Making a Prismatic Monocular

18 11 2009

I hope readers won’t be disappointed to learn that this post does not take them through the actual making of a monocular. Rather, it shows how to chop up a pair of 1960s binoculars into two monoculars, without leaving any ugly bits sticking out. These Porro prism binoculars may now be had very cheaply, as they have been displaced by roof prism binoculars which are much easier to carry in the pocket, even though their light grasp may not be very good and their magnification is in some cases unrealistically large.

While Tamaya at one time offered a sextant provided with a 7 x 50 monocular, the diameter of the index shades and mirror would have limited the usefulness of the tremendous light grasp of the 50 mm objective, though it gave a very good view of the horizon. I found that despite its magnification of seven times, its 7 degree field of view made respectable sun shot results possible, even when bouncing about in a 15 ft dinghy. Freiberger offered an 8 x 30 option with their drum sextant.  6 x 30 would probably be the ideal, but such binoculars are rare and the best compromise is probably 7 x 30 or 7 x 35. The latter admits about 36% more light than the 7 x 30.

First separate the two halves of your chosen pair of binoculars. At each end of the central axis will be found a large brass screw, its head concealed either by a washer with a central screw, or sometimes by a threaded cap.

Remove the caps or washers. The large screw at the eyepiece end may have a locking screw. If so remove it before undoing the large screw.

Removing the large screw at the eyepiece end allows you to detach and separate the two eyepieces…

…while removing the screw at the objective lens end allows you to withdraw the central shaft and separate the two bodies.

Only one of the two halves has an adjustable eyepiece. Select this half for attention and put the other one aside. You can convert the eyepiece to an adjustable one and this may be the subject of a future post.

Next, unscrew the objective lens assembly and the eyepiece tube from the body. This may need brute force, but on no account grip the parts directly in a vice or, worse, a Stilson wrench. First try wearing rubber kitchen gloves to enhance your grip. If really stuck,  in a piece of wood bore a hole the same size as the bit you want to grip and then split the wood along one radius. You can then put the part in the hole and squeeze it closed in a vice with less risk of damaging the lens.

Then remove the end plates. Usually these are secured only by a single screw.

Then it is the turn of the prisms. These are usually held in place by a leaf spring, one end of which sits in a slot  inside  the body, and the other end is fixed with a screw.

 In very cheap binoculars, where the prisms fit only loosely in their seats, you may find a blob of mastic which you will have carefully to remove as well. With luck, once you remove the leaf spring, the prism will drop out into your hand. If it does not, beware of using a metal tool to prise it loose, as the glass chips very easily. If your fingers are not strong enough to get it loose, use a slip of wood as a lever. Note too that the prisms are not always of the same size, that at the objective end usually being the smaller of the two.

Once you have removed both prisms from the body, you can more safely set to work with a hacksaw and amputate the bits that stick out.

The result is not pretty, so peel off the faux leather and file the whole to a smooth and pleasing contour. Often the “leather” is so firmly attached that you will have to cut it off sliver by sliver with a sharp knife. The aluminium is not usually of a free-machining quality and it tends to clog files easily and then the work-hardened stuck bits or “pinings” scratch the workpiece. The traditional preventive remedy is to rub the file occasionally with chalk, though I cannot say that I have found it satisfactory, nor do I find that scraping the “pinings” out with a piece of sheet brass much better. Using a sharp point to prise out the clogged bits is the quickest remedy.

You can then re-cover the body with a piece of leather cloth, the thinner the better, as the end plates have to fit back over it at each end.

Clean the prisms with iso-propyl alcohol and replace them and their retaining springs, noting that the side of the prism with the apex cut off faces inwards, followed by the end caps. If the leather cloth is thick, this may be a bit of a struggle, and it is helpful to use a piece of shim as a sort of shoe horn, to prevent crinkling as the lip of the cover is wangled over the cloth. Now, move on to the eyepiece assembly.

If you unscrew the plastic eyeguard, you will find that it conceals three tiny grub screws. Screw the eyepiece in to the maximum extent and then slacken off the screws. It doesn’t matter if you remove them completely, but they are easy to lose and difficult to replace. Once you have backed them out, it should then be possible to remove the knurled focussing ring and then to unscrew the eyepiece itself, with its three-start thread and sticky grease. The next photo shows this disassembly. Unfortunately, the “nut” is shown reversed end for end, but by the time I discovered this, it was too late to go back and repeat the photograph.

