progress.

Figure 7 : Dividing the drum

 Normally, dividing would be carried out on a separate dividing head or the divisions would be rolled into the surface, but I often use a home-made attachment on the headstock of the lathe itself as it ensures concentricity (Figure 8). A worm engages with a  worm wheel having 360 teeth on the back end of the lathe spindle, so six turns of the worm advances the drum through one division. It is necessary not to lose concentration during this dividing process as the discovery that you have lost count somewhere is usually delayed until you have cut the last division and find that it is too small or too large.

Figure 8 : Headstock dividing.

The next task also needs concentration if exclamations such as “How very unfortunate!” are to be avoided when a wrong or upside down number is punched. Number punches are best guided by some sort of jig if an amateurish result with uneven alignments is to be avoided. Figure 9 shows a primitive possibility which consists of a square bar with a hole in it and a 6 mm screw whose end provides a flat surface within the hole to prevent the punch from rotating. The dividing attachment and the top slide of the lathe are used to position the numerals. Some numerals need harder blows than others and this requires a little practice on a piece of scrap material to get things right.

Figure 9 : Punching numbers.

With divisions and numbers safely out of the way, the part can be turned to its final shape, the central hole drilled and reamed and the combined drum and thimble parted off. The original drum was white with black markings, but since taking the photographs for Figure 10, I have been persuaded by my wife  to prefer white markings on a black ground. The ground is sprayed on first and allowed to cure thoroughly. The divisions and numerals are them carefully scraped free of paint and filled in by painting over with the contrasting colour and wiping off  with a single layer of thin rag stretched over a finger tip. Thicker or loose rag tends to wipe the paint from the bottom of the divisions or numbers. Figure 10 shows the result.

Figure 10 : New drum.

I have covered making new rectangular mirrors in a previous post (New Sextant Mirrors for Old, 11 February 09). Cutting cirular mirrors for the horizon mirror will be the subject of a future blog when I have fully developed the method.

Overhauling the monocular revealed an interesting detail, presumably based on experience of  keeping the instrument waterproof in the very adverse conditions found on  U-boats during WW II. Figure 11 shows the construction of the objective lens mounting and the front plate of the monocular. Engineers will recognise this as a form of labyrinth seal in which contaminants (in this case sea water) have to follow a circuitous route, meeting mechanical barriers and thick layers of grease on the way.

Figure 11 : Labyrinth seal of monocular objective.

Spray painting the frame and other individual parts completed the restoration. I use CRC Black Zinc, as it is tough, relatively quick curing and has a semi-matte finish very much like the original. I have covered ways of masking shades and other parts in the Sextant Restorations category. I always have to restrain my impatience and allow at least 24 hours for the paint to harden up enough to allow reassembly, but the paint takes a few more days to reach full hardness.

Normally, reassembly and tidying up the case would complete a restoration, but I recently completed a sextant calibrator that allows me to calibrate a sextant in about half an hour (Chasing Tenths of an Arcminute), so I checked out my new-looking sextant. The results shown in Table 1 were not compatible with C Plath’s high reputation and it was very unlikely that the sextant had left their factory with such large errors, exceeding a minute in two instances.

Table 1 : Sextant errors, first run.

Errors like this, increasing rapidly as the sextant reading increases, suggested that the axis of the index arm bearing was not at right angles to the arc and, by implication, the frame. A quick check on a surface plate with cigarette papers showed that the limb was slightly bowed, concave to the front, and the machined rear surface of the framewould not sit squarely on the plate without rocking slightly. The frame, it seemed, was bent but in which direction? I removed the index arm bearing to check that it was seated properly in the frame and it was. I normally advise against  doing this without very good reason, but I felt that I could scarcely make the instrument worse, so I went ahead and checked. Finally, I made a mandrel to fit the taper in the bearing and checked it with a square against the frame (Figure 12).

Figure 12 : Leaning mandrel.

While it was only possible to check where there was frame on which to sit the square, it appeared that the rear edge of the frame was bent, so I held the zero end of the limb in a vice and gave a hard pull backwards on the apex of the sextant. My first attempt was lucky, as Figure 13 shows. The limb now trapped cigarette paper throughout its length and the frame no longer rocked on the surface plate.

Figure 13 : Uprighted mandrel


Table 2 : Sextant errors, second run

  Recalibrating it gave the results shown in Table 2. While the errors above 90 degrees are perhaps rather large for this class of instrument, in practice only a Lunartic or a surveyor would complain about them, and for its era are perfectly acceptable. Certainly, it is “Free from error for practical use“, which is all that C Plath was ever prepared to say.

So, dropping a sextant with a bronze frame can bend it, as well as causing other, more obvious damage, but it need not be a death sentence, with the instrument condemned to hang on a living room wall or behind a bar with a nautical theme. With love and care and some surgery, it can regain its good looks and live a normal life again (Figure 14).

Figure 14 : Sextant no. 3****6 returns to a normal life.

If you have enjoyed reading this account, I am sure you will enjoy reading my book “The Nautical Sextant“, and your purchasing it will help me to ensure that more sextants are restored to a normal life.

German WW II Gyrosextant (See-Kreisel-Sextant)

23 01 2019

Figures 5, 6, 12, and 13 may be enlarged by clicking on them.

During the Battle of the Atlantic, which for Britain began on 2 September 1939 and ended on 8 May 1945, without a break, Britain was nearly brought to its knees by submarine warfare. The battle began to turn away from Germany’s favour in mid-April, 1943, when for the first time convoys could receive continuous air cover between Britain and North America. It soon became apparent from increasing submarine losses that German submarines could no longer safely remain surfaced during daytime. While the Schnorkel ventilating tube mitigated this problem to some extent, by early 1943, the protection given by darkness was removed by the Allied development of airborne centimetric radar and the Leigh Light which illuminated the sea for attack once an aircraft had been guided to the submarine by radar.

In spite of having a Schnorkel, a submarine still had to surface to make observations of the stars, moon and planets at night, but as the horizon is generally not visible at night, an instrument with an artificial horizon had to be developed. I have described the SOLD KM2 bubble sextant in my post of 5 November, 2013. Unlike a large aircraft, which has at least partly predictable oscillations in flight, anyone who has taken a bubble sextant to sea will know that the accelerations as revealed by the bubble vary wildly and unpredictably.  If the instrument is provided with a read-out that integrates observations over two or three minutes, as is the C Plath SOLD, it may be that results will be better than with instruments that simply average many observations at an interval that may coincide with the frequency of, say, rolling of the submarine. This potentially can lead to very large errors indeed.

When a bubble sextant is subject to an acceleration, all the fluid in the spirit level is affected. As you will see when I describe the gyro unit, when the Kreiselsextant is subject to an acceleration, the only connection between the sextant and the gyro is via its low friction, small area bearing.  Effectively, the gyro is almost detached from the sextant and retains the direction of its axis of spin in space.

Figures 1 and 2 show the left and right hand sides of the Kreisel sextant. Apart from the gyro unit and minor changes to the light path, it is almost identical to the SOLD sextant, so I will describe only the gyro unit and the consequent light path changes in what follows. Readers interested in the interior of the instrument may consult my post of 5 November 2013.

ga left side

Figure 1: Left hand side.

ga rhs

Figure 2: Right hand side

Figure 3 shows the gyro rotor sitting on it bearing in the bearing housing. The upper part is cross bored on a diameter, one end of which carries a collimating lens and the other a graticule at the focus of the lens, so that light rays emerging from the lens are parallel. When viewed, the image of the graticule thus appears to be at infinity. The lower part has 35 crescents, or buckets machined into its edge, so that when air is blown into them the rotor is caused to rotate. As the gyro rotates at hundreds of time a minute, the image projected into the sextant flickers only a little.

gyro in situ

Figure 3: Gyro rotor on its bearing.

Figure 4 shows the image obtained of the graticule when viewed through the collimating lens. The central pair of lines are intended to be used for star observation, while the outer pair are used for moon and sun observations. The out-of focus vertical dark line is of course the central bearing spindle.


Figure 4: Graticule seen through collimating lens.

Figure 5 is a cross sectional drawing of the rotor and its housing, copied from a British analysis of the sextant reported in August, 1945. I have added the light path, The rotor has a  hard steel spindle through its centre coming to a point of 0.13 mm radius.  Together with a concave artificial sapphire of 3.7 mm radius, it forms a low friction, self-centring  bearing for the rotor. The carrier for the sapphire is spring loaded within a lifting tube that can be raised to lock the rotor against the top of its housing.

gyro section labelled

Figure 5: Sectional drawing of gyro.

Figure 6 shows details of the gyro bearing and the lifting tube used to lock the rotor.

gyro bearing detail

Figure 6: Gyro bearing detail.

Figure 7 shows the window through which rays exit the rotor into the body of the sextant. Circled in white are two of the six tiny holes or nozzles through which air is projected into the buckets at high speed at about 40 degrees to tangential to make the rotor rotate at high speed. They are 1 mm in diameter.

gyro window to sext

Figure 7: Detail of interior of housing.

Air enters the nozzles from a gallery which is connected to an air inlet shown in Figure 8 below. The air leaves the gyro housing through 8 holes drilled in the circumference of the bearing housing. A flap covers a viewing window through which the motion of the gyro may be viewed to check when it has settled into a steady motion.

gyro extterior labelled

Figure 9: Exterior of gyro unit.

Figure 10 shows the lamp carrier for the gyro unit. It screws into the gyro unit. When I bought the sextant it came with three 3 volt bubs with the miniature bayonet base shown, all unfortunately defective. In this voltage they now seem to be unobtainable, but I was able to find some 6 volt versions, perhaps the last dozen on the planet, and had to make up a makeshift 6 volt battery pack that would fit into the sextant in order to try it out (Figure 11).

gyro lamp carrier closeup

Figure 10: Detail of gyro lamp carrier.

battery compartment

Figure 11: 6 volt battery pack in place

Figure 12 is of another drawing from the British 1945 report, showing the full light path. After being collimated at the rotor the image of the graticule is, I think, reverted  by a pair of lenses, one applied to the face of a 90 degree prism and the second beyond a fixed mirror. This second lens appears to be at the focus of a further collimating lens that brings the rays parallel again, to be viewed in a Galilean telescope. (I am not confident that I have correctly described the function of the lenses between the two collimating lenses. If any  reader can enlighten me further I should be glad to hear from them in the “Comments” section.)

lightpath 2

Figure 12: Light path through sextant.

Figure 13 shows the instrument in its case. At top left is a carrying handle used to carry the instrument up through narrow confines of the conning tower hatches. Although it would not normally be used in daylight with a natural horizon, it is nevertheless provided with a set of four shades so that daylight observations of the sun or moon could be carried out on the uncommon occasions when the bodies are visible but not the horizon, e.g. in ice. A light shade would be needed for night observations of the moon when near full.

1 case inside

Figure 12: Sextant in its case, with accessories.

Below the shades is a charging adapter and blue bulb used as a dropping resistance to allow the white nickel-iron-alkali battery to be recharged from a 110 volt direct current supply. Proceeding clockwise, there is a spare gyro bearing and a bank of four spare bulbs. The sextant is held in the case by a bracket that folds down from above, which is then locked in position  by a transverse bar. The sextant itself weighs just over 3 kg, but with its case it is a hefty 8.6 kg.