Disassembly now allows you to amputate the arm sticking out from the nut and either to file the outside smooth or turn it smooth in the lathe. You then have to drill and tap three holes spaced at 120 degrees for grub screws. I used 8 BA size, though there was room for Metric 2.5 or even M 3 . Their purpose is to allow the nut to be securely attached to the eyepiece tube as shown in the next picture.

To reassemble, screw the eyepiece tube into the body, taking great care not to “cross” and damage the very fine thread. Screw the eyepiece fully in to the nut and push the assembly over the eye tube. Then put the focussing ring into place over the end of the eyepiece, wangling until it sits squarely. If you do not screw the eyepiece fully in to the nut, you will probably find it impossible to get the focussing ring into place, as there is an eccentric lip turned on the inside of the ring. Tighten the focussing ring grub screws and then temporarily tighten the nut grub screws to check that the focussing operates smoothly. Final positioning of the nut comes later, after making and fitting the bracket for attaching the monocular to the sextant.

In a previous post I gave the briefest outline of making a monocular mounting bracket for a sextant. In this post, I give more details, for a mounting for a Freiberger Prazisionsmechanik Drum Sextant, which has a male vee on the sextant rather than the female vee on C Plath and their many derivative sextants. There is no getting away from having to do some metal machining and if you have no workshop, these notes may serve to guide your friendly local jobbing machinist.

First you need to make a few measurements to allow you to mark out your metal for machining. No great precision is required. The outside diameter is for neatness,  and fits only the air. The inside diameter will be the outside diameter of the objective lens mounting thread plus about 0.2 mm, with a similar plus allowance for the counterbore, which may need to be tapered to fit the outside of the lens mounting.

The width of the stem can be taken off from the sextant mounting bracket plus a millimetre or two and its length should be such as to bring the centre of the monocular to the same height as the centre of the horizon mirror is above the frame, plus , say, 10 mm, for when you want to see more of the horizon by raising the ‘scope. Once you have these various lengths, you will be able to mark them out on a piece of 6 mm aluminium alloy (or brass) plate, of a free machining variety for preference.

The workpiece then has to be centred in the lathe in the four jaw chuck, drilled through and bored out to an easy fit for the outside diameter of the thread of the objective lens mount. It is then counterbored to a depth of 5 mm, leaving 1 mm thickness for attachment purposes. Depending on the shape and length of the lens mount, it may be necessary to taper bore the counterbore, so that the shoulder on the mounting can seat properly on the bottom of the counterbore.

If you are going to produce the vee groove by shaping, you now have to drill a shallow hole at the upper end of the groove for the tool to have somewhere to go at the end of each stroke of the shaping machine ram. It is also possible to make the groove in the vertical milling machine using a 90 degree countersink cutter, in which case no run-out hole is needed (you can see where I have made a couple of trial cuts on the edge of the workpiece). I preferred to use a shaping machine with a 90 degree form tool deepening it progressively, and using the sextant bracket as a gauge, until the flat on the sextant bracket sat properly.

The last machining operation is to mill out the slot for the stud on the sextant bracket, checking that the stud fits without interference when the two parts are mated, and if necessary increasing the width of the slot.

This concludes the machining and it is now necessary to return to the craft skills of sawing and filing the outside so that it looks right. The outside diameter does not have to be perfectly circular, it just has to look that way. Because I have had a lot of practice with a piercing saw, I removed most of the excess metal with that. I have little experience with the band saw, but I have the impression that the radius may be too tight to do the job. It can of course be centred on a rotary table and the outside milled, but it can be sawn out in the time it takes to set up a milling machine to do this. The next picture shows the sawing half-completed…

and the next the completed article wearing a coat of paint.

If you’re going to touch up the rest of the paint work, this is perhaps the point at which to do it, allowing the paint at least 36 hours to get good and hard before handling the parts again. To assemble, pass the objective lens and its mount through the new mounting bracket and screw it into place. As about a third of the diameter of the female thread inside the monocular body is missing, it is exceedingly easy to cross-thread as you do so. It is as well to get a feel for what the thread feels like when screwing easily and fully home, before trying to do it with the bracket in place.

The new bracket leaves the objective projecting a millimetre more than before, so the eyepiece has to be repositioned. I suggest you set the eyepiece focus to its mid point, loosen its grub screws and rotate the focussing ring to zero. Re-tighten the screws firmly and then, while wearing whatever spectacles you may  normally use when taking sights, loosen the “nut” grub screws and slide the eyepiece in or out until the ‘scope is sharply focussed on a distant object. Re-tighten the grub screws and the job is complete.