Figure:13  Instructions

The instructions, pasted to the inside of the lid, are of course in German, and there appears to have been at least two versions. It seems that the instrument in its case was to be set down on a table with the lid open and horizontal. An air supply probably from a foot pump, was then attached to the air inlet and pumped rapidly until the whirring of the rotor reached a high pitch, when the rotor was then allowed to settle down for three minutes, though this appears later to have been altered to five minutes. During this time, precession of the rotor settled down so that its vertical axis was aligned with local gravity, and the light path through it horizontal. The integrator was then wound up, the instrument eased carefully out of its case and the carrying handle clipped into place (Figure 14), all the time keeping the sextant upright and avoiding knocks or sudden movements.


Figure 14: handle in place.

Even removing it from its case needs great care, as it is a narrow fit and it is all too easy to catch it on some projection. The sextant is then passed from hand to hand up the conning tower, all the time avoiding sudden movements and knocks. On reaching the top, the handle is then unclipped. The spring is strong and again it is quite difficult to do this without upsetting the gyro. The main switch on top of the left handle was then switched on and the gyro lighting control in the left handle (Figure 1) adjusted and a view of the graticule obtained. The gyro lighting comes on only when the integrator is wound up and goes out when the integrator runs down, signalling the end of the observation period to the observer. The lighting of the integrator read out then comes on and its intensity can be adjusted using the scale lighting control (Figure 2). Pressing the button switch on the scale lighting cover (Figure 2) causes the integrator lighting to go out and the lighting to the remainder of the scales to come on.

The cone of visibility of the graticule is quite small and it needs a little practice simply to obtain a sight of  it on dry land. I imagine it would need much practice to see it and then make it coincide with a star on a submarine at sea, but we know from at least one voyage report that this was done successfully, though we do not know with what degree of accuracy. Hand held, on dry land, the mean error of 50 observations of the sun was 10.9 arc minutes, with a standard deviation of 8.86 minutes.

As spinning tops as toys have given way to electronic games at all ages, some readers might wonder how it is that the gyro comes to define the vertical with its axis and hence provide an artificial horizontal via the graticule and collimating lens. Another way of putting the question might be: why does a spinning top stand up, but this is somewhat complicated because at rest in this gyro, the rotor is stable, as its centre of mass is 3.5 mm below the bearing.

The law of conservation of angular momentum decrees that undisturbed, a rotating body will continue to maintain the direction of its axis of rotation unless a force acts upon it. The only available forces are gravity and friction in the bearing and, as I have noted above, the end of the spindle is spherical, with a radius of 0.13 mm, so that when leaning, the centre of mass does not coincide with the centre of the spindle. This with gravity creates a couple, which leads to the axis describing a cone, or precessing until the centre of mass is coincident with the centre of bearing, in which position it is upright.

Readers of a mathematical bent (which I am not), will find a more lengthy and satisfying explanation in most university level textbooks of physics and mechanics. In 1890, J Perry published an entertaining little account of a popular lecture he had delivered, “Spinning Tops,” in which the words, “vector,” “angular velocity” and “torque” do not appear. General readers may find in this a more accessible account.




Ilon Industries Mark III Sextant

1 10 2014

John Pazereskism kindly sent me an e-mail, but my replies to him have bounced. If you’re reading this, John, please send me an e-mail from another address.

Of the many small-size sextants, possibly the rarest are those by Ilon Industries Inc of Port Washington, N.Y.  Victor Carbonara, a prolific inventor of navigational instruments prior to  and during the Second World War and one-time president of Kollsman Instruments, manufacturer of altimeters and bubble sextants, may have had something to do with its design. However, I have not been able to discover any details about Ilons. I was very pleased, then, when an Australian friend, one of the many friends I have yet to meet, entrusted me with his Ilon Mark III sextant and invited me to describe it, deconstructing it in the process if need be. The sextant is in a stout leather case with a metal zip fastener (Figure 1)

Figure 1: Case

Figure 1: Case

The interior of the case is lined with red velvet, with compartments for the sextant body, an adjusting key, the handle, a sighting tube and a tiny prismatic telescope, as shown in Figure 2. A clearer view of the individual parts is given in Figure 3, which shows the parts outside the case.

Figure 2; Kit of parts in the case.

Figure 2; Kit of parts in the case.

Figure 3: Parts out of case.

Figure 3: Parts out of case.

Figure 4 names some of the parts of the sextant for those who are not very familiar with sextant structure. The telescope is six power with an aperture of 15 mm.

Figure 4: Some of the main parts.

Figure 4: Some of the main parts.

While it is quite usual for sextants to be stored without their telescope in place, it is very unusual for the handle to be a separate part.  The sextant has a front plate which carries the telescope, the index arm with its attached micrometer mechanism, a rack with which the micrometer worm engages, the arc and the two mirrors. The front plate  is attached via two pillars and a plate through which the telescope passes, to a back plate to which are attached the shades and the handle. Figure 5 gives some clues as to how the handle is attached. The upper leg of the handle has a cross pin through it and this is inserted into a slotted hole and rotated through about 30 degrees, at the same time engaging a reduced diameter of the lower leg in a plain hole in the back plate. On tightening the knurled locking nut, the handle is located and  held with sufficient firmness to the back plate.

Figure 5: Sockets for handle.

Figure 5: Sockets for handle.

Figure 6 shows the handle locked into place and also shows the means of attaching the telescope or “zero magnification” sighting tube. The ‘scope or tube screws into a dovetail slide which engages with dovetails machined in the plate that holds the rear of the the two plates together. The ‘scope can be moved transversely to admit more or less light from the horizon, depending on conditions, and then locked into place by means of a locking nut bearing on the upper gib strip. This takes the place of the conventional “rising piece”. While the human eye can just about detect a doubling in light intensity, quite small changes in intensity can improve contrast between the sky and the horizon at twilight significantly.

Figure 6: Handle locked into place.

Figure 6: Handle locked into place.

Figure 7 shows how the index and horizon mirrors are tucked away safely between the plates. It also shows how up to three index shades and two horizon shades can be rotated on brackets attached to the inside of the back plate to reduce light intensity from the observed body and the horizon respectively.

Figure 7: Shades and mirrors.

Figure 7: Shades and mirrors.

The radius of the arc is a mere 60 mm (about 2.4 in.) It is traversed by an index arm outside the front plate and which rotates about a short plain bearing, the other side of which is a plate carrying the index mirror bracket. A magnifier built into the lower end of the index arm helps in reading the whole number of degrees (Figure 8).

Figure 9: Micrometer index.

Figure 8: Micrometer index.

A rack is machined into the rear of the front plate and is engaged with the worm of the micrometer mechanism (Figure 9). An 18 mm diameter micrometer drum allows single minutes to be read off with ease. The mechanism follows the practice of Heath an Co in swinging the worm out of  the plane of the rack by means of a spring loaded release catch, so that the index arm can then be moved rapidly.. The swing arm that carries the worm in its bearings rotates between cone-ended screws that can be locked in place when all backlash has been removed.

Figure 8: Micrometer mechanism.

Figure 9: Micrometer mechanism.

Axial play of the worm shaft is removed by an adjustable bush that is also locked into place when it is judged that there is no axial play of the worm in its bearings (Figure 10). A tongue (best seen in Figure 9) forming part of the rear swing arm trunnion bearing projects to form a keeper that prevents the index arm lifting off the front plate.

Figure 9: Detail of micrometer mechanism.

Figure 10: Detail of micrometer mechanism.

Figure 11 shows the light path, in red from a heavenly body at about 60 degrees altitude and in yellow from the horizon. Both mirrors have their reflective surface on the front and presumably this was chosen so that they could be cemented firmly into their brackets without the need for clips, which, at this scale would be extremely small and fiddly to fit. The horizon mirror has no clear-glass portion and the light from the horizon simply passes over the top of the mirror to combine with the rays from the observed body. In some respects, this is a disadvantage as the clear portion of the more usual horizon mirror reflects about 10 percent of the light coming to it from the heavenly body and increases the overlap in the view of the body and the horizon.

Figure 10: Light path.

Figure 11: Light path.

Figure 11 shows some of the structure of the horizon mirror bracket. Horizontal and vertical slots almost meet, leaving a slightly flexible diaphragm of metal between, so that the mirror can be adjusted in the vertical plane by two screws, the nearer one of which in the photo, as it were, pushes, while the other pulls. The two are adjusted against each other to correct side error. This is seen in a different view in Figure 13. The index mirror bracket is slotted in a similar way to allow adjustment for perpendicularity in the usual way.

Figure 12: Detail of horizon mirror bracket.

Figure 12: Detail of horizon mirror bracket.

To adjust for index error, the whole bracket rotates about a screw through the plate and two adjusting screws bear against each other on the opposite sides of a post. When adjustment is complete, the screw through the plate is locked.

Figure 13: Horizon mirror adjustment.

Figure 13: Horizon mirror adjustment.

It is not clear for whom this interesting little sextant was intended. Produced in small numbers, it must have rivaled a full-sized sextant for cost. There are plenty of box sextants around, but the Ilon is much easier to read than the crowded vernier scale of a box sextant and though the latter were useful to surveyors for reconnaissance surveys and to artillerymen for setting up their guns, improvements in survey instruments may well have made the sextant redundant for these purposes. The professional seaman is unlikely to have wished to be seen with anything other than a full-sized instrument, leaving only the well-heeled yachtsman or the collector as a possible purchaser. Perhaps this ingenious little instrument was too good for its own good, as they appear to be excessively rare, suggesting perhaps that it did not sell well.

I am grateful to Murray Peake for the loan of his instrument.

3 October 2014: In response to this post, Alan Heldman remarks that the design “…would easily lend itself to giving the user the option of putting the handle on the left side as well as the right side. With the handle on the left-hand side, the user could easily work the micrometer screw with his right hand.” There does not seem any reason why the handle could not be retrofitted to the left plate, though it would have to be almost horizontal and high up to clear the index arm.

Chris White writes “ Just saw the Ilon sextant article. Victor Carbonara was my grandfather and i worked at Ilon for a couple years in the 1970’s. I actually assembled and sold a number of these sextants from the parts inventory years after they had been discontinued.

It is a great sextant for marine use. Fussy to build due to the short arm requiring higher accuracy. “

Adapting to LEDs 1: miniature screw bases.

4 09 2013

It is nearly six months since I last contributed to this blog. I have been busy persuading reluctant chronometers to work properly and writing posts to my chronometer blog at This is not to say I have been ignoring sextants  and I have two or three restorations to write about, soon, I hope.

Incandescent light bulbs are beginning to fade into obsolescence and to be replaced by light emitting diodes (LEDs). At first available only with a rather feeble red light, now a variety of colours, including white, is available at high intensity. Recently, I came into possession of a C Plath professional bubble artificial horizon, my reward for restoring two others to working order. However, mine was lacking a working bulb. Plath sextants lit their readouts using miniature bulbs with screw bases and these are now very hard to find. may have a small supply, but once these are gone I have no idea where others may be had. For this reason, I have looked into adapting the instruments, sextant and its artificial horizon unit, to use LEDs. The base of the bulb is 5 mm in diameter, but fortunately 3 mm diameter LEDs are available.

To experiment, I bought half a dozen 3 mm LEDs with white output of 4000 mCd (millicandelas) with a supply voltage of 3.6, and with 35 degree angle of radiation rather than the more usual 11 degrees. At full output, 4000mCd is uncomfortably bright and I chose the larger output angle to give a wider and more even area of illumination within the bubble unit. It was also very adequate for the scale illumination.

The first step is to salvage the base (Figure 1). Since the bulb was burned out, I grasped it with pliers, intending to crush the glass, but in the event  the globe came free intact, except for one of the wires.

Figure 1: Removing old globe.

Figure 1: Removing old globe.