The next picture shows the completed monocular with its new mounting bracket (the paintwork is better than it looks…).

and the final picture shows it in place with its new companion.

It is as well to check that the axis of the ‘scope is parallel to the plane of the sextant. In the Freiberger, this is easy to do using a small square raised a little on a parallel to clear the sextant mounting bracket. Remove the thin metal protective cover from the end of the objective lens mount and check that the face of the lens mounting is square to the frame. If it is not (and mine was not), judicious filing of the underside of the sextant bracket a little at a time is probably the quickest way of putting things right.


11 December 2009

The limb of my second-hand Freiberger was painted a darker grey than the rest of the instrument and, as the paint had worn in places, I decided to replace it and at the same time refresh the graduations. My technique is to scrape out all the old paint first, using a scriber, and then to mask the rest of the sextant before respraying the limb. Although it probably isn’t necessary, I then scrape out the new paint too. When the paint has thoroughly hardened, after 36 to 48 hours, I brush on some white acrylic paint, let it dry for only a very few minutes and then wipe it off with a barely-damp rag. The next photo gives a close-up view of the result.

Once I had done this, my wife suggested that an all-black monocular looked out of place (whatever one might feel about the national rubgy team, the All Blacks), though this was how Freiberger supplied them. I did try painting the black leather cloth, but it didn’t stick very well. On a trip to our local city, 200 km away, I found some grey leather cloth of exactly the right shade, so I used it to replace the black, and repainted the objective lens mount and end plates.  What an Australian friend calls “My fraulein” now looks very fashionable.

I felt it deserved a proper home and made a case for it out of kwila, a South Pacific hardwood that resembles mahogany a little. I rebated the corners and the base, but kept to the traditional screwed-on top. CRC Black Zincit gave just the right touch of austerity to the case fittings. The base and corners have been pinned with brass pins whose heads have been punched below the surface and concealed with filler

If you have enjoyed reading this and others of my blog posts, you will probably also enjoy reading my book, The Nautical Sextant, which is a fund of information about sextant structure and function.

Making a sextant monocular mounting

28 01 2009

In the Yaho sextants group recently, Mike Bowman of Darwin Australia asked about the availability of a prismatic monocular for his Freiberger Trommelsextant and Federico Rossi, writing from Italy, wondered about the possibility of cutting a pair of binoculars in half and mounting a half on a sextant. Had anyone experience of this?

Until this afternoon, I would have answered in the negative for myself, but wanting a break from writing my new manual on restoring the A10 and A 10A bubble sextants, I retired to the workshop for a few hours to see if I could prove a concept I had in my mind. It would be very difficult to retrofit a mounting like the one shown in the next picture, in which a huge 7 x 50 monocular is mounted on a 1957 Tamaya sextant as original equipment

Tamaya 7 x 50 prismatic monocular

Tamaya 7 x 50 prismatic monocular

The mounting of the very old 10 x 30 monocular shown in the next picture gave me the germ of an idea that started me rooting around in one of my many treasure chests for a suitable piece of aluminium alloy plate.

Ancient prismatic monocular

Ancient prismatic monocular

In this ‘scope, the monocular backplate is continued as a fork that has a vee machined on one side. It was probably originally mounted on a top-of-the-range Heath and Co sextant, made around the beginning of the twentieth century, and there is no room for the fore-part of the monocular in a modern sextant, so I decided to use the objective lens mounting to hold a vee and flat type of mounting, to match the Tamaya shown above, as well as nearly all modern sextants.

I had only scraps of 4 mm plate in a suitable material, so screwed two pieces together to save having to convert a 12 mm hunk of material mostly to metal chips. I won’t bore you with all the steps, but the mounting started out as in the following picture.

Starting point

Starting point

After about three hours of machining, sawing and filing, it ended up like this:

Finished article

Finished article

The 4 mm plate is counterbored to a depth of 3 mm to accommodate the outside diameter of the objective lens mount while the threaded part of the lens mount passes through the smaller diameter hole and screws into its rightful place in the monocular, with an increase in optical path length of only 1 mm.  The next picture shows it in place.