Figure 2 shows the wire remnants being removed with the aid of a soldering iron. To avoid burned fingers, It is helpful to have a third hand in the form of a mounted crocodile clip to hold the base. When the central wire has been removed, a quick puff of air while the solder is still molten will leave a hole for the new central wire.

Figure 2: Removing remnants of wires.

Figure 2: Removing remnants of wires.

It is then necessary to remove old cement and a scriber, preferably carbide tipped, can be used to chip it away. Figure 3 shows the process complete.

Figure 3: Salvaged base.

Figure 3: Salvaged base.

A notch must now be filed or sawn into the edge of the base to accept one of the LED wires. Figure 4 shows this being done with a triangular Swiss file, sometimes called a “three-square file” for an unknown reason. A junior hacksaw blade will do the job just as well.

Figure 4: Cutting notch for positive wire.

Figure 4: Cutting notch for positive wire.

The notch must be deep enough so that when the wire is bent at right angles close up to the base of the LED, that latter will sit just inside the salvaged bulb base. This insures that the tip of the LED will project no more than the original globe (Figure 5). While an incandescent bulb will work whatever the polarity, an LED will not. In my fine old C Plath sextant with the batteries inserted positive upwards, the central contact of the base in the scale lighting  is also positive, but in the bubble unit, it is negative, so if you have both, the LED lamps will not be interchangeable. However, since the life of an LED is very long indeed, once fitted, they can be forgotten unless they work loose in their holders. At any rate, the longer lead of the LED is positive. As Figure 5 shows, the correct lead is selected to fit in the notch and the other lead is led straight through the centre.

Bulb adapt 006

Figure 5: Leads in place.

Figure 6: Leads soldered.

Figure 6: Leads soldered.

Figure 7: LED adaptation alongside original globe.

Figure 7: LED adaptation alongside original globe.

If, as in Figure 7, the LED is slightly askew, it can easily be persuaded to point in the correct direction. The very cautious may secure it with a blob of Araldite or similar, but the leads anchor it very securely and no further action is really necessary.

An Improvised Dip Meter

5 04 2012

On 19 March this year (2012) on NavList, Alex Eremenko reported some strange results for observations made by him and a friend from the shores of Lake Michigan. Much discussion followed about abnormal refraction conditions that can cause large errors in the dip of the horizon and the possibility that clocks corrected by radio signals could occasionally be in error by a whole minute. As correcting the observations for an error of a whole minute in time then gave results that were uniformly as good as these experienced observers normally obtained, it seemed to Alex (and to me) that the clock hypothesis was the correct one. However, discussion of the matter then moved on (28 March) to how to determine whether there is abnormal dip of the horizon, a condition likely to occur when there is warm air over cool water, which is particularly common and severe in arctic regions. Uncertainty about dip swamps most other potential errors in measuring the altitudes of heavenly bodies at sea.

Figure 1: Dip with and without allowance for refraction

For those not familiar with the concept of dip, Figure 1 shows that the height, h, of the eye of an observer O affects the apparent horizontal. This is shown in dotted red. But, especially close to the horizontal, light does not travel in straight lines, because the density of the atmosphere decreases with height. Generally the light path, shown in full green, is bent or refracted so that it is convex upwards. This has the effect of making the distance to the horizon greater and the angle between the apparent and true horizontals, the dip, smaller. However, when atmospheric conditions are abnormal it can be much greater or even reversed. For a much fuller treatement of dip and its abnormalities, see

 If the angle between the horizon in front of the observer and the horizon behind him can be measured, then dip can be deduced directly, as it is half the value of that angle.This is not a new problem. In 1900, John Blish of the United States Navy applied for a patent for an attachment to add to a normal sextant. The patent can be viewed on Google Patents by searching for Patent number 714,276. Figure 2 shows one of the patent drawings.

Figure 2 : Blish prism attached to nautical sextant.

Essentially, the device is a prism that diverts light rays through 180 degrees so that the horizon directly behind the observer can be viewed at the same time as the horizon in front and the amount of dip read out directly from the sextant’s scale.

Several other inventors devised instruments or attachments to do the same thing. Among the more complex dedicated instruments was one patented by Boris Gavrisheff in 1961 (US Patent number 2,981,143). A telescope views via two prisms light coming from one horizon behind the observer at the same time as the light from the horizon in front of the observer. One of the prisms is rotatable so that the deviation from 180 degrees, i.e. the dip, can be directly read off a micrometer drum. A third prism, labelled 12 in the diagram, diverts the rays into a telescope (Figure 3).

Figure 3 : Gabrisheff’s dip meter.

It occurred to me that expensive prisms are not needed. They are not used in nautical sextants though for some reason most American bubble sextants used them. I had a box of wreckage from three Hughes and Son Mark III survey sextants that a kind friend gave me, so I set about building a dip meter as an exercise to illustrate the principle rather than as a serious sea-going device, though it could certainly be made more robust for sea-going use. An index arm extension carries a mirror that receives light from the rear horizon over the top of the observer’s head and diverts it through 45 degrees into another mirror that diverts it another 45 degrees into the telescope. The telescope receives light from the front horizon over the top of the second mirror so that, provided the mirrors are correctly aligned, both horizons can be viewed at once and the deviation from a straight line be measured using the micrometer drum. Figure 4 shows the layout of parts and Figure 5 the path of the light rays from front and back.

Figure 4 : Parts of the dip meter

Figure 5 : Ray path of dip meter.

Figure 6 shows how the dip meter is adjusted. Two autocollimators are set up facing each other and their axes adjusted so that they coincide in the vertical and horizontal planes and are parallel to the surface of the surface table. One is then “shuffled” sideways using a mirror, so that its axis is displaced from but remains parallel to the axis of the other. The plane of the sextant’s arc is set parallel to the table using a dial indicator and shims under the feet of the sextant. The sextant is set to zero and the two mirrors adjusted until the images of the crosswires of the autocollimators coincide when viewed through the telescope. The dip meter is then ready to measure the angle of dip directly.

Figure 5 : Light paths from the autocollimators

Figure 6 shows how the mirrors are adjusted, the same way that an horizon mirror in a standard sextant is adjusted, with screws bearing on the back of the mirror opposite spring clips bearing on the front.

Figure 6 : Mirror adjusting screws.

If you enjoyed reading this post, you will enjoy reading my book The Nautical Sextant, available from the joint publishers, Paradise Cay and Celestaire and via Amazon. Readers in Australia and New Zealand may Contact me, as I am able to offer them a discount on the published price.

Heath and Company’s best vernier sextant

10 12 2011

Previous posts in this category include:  “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”, “A British Admiralty Vernier Sextant”, “An Hungarian Sextant via Bulgaria” ,  “A Half-size Sextant by Hughes and Son” and “A Fine C Plath Vernier Sextant.”

Clicking on the figures will enlarge them and allow you to see more detail, while clicking on the back arrow (top left) will restore the post.

Several years ago, when I had first started to restore nautical sextants, I bought a Brandis vernier sextant on e-bay. I was dismayed when it arrived to find that it appeared to be loose inside a case that did not  belong to it and, worse, the case was jammed shut, perhaps explaining why the seller had not followed my usual request to put packing around the sextant inside the case. Eventually, I was able to get the case opened without damaging it and found that, improbably, the Brandis sextant had escaped all damage. The rosewood case, bound in brass, belonged to a Heath and Co pillar sextant that, as befits such a high-end product, had been provided with every possible accessory, though the only one present was an early 10 x 20 prismatic monocular. I restored the case and put it aside, against the day, yet to come, when I could acquire the sextant to go with it. However, a few weeks ago I acquired a somewhat later Heath and Co top-end product, an 8 inch (200 mm) radius vernier sextant, equipped with their patent “Hezzanith” endless tangent screw automatic clamp and a set of telescopes that was complete except for a prismatic monocular and the rising piece to go with some of  the other telescopes.  The sextant had its own case, so I still have a spare case for a Heath and Co Pillar sextant, and could be persuaded to part with it if offered the right price…

Heath and Co were granted a patent for their automatic clamp in 1910, so the sextant was no earlier than that, but it also had a Class A inspection certificate from the National Physical Laboratory in Teddington, dated January, 1921, so that its date can be fixed to within a dozen years (see Figure 1)

Figure 1 : Inspection certificate.

The mahogany case (Figure 2) had been protected from much damage by being contained in a stout cowhide satchel. It came as no surprise that most of the stitching had rotted and given way, nor that the leather of the lid hinge had dried out and parted company with the rest of the satchel. I spent a few quiet afternoons restitching the case by hand and gluing strips of leather to repair the broken hinge. Nothing can be done to restore the finish, however, and illustrating the satchel will have to await a post script. While the top of the case had, as is usual, been attached with glue and screws, I was surprised to find that shortcuts had been taken with the bottom: it had been attached by glue and brass panel pins, both of which, after over seventy years, had given way in places. Some of the drawer dovetails at the corners had also given way, so I re-glued everything and replaced the panel pins with brass screws. The “furniture”: brass handle, keyhole escutcheon, piano hinge and hook latches, responded to 600 grit emery paper, followed by metal polish.

Figure 2 : Exterior of case as found.

The details of the hook latches are a little interesting, as they incorporate a safety lock (Figure 3), similar to those found in some early post WW II Tamaya sextant cases. A springy brass sector plate is screwed to the case underneath the hook and when the hook is swung into the closed position, the plate springs up behind the hook, so that it cannot be accidentally un-latched without first depressing the plate.

Figure 3 : Safety hook latches.

A “belt and braces” (belt and suspenders in US) approach was taken to securing the sextant in its case. The pocket and boxwood latch is commonplace, but Heath and Co added the refinement of a brass pillar that  locates the handle in the pocket, and which has a spring-loaded tongue that projects above the handle to secure it. Pressing a button at the rear of the case (Figure 4) withdraws the tongue and releases the handle. The figure also shows that the legs rest upon a springy brass plate that protects the bottom of the case from the legs and also prevents the instrument rattling within its bonds.

Figure 4 : Release knob.

Figure 5 shows the sextant in its case before restoration. At some time, the original black lacquer had been over coated with black paint which had begun to flake off.  Beneath the paint was widespread verdigris that fortunately had progressed no further than a light surface coating. The frame, mirror brackets, shades mountings and legs are all of bronze, while the index arm is a single plate of heavy brass. Catalogues often describe sextants as having brass frames, but brass is an alloy of copper and zinc, without the resistance to corrosion of the copper and tin alloy that is bronze. The silver arc has a radius of about 200 mm (8 inches) and weighs a hefty 1.8 kg (4 lbs) without any telescope mounted. The size of the mirrors is large for the era. The index mirror measures 38 x 57 mm while the horizon mirror is 30 x 40 mm.  The large star telescope “sees” a relatively small area of the reflected image, but has a wide view of the horizon through and around the unsilvered part of the horizon mirror.

Figure 5 : Interior of case as found.

There is a substantial set of telescopes (Figure 6). Of especial note is the 4 x 52 mm Galilean or “star” telescope that, despite its impressively large objective lens, has a measured field of view of only 3.5 degrees. The other star telescope is only a 3 1/2 x 19 mm instrument that is very little different from those in use a hundred years earlier. While lacking the light grasp of the large star telescope, the 4 x 30 inverting telescope has more than twice the field of view to compensate. The 11 x 19 mm inverting scope again belongs to another era and even by 1921 was probably very seldom used. The kit is completed by a zero magnification sighting tube and a pair of eyepiece shades, to which I have added the 10 x 20 mm prismatic monocular with its field of view of about 3 degrees.

Figure 6 : Telescope kit.