Mounting in place

Mounting in place

It works well, but the weak point of the system, apart from the ease with which the threads of the objective lens mount can get crossed, is that when you halve most binoculars, you also have to halve the focussing arrangment. You can find binoculars with individual focussing, but these tend to be vintage WW II US Navy ones, worth almost as much as a second hand sextant of the same vintage. I haven’t for the moment any suggestions, but you can always let me have your ideas in the comments section; and you could encourage me by buying my book, The Naked Nautical Sextant and its Intimate Anatomy.

Another Sounding Sextant

17 11 2018

A little while ago I acquired yet another sounding or survey sextant for a relatively small sum. It is based on a Cassens and Plath nautical sextant. As with most sounding sextants, it has no shades, but where the index shades would normally be mounted is a leg and where the horizon shades would be mounted is a bracket for a pentagonal prism or “penta prism”.

Over two hundred C&P surveying sextants were obtained by the US Coast Guard Service from Weems and Plath around 1978, provided with a handle that would make holding the instrument horizontally easier and stripped of the lighting system, to save unnecessary weight. In my instrument, which bore a USGCS label, the lighting system is intact and there is no provision for a modified handle.

Figure 1 shows the state of the instrument as received and it had plainly not been well loved in the autumn of its life (by the way, the hand holding it is not mine).

As found labelled

Figure 1: C & P Sounding sextant as found.

My usual method is to strip the sextant down to the last screw and washer and then to clean and repaint everything, stripping all the old paint off if necessary. As I go, I fix electrical faults, renew wiring, replace mirrors , clean optics, and re-grease moving parts. As I have elsewhere in the blog described these activities, I will not go into them here, but instead focus on the main point of difference from other sounding sextants: the pentaprism. I have given a very brief account of the use of sounding sextants in the post for 26 April, 2009, and this should be read in conjunction with the comment kindly sent by Peter Catterall.

In a pentaprism, the emerging ray is at right angles to the incident ray, and the angle between the two rays (really two parts of one ray) is independent of any rotation of the prism about an axis parallel to any of its faces.  The image is not inverted or reverted. However, if the prism is rotated about another axis, the incident and emergent rays will not be at a right angle. Although there are two internal reflections in a pentaprism, they are not total internal reflections as, say, in a 90 degree Porro prism, and so the reflecting surfaces have to be silvered. If the paint film and underlying silvering gets damaged, the damage will be apparent in the view through the prism.


Figure 2: Position of the pentaprism.

Figure 2 shows the location of the pentaprism behind the clear glass of the horizon mirror. It is located in a spring-loaded bayonet socket by means of a peg, which allows it to be placed in two positions (Figure 3). Rotating it anticlockwise locates it in the position shown in Figure 4.

Pentaprism base

Figure 3: Pentaprism base

When located in this position, if the index arm is set at 90 degrees, the two light paths should be parallel, so that a distant vertical object should form a continuous vertical, straight line when the instrument is held with the frame horizontal. For this to happen the faces of the prism must be at a right angle to the frame of the instrument, so three adjustment screws are provided to bring this about. It is a great deal easier to do this if a 2 mm diameter torus of thin, soft copper wire is placed centrally under the face opposite the adjusting screws, so as to allow a little rocking  to take place. This is a little simpler than following the official advice promulgated in the US Coast Guard Service manual, available on line here:  The prism is held in place by two rectangular “springs” which offer quite a lot of resistance to the movement of the prism when adjusting it, so it is easier simply to leave them a little proud of the prism faces and rock the prism as I have suggested. The adjusting screws then do double duty of adjusting and retaining with the springs as back-up retainers.

Any index error of the sextant must of course be allowed for, in addition to any error found with the prism in place, and normal checks for perpendicularity of the index mirror and side error made and corrected.


Figure 4: Position of prism to check index error.

Figure 5 shows the prism in its orientation in normal use and you can see that with the index arm set at 30 degrees, the rays diverge by 90 + 30 = 120 degrees, the practical limit for a normal sextant, where the reflected image is reduced to a narrow slot.

30 degrees

Figure 5: 90 + 30 degrees = 120 degrees.

Figure 6 shows how 180 degrees can be measured by setting the index arm to 90 degrees. The ability to measure large obtuse angles improves the strength of position lines when fixing the position of aids to navigation.

90 degrees

Figure 6: 90 + 90 degrees = 180 degrees.

Although the stout case could be mistaken for solid wood, it is in fact some sort of laminated wood, as witnessed by the delamination of the outer layers of the top and bottom. It seems strange that an instrument destined for use in a damp and sometimes wet atmosphere should not at least use marine grade laminates for its case. The corners are keyed mitre joints which give both a very neat appearance and very adequate strength. Note that the key should be sited as close to the inside angle as feasible, as shown in Figure 7.

Case corner

Figure 7: Keyed mitre joint in case.

Figure 8 shows the instrument less its telescope in its case. It cannot be stowed with the pentaprism in place, though with a little more thought, the pocket for the sextant handle could have been rotated anti-clockwise and moved to the left a little to give room for both the prism and the originally supplied prismatic monocular.

In case

Figure 8: Instrument in its case

The telescope with my sextant is a standard 4 x 40 C and P offering except that there is a glass in front of the objective lens that acts as an astigmatiser with stars, drawing out the point sources into lines (Figure 9). It has no effect on extended images. I cannot imagine how this works, so if any reader knows, I should be glad if they would share their knowledge with me.

I hope to be able to add two or three more posts to this blog before the end of the year, after which it will the turn of a marine chronometer to be described on my other web site, .




C Plath Yachting Sextant

14 06 2015

This post was preceded by “Making a shades adjusting tool” and “Eighty years of Carl Plath Sextants”. Other posts on C Plath sextants may be found by entering “C Plath” in the search box on the right. All figures may be enlarged by clicking on them. Return to the text by using the back arrow.

Several makers, including C Plath, made sextants directed at the yachting market with more or less success. There seems to be a fair number of Freiberger yachting sextants around, but I have only ever seen two Plath Yachtsman sextants. In the years after WWII, many full-size sextants must have flooded the market, especially the USN Mark II sextant and those made by Henry Hughes and Son. The latter also made half size sextants for use in sea planes and presumably they were attractive to yachtsmen, as some have survived. A variety of plastic sextants derived from the Maritime Commission version for lifeboats came on to the market and evolved into instruments that looked like “proper”metal sextants, though few were rigid enough to behave like one. Francis Barker produced a box sextant labelled “Small Craft Precision Sextant” intended for sale to yachtsmen, but despite having been provided with a horizon shade and an eyepiece shade in addition to the usual index shade, I doubt that it found much favour with nautical users. A box sextant is a fiddly instrument at the best of times and it is difficult enough to take sights from a rolling yacht. Ilon industries made an ingenious little micrometer sextant provided with a tiny prismatic monocular ( that may have found favour with the well-heeled and Tamaya made a light weight 5/6ths sized micrometer sextant. The French firm of Roger Poulin made an interesting little sextant that was plainly aimed at the yachting market and I have described it here: .

It is not clear whether the yachtsman wished a smaller sextant because of lack of space aboard yachts or because a smaller sextant might be cheaper than a full-sized version. At any rate the saving in space and weight must have been insignificant, and the savings made by buying a smaller sextant cannot have been great when compared with the cost of the vessel.

Unlike the Freiberger Yacht Sextant (, which attempts in a way to echo the full sized instrument, the frame of the C Plath sextant is monolithic and exceptionally rigid. Figure 1 shows a general view of the front. The bases of the index and horizon mirror brackets are identical though the horizon mirror itself is half silvered. Both are circular, presumably because it is easier to seal the mirrors against the intrusion of salt water behind them, but as can be seen in some of the later figures, the index mirror has suffered around the edges. The two index shades and one horizon shade are adequate in most circumstances. Their brackets are simple and no provision is made for adjustment of friction. A notch in the edge of the frame allows the horizon shade to be folded completely out of the line of sight.

The rack in which the micrometer worm engages in machined into the edge of the limb, together with a slot for a keeper to keep correct engagement. The radius of the rack is about 140 mm (5.5 ins) and the instrument weighs 1260G (2lbs 12 oz).

Figure 1: General view of front.

Figure 1: General view of front.

The telescope has a simple draw tube for focusing, and  has an aperture of 25 mm and a power of about 2.5 diameters, giving a field of view of a little over 6 degrees. This is about the same as one gets from a 4 x 40 mm telescope of a full-sized instrument. Though a C Plath leaflet says the aperture is 30 mm with a magnification of x 4, the inside diameter of the tube in front of the objective lens of my sextant is only 27.5 mm and it has to sit on a shoulder, so the aperture behind the lens is only 25.1 mm. The measured magnification is about x 2.5.

The telescope is not demountable, a disadvantage on a small vessel when it is rolling and pitching, as with a standard field of view it can be difficult to acquire the heavenly body and bring it down to the horizon. Removing the telescope altogether makes it much easier to find the body and to bring it down, when the telescope can be replaced and the horizon swept to re-acquire the body. However, the telescope mounting is very robust so that it is not only resistant to knocks, but the sextant can safely be picked up by the telescope without fear of damaging or displacing it. The micrometer mechanism is well protected against knocks and the release catch is simple to operate. Figure 2 shows a rear view of the instrument.

Figure 2: Back view.

Figure 2: Back view.

The frame is closed off at the back by a back plate, which is attached to the frame by three screws and a leg. The handle, adapted from a full-sized instrument battery handle, is attached to the back plate via pillars by two countersunk screws. Removing the back plate reveals the index arm as shown in Figure 3. Note that if the sextant gets drenched in salt water, it is an easy matter to rinse out the interior with fresh water without necessarily removing the back plate.

Figure 3:  Rear view without back plate.

Figure 3: Rear view without back plate.

The index arm is in two pieces: a stout rectangular bar attached to the index mirror bearing at the top; and  a plate that I have christened the index arm expansion at the bottom. This plate carries the micrometer mechanism. I have labelled the screw for attaching the horizon mirror and the swing arm keeper in Figure 3 for future reference below. Also seen are the two stout screws that attach the telescope to the frame.

Figure 3: Index arm bearing.

Figure 4: Index arm bearing.

The anatomy of the index arm bearing is revealed in Figure 4. A micro-finished journal runs in a parallel bearing machined directly into the frame, with two PTFE washers acting as spacers and also taking any minor thrust forces that may arise. A flange above the journal carries the index mirror in its bracket, while a spigot below attaches the index arm. Figure 5 shows how the upper end of the index arm is split, with a pinch screw to close it around the spigot. This allows adjustment of the mirror in the horizontal plane as well as axial adjustment to take up any axial movement in the bearing.

Figure 5: Upper end of index arm.

Figure 5: Upper end of index arm.

Figure 6 shows how the index mirror is adjusted for perpendicularity and the horizon mirror for side error (the horizon mirror is illustrated) . The mirror bracket is rocked by means of two screws about two ball bearings sitting is depressions to form an axis of rotation.

Figure 6: Mirror bracket adjustment.

Figure 6: Mirror bracket adjustment.

As the reflective surface of the index mirror lies a little ahead of the axis of rotation of the index mirror it is necessary to use two vanes to raise the line of sight to somewhere near the centre of the mirror, as otherwise a minor error in perpendicularity may be introduced. Figure 7 shows how two small dominoes have been used, but any two identical objects objects of about the right height may be used, such as pieces cut from aluminium or steel angle, large hexagonal nuts or large rollers from a scrapped roller bearing. One is placed on the limb of the sextant at zero and the other at about 90 degrees. The index arm is then rotated until a reflected view of the second vane is seen alongside a direct view of the first, when the mirror is adjusted to bring their tops into line as shown. In many sextants, including this one, it may be necessary to remove the telescope and/or index shades to obtain the required view.

Figure 7: Adjusting index mirror for perpendicularity.

Figure 7: Adjusting index mirror for perpendicularity.

When adjusting the horizon mirror to remove index error, the screw arrowed in Figure 3 is slackened and a tommy bar used in the hole visible on the right in Figure 7 to rotate the whole base. This is a relatively coarse way of adjusting and may involve much trial and error, but once done, the whole set-up is rigid and not likely to drift out of adjustment in a way that is so annoying with plastic “instruments”.  Removing side error has already been mentioned in the paragraph following Figure 5. Note that index error cannot be removed by using the sun, as the single horizon shade is not dense enough for this method. There is no adjustment available for collimating the telescope, but quite large errors of collimation have relatively little effect on the accuracy of readings, especially for the class of sight likely to be made with this instrument. In any case, this is taken care of at manufacture and would require very rough handling indeed to disturb.

The micrometer mechanism is robust and well-protected. Figure 8 shows it detached from the index arm. The black release catch on the right in fact remains stationary when disengaging the worm and it is the horn extending down and to the left  on the plate that rotates when it and the black catch are squeezed together.

Figure : Micrometer mechanism detached from index arm.

Figure 8: Micrometer mechanism detached from index arm.

In Figure 9, the front plate which carries the fiducial lines for the degrees scale and the micrometer has been removed to show the swing arm chassis. This carries the micrometer worm in a plain parallel bearing, the axial play of which is taken up by a leaf spring. A swing arm extends upwards and to the right to a bearing in the form of a shouldered screw, about which the chassis rotates. A stout helical spring keeps the worm in engagement with the rack machined on the edge of the limb of the sextant.

Figure : Front plate removed to show interior of micrometer mechanism.

Figure 9: Front plate removed to show interior of micrometer mechanism.

Figure 10 shows these parts more clearly. In addition, there is a rectangular keeper that guides the index arm expansion and keeps the worm in correct engagement. It slides in a slot machined in the limb below the rack.

Figure : Micrometer mechanism exploded.

Figure 10: Micrometer mechanism exploded.

A further, circular, keeper ensures that the swing arm chassis cannot lift off the face of the index arm expansion. The spigot on the keeper slides in the oval slot and the keeper is retained in the chassis by means of a screw whose tapped hole is shown in Figure 11, centre, which illustrates the bearing surfaces for the swing arm chassis. The keeper can be seen in place in Figure 3, above.

Figure : Swing arm bearings.

Figure 11: Swing arm bearings.

The sextant frame, being made of aluminium alloy, is inherently resistant to corrosion, but parts that do not run together have a tough coating of blue paint. Other parts are made of bronze and all the screws and springs are of stainless steel. If the sextant should receive a soaking, it is a simple matter to rinse it with fresh water and allow it to dry, as all the parts of the interior are accessible. Nevertheless, at overhaul it would be wise to use waterproof marine grease  for all moving parts except for the rack, which should receive SAE 30 lubricating oil, brushed into the rack with surplus being brushed and wiped off.

The case provided was, like so many other sextant cases over the last fifty years, made of plywood. Quite why the makers did not usually specify marine grade ply is a mystery, as many of them, including those from C Plath, suffered from delamination if stored damp. It was stored face down in the case, leaving the handle ready for use, but as it cannot be set down on a table face down, this is a limited advantage. Perhaps though, it was to discourage users from leaving it in a position on a table to slide onto the floor. The general rule is that a sextant should be in the user’s hand or in its case, relatively easy to follow on a yacht, but more difficult on the bridge of a large ship. All in all, this is a robust sextant, well suited to its task.

Dr Andreas Philipp writes that at least 900 of these sextants were made from 1968, starting with a serial number of 101. They were sold mainly in the USA.


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 (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

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.

Freiberger Drum Sextant (Trommelsextant)

10 08 2011

This post is preceded by one on the Freiberger Yacht sextant and two on the Freiberger Skalensextant

Readers looking for a manual that helps with maintenance and repair of the Freiberger Trommelsextant (drum sextant) will find my SNO-T Sextant Manual very useful, as the design of the one is based on the other. While the manual describes the SNO-T, it also gives an account of the Freiberger drum sextant where its design details differ. See under “The USSR SNO-T sextant” and “Buy”

The firm of Freiberger Präzisionsmechanik has been in existence since  late in the eighteenth century and by the 1870s had a large workshop employing over eighty people in the manufacture of surveying instruments. At the end of WW II it was overrun by  Soviet forces and dismantled, leaving only fifteen workers to carry out maintenance on surveying equipment. It was refounded in 1950 and in that decade the trommel sextant was developed. As far as I know, the firm had never previously made sextants.

The sextant is unusual in several respects. While the over-all shape and placement of shades and mirrors is conventional, it has a die-cast aluminium alloy frame, which combines lightness with a strength and hardness near to that of mild steel. The ladder or three circle patterns of its main competitors were ignored. Material is concentrated around the edges and the whole stiffened by a central web (Figure 1). The worm runs in a rack machined directly into the edge of the limb, thus avoiding the complication of attaching a bronze rack to an aluminium frame.  The substantial but unseen bronze worm seems to run very well against the alloy rack.

Figure 1 : Freiberger drum sextant, front view.

The very substantial index arm lies behind the web of the frame and is bridged by a casting to which is attached the handle (Figure 2). The upper end of the index arm is screwed to a large diameter bronze journal (the part that rotates in a bearing) that rotates in a bearing machined directly in the frame, thus abandoning the narrow, tapered journal and bearing in use since the third quarter of the eighteenth century. C Plath later dallied with such a bearing in their bronze-framed instruments, but they soon reverted to the tapered form.


Figure 2 : Freiberger drum sextant, rear view.

 The micrometer mechanism is concealed within an alloy casting attached to the lower end of the index arm. The cylindrical worm runs in eccentric bearings in a bronze casting that itself rotates against the force of a helical spring in bearings machined in the alloy casting. Thus, when the bronze casting is rotated, the worm swings out of engagement with the rack. The closeness of this engagement can be adjusted by means of a tangential screw whose head is just visible in Figure 2 to the left of the drum. While Freiberger chose to swing the worm out of the plane of the rack, nearly every other maker swung it out of engagement in the plane of the rack, following the pattern devised by C Plath in about 1907.  The latter method must certainly have been cheaper to manufacture, even allowing for the unecessary complexity of the worm shaft bearings in some pre-war marques. However, Freiberger’s method totally encloses the worm and solidly supports the shaft at both ends, so it is hard to imagine the shaft getting bent by an accidental knock, as had happened to at least three conventional instruments that have passed through my hands.

The sextant was usually provided with a 3½ x 40 Galilean telescope only. My own instrument has a 7 x 35 monocular, which gives a superior field of view as well as making the point of coincidence of the body with the horizon easier to determine. The vee and flat of the mounting are the reverse of all other makers, so their telescopes cannot be interchanged.

The USSR imitated Freiberger’s design in their SNO-T sextant, albeit in an instrument of slightly smaller radius and one provided with an unusually full complement of tools and spares. The edge of the SNO-T frame is  8 mm thick (compared to 3 mm), making it an even more rigid and robust instrument than the Freiberger. The bare sextants weigh 1300 and 1200 grams respectively.

British Admiralty Vernier Sextant

23 06 2011

Previous posts in this category cover:  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” and “A fine sextant by Filotecnica Salmoiraghi.”

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

Figure 1 : Inspection certificate of sextant number 222**

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

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

Figure 1 : Admiralty Pattern sextant and accessories

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

Figure 3 : Galilean star telescope

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

Figure 4 : Inverting telescope

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

Figure 5 : Checking squareness of face of telescope ring.

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

Figure 6 : Rising piece mechanism.


Figure 7 : Attachment of rising piece mechanism to frame.

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

Figure 8 : Extract from patent on sealing mirrors

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

Figure 9 : Horizon mirror mounting exploded.

Figure 10 : Rear of horizon mirror sandwich.

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

Figure 11 : Magnifier mounting

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

Figure 12 : Tangent screw mechanism in situ.


Figure 13 : Tangent screw mechanism exploded.

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

Figure 14 : Sextant in case

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

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.

A Fine Sextant by Filotecnica Salmoiraghi of Milan

5 10 2010

The preceding posts covers “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”  and “A Sextant 210 Years On”.

A little while ago, I felt privileged to be able to overhaul an uncommon sextant, made by La Filotecnica Salmoiraghi of Milan in Italy. A first glimpse of the case hinted that the instrument inside would be of very high quality, as attention to detail had been paid in making the case. A heavy cast brass latch and two chromium plated brass hooks held the case closed, the slots in the screw heads that held the top of the lid in place were all lined up parallel to the edges and the hinge was a piano hinge extending nearly the whole breadth of the case. As final details, the brass key bore the maker’s initial (Salmoiraghi) and a brass plate was let into the front for the owner’s name.    

Figure 1 : Exterior of case

  La Filotecnica was founded in 1865 by Ignazio Porro ( 1801 – 1875), the inventor of the prismatic binocular. Porro served in the Italian Army before retiring with the rank of major, in 1842. He worked in Paris for about fifteen years, producing a variety of surveying and optical instruments before returning to Italy in 1861. He founded the Officina Filotecnica at first in Turin and then, in about 1865, in Milan. The factory combined production with what we would probably call nowadays a school for apprentices. Porro seems to have had a poor head for business and in 1870 one of his pupils, Angelo Salmoiraghi (1848 – 1939), took over the ownership of the business after a short period in management. Under Salmoiraghi as La Filotecnica Salmoiraghi,  the firm became one of Italy’s leading producers of optical and precision instruments. Today, the firm, trading as Salmoiraghi e Vigano, manufacturers principally spectacles and markets optical goods made by other firms..    

Figure 2 : Name plate.

Opening the case reveals a clever way of  retaining the sextant in the case. Unusually, the instrument can be placed directly in the case without letting go of the handle. A further very practical aspect to the design of the case is that the sextant can be replaced with the telescope in place. Closing the lid