 Those telescopes not provided with a forked rising piece have interrrupted screw threads, to allow them to be mounted on the instrument  thread with less than one sixth of a turn. The rising piece for these ‘scopes was missing, so I had to make a new one from scratch. This can be seen in Figure 7 , below, but I have saved the description of how to make it for my next post, under the “Interesting Overhaul Problems” category.  The plain fork fits into a substantial and close-fitting slot in the telescope bracket and is retained there by a nut and a large knurled washer. The washer has a short slot cut in it at 45 degrees to a radius and could presumably have engaged with a button on the telescope fork to act as a crude way of making fine adjustment to the position of the fork, by rotating the washer. However, the large star telescope has no such button and only traces of the button remained on the prismatic monocular, following its adaptation to another instrument.

Figure 7 : Telescope mounting.

The index arm bearing is typical. A slender bearing fits closely in the frame  and a tapered shaft or journal rotates within it. The end of the shaft bears a square that fits inside a square in a washer, while a screw adjusts fit and removes end play. It is worth noting (and repeating) that this screw is used for taking up play only until the faintest trace of resistance to rotation is felt and is then slacked off a little. It must not be screwed up hard as this will very likely cause the bearing to seize, if it does not first twist off the head of the screw. The purpose of the square is to prevent rotation of the shaft being transmitted to the head of the screw. A cover acts as a third leg for the sextant.

Figure 8 : Index arm bearing.

  The mirror mountings are standard, following the pattern described by Peter Dollond in a letter of 1772 addressed to the Astronomer Royal, Nevil Maskelyne. In the letter, Dollond describes how the mirrors are supported at only three points at the back and are retained in their brackets by three spring clips that bear on the front directly over the points. Dollond claimed to have devised the system. Whatever the truth of this, he was granted a patent for it on 22 May 1772 (no. 1017), though one should bear in mind that in the eighteenth century at least, patents were not about priority of invention but gaining a monopoly of use. One of the screws on the index mirror mounting allows it to be brought perpendicular to the plane of the arc and on the horizon mirror, one screw brings it parallel to the plane of the arc while the other one makes it parallel to the index mirror when the sextant reads zero. In this instrument Heath have made a slight refinement to protect the thread of the adjusting screws by providing a counterbore which fits over a boss at the rear of the bracket and which can be filled with a soft rubber washer or with grease (Figure 9). A front view of the clips is shown in Figure 10.

Figure 9 : Horizon mirror mounting.


Figure 10 : Horizon mirror clips.

Figure 11 shows how the horizon shades are mounted and the same arrangement is used for the index shades. The shades are mounted on a tapered shaft and are separated by washers which also have tapered holes in them. When the shaft is inserted into the bracket and through the sandwich of shades and washers, it is prevented from turning by a pin that passes through its head into the bracket. As the adjusting screw is tightened, the washers and shades are forced further up the taper, thus increasing the friction. There is enough friction between the washers and the shafts to prevent them from turning, so that rotational forces from moving one shade are not transmitted to the next. Unusually, in addition to the four index shades, there is an astigmatiser. This is a weak primatic lens that draws out the image of a star into a fine line. In some circumstances, this can make it a little easier to bring a star down to the horizon and, if correctly mounted, can indicated whether the frame of the istrument is tilted relative to the horizon. However, its main use was probably when employing an artificial horizon, when the line of the reflected image would be made to bisect the round direct image of the star, or the image of the bubble when using  a bubble artificial horizon. The latter had only recently been invented at the time this sextant was made.

Figure 11 : Horizon shades

Cheaper vernier sextants generally simply mounted the magnifier at approximately the correct viewing angle and focussing was carried out by sliding the magnifier up or down in a sleeve at the end of a swing arm centred about one third of the way up the index arm. Heath’s rather elaborate and delicate swing arm carries trunnion bearings that allow the magnifier to be tilted so that the view through the magnifier can be centred at any point along the vernier scale (Figure 12).

Figure 12 : Scale magnifier.

Figure 13 (below) shows the intact catch fitted to the rear of the index arm expansion on the left and the exploded structure on the right. A swing arm plate carries the bearings for a worm and its shaft and is itself carried on trunnions that run in bearings mounted on the index arm. Click on the photo to see an enlarged view. These bearings also double as keepers that prevent the index arm from lifting off the front of the limb. Close inspection of the right hand side of the illustration will show that these keeper-bearings have bosses that fit into bushes within holes on the index arm. The holes in the bushes are eccentric, so that the position of the bearings of the swing arm plate can be adjusted to remove end float of the plate and to bring the worm into correct engagement with the rack. End float of the worm itself is removed by adjustment of a cone-ended screw that engages with a centre in the end of the worm and that is locked by a knurled lock nut

Figure 13 : Release catch mechanism.

When the release catch button is squeezed, the worm and its mounting is swung out of engagement with the rack so that the index arm can be placed rapidly and approximately in position, after which the worm is used to make fine adjustments. Because it is so short, the pitch of the worm is rather difficult to measure, but it appears to be of around 0.8 mm (32 t.p.i.). After receiving their patent (No 17,840 of 10th March, 1910), it seems that it took Heath and Co another fifteen years or so to make the obvious next step and make the pitch such that one turn of the worm moved the index arm through half a degree, or 1 degree of sextant reading. This probably had more to do with conservatism than with technique, as the rise of the motor industry around the turn of the century had stimulated the production of  accurate gear hobbing machines. There is some evidence that C Plath of Hamburg had produced a very similar release catch mechanism somewhat before Heath did so, and they certainly continued to do so into the 1920s, until their micrometer sextant gained popularity and ousted the vernier instrument. Neither firm could of course claim priority for the worm and rack which was certainly known to 1st century Greeks. Heath’s claim was for the method of mounting  a “spring urged plate upon which the traversing screw is mounted…in such manner that the traversing screw can be taken and held out of gear...”  Had Plath patented their micrometer sextant in 1907, when they first advertised it, this is probably precisely the claim they would have made. Figure 14 shows the restored instrument in its case. If you have enjoyed reading this post, you may enjoy reading my book “The Nautical Sextant”, available through good booksellers, from Amazon and direct from the pjoint publishers, Paradise Cay Publications and Celestaire.

Figure 14 : Restoration completed.

A Half-size Sextant by Hughes and Son

29 09 2011

Previous posts in this category include:  “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”, “A British Admiralty Vernier Sextant and “An Hungarian Sextant via Bulgaria.”

Clicking on the figures will enlarge them and allow you to see more detail, while clicking on the back arrow (top left) will restore the post.

Hughes and Son made sextants and other navigational instruments from the middle of  the nineteenth century until 1947, when they merged to become Kelvin and Hughes. Prior to and during the Second World War they made a wide variety of aircraft instruments, among which was a small sextant intended for use in seaplanes such as the Sunderland flying boat, perhaps not so much for celestial navigation as for taking anchor bearings and amplitudes for checking the magnetic compass. Quite why an ordinary nautical sextant was not issued is unclear, as the small sextant in its case weighs only 600 G less than a full-size Hughes sextant of the same period that weighs in at 3.9 kg. There was scant advantage in volume either : the smaller instrument’s case is 200 x 200x 140 mm against the full-size case of 275 x 260 x 145 mm. The sextants were made under an Air Ministry contract and, like the Mark IX series of aircraft bubble sextants, were issued in a case made of a heavy dark brown plastic material similar to paxolin. It is probable that in a period when imported timber was at a premium and skilled woodworkers were engaged on aircraft production, the plastic cases, made of 5 mm sheets pinned together with brass nails, were seen as a satisfactory solution. The examples in Figure 1 show a full-sized Hughes and Son nautical sextant and its little brother  along side it. The smaller one was made in 1943 and eventually made its way to Australia, where it was sold by T.M.Burroughs of Flinders Street, Melbourne  to the Third Officer  of a ship (whose name I cannot decipher) in May 1948 for the sum of twenty pounds. This was about the going rate for a full-sized sextant: one in my possession was sold new  in 1945 for eighteen pounds, with an extra four pounds for a large aperture telescope. There are very many so-called reproduction or “replica sextants” of similar size on the market, but this is a fully functional and accurate instrument able to perform at nearly the same level as a full-sized instrument.

Figure 1 : Two Hughes and Son sextants

Figure 2 shows the instrument in its case. The handle is the same as for the larger instrument, on the side of the case, and the latches are very similar to those used for Hughes cases of the 1930’s. The sextant’s legs sit in mahogany pockets and it is further restrained by two pads in the lids, all typical of Hughes’s full-size practice.  There is a Husun (Hughes and Son) calibration certificate in the lid and pockets for the oil botttle, adjusting pick and a key for the box lock. I have seen an example from 1942 in a mahogany case with otherwise identical furniture and layout.

Figure 2 : Interior of case.

The front view of the sextants, seen in more detail in Figure 3, reveals that the shades, mirrors and micrometer mechanism are all  full sized, while the x2 fixed focus Galilean telescope has an aperture of 20 mm against that of 30 mm for the larger sextant.

Figure 3 : Front (left hand) face of sextant.

The telescope has no rise and fall mechanism and is attached to the frame by a single screw that passes through a boss in the base of a shaped column. A dowel pin locates it so that it points in the correct direction. This pin can be seen above the boss in the close up photograph of the telescope column in Figure 4.

Figure 4 : Location of telescope in frame.

The rear (right hand) view is shown in Figure 5.

Figure 5 : Rear (right hand) view of sextant.

  The handle, like the telescope,  is mounted on a single column, but instead of being restrained from rotation by dowels, the column has squares on each end that fit into sockets in the handle and sextant frame, being held there by single large screws (Figure 6). Notice too that the index arm bearing is concealed by a stout brass cover that screws over it and doubles as a third leg.

Figure 6 : Method of locating handle.

In the view of the micrometer mechanism (Figure 7, below), note the fine pitch of the worm which allows a full-size drum to be used. The worm shaft has a tapered thrust bearing and a parallel portion, which run  in a single block of bearing, with preload applied by a U-shaped spring. The worm shaft is in two parts, to allow assembly. The release catch operates a cam which swings the assembly out of mesh with the rack.

Figure 7 : Micrometer mechanism.

 The mounting of the shades, particularly the index shades, is a little unusual (Figure 8). Normally, the shades are mounted on a shaft that is prevented from rotating, and the shades are separated by washers that are also prevented from rotating, so that when one shade is rotated into position rotation is not transmitted to adjacent shades, and they do not follow. This is very convenient when taking sights, as it is easier to find the sun with a relatively light shade in position, when a darker shade can then be swung into place without one having to take one’s eye off the quarry.  In this little sextant, there are no washers. Instead, slots have been milled in the mounting for each shade. These can be closed up by means of  nut on the end of the mounting pin or shaft, and the latter is prevented from rotating by a crossed taper pin through its head. In the 1942 sextant mentioned above, the shades mounting follows Hughes’ standard practice.

Figure 8 : Index shades mounting

The horizon shades are mounted on a single shoulder screw and are separated by red fibre washers that fit tightly on the screw. A Belleville washer, which behaves as a short, stiff spring, provides friction and the screw is locked by a shallow nut (Figure 9).


Figure 9 : Horizon shades mounting.

Figure 10 shows the bare frame of the instrument.

Figure 10 : Bare frame.

A kind French correspondent, whose name I have unfortunately lost, sent me a photograph of a similar sextant in his possession (Figure 11). It is identical except that it is named Heath and Co. and carries a telescope in the style of that company. It may be that the Ministry of Aircraft Production or the Air Ministry during the exigencies of World War II imposed some degree of cooperation between the two makers, or perhaps Heath and Co acquired some instruments as war surplus and added their own name and telescope.

Figure 11 : Heath and Co. seaplane sextant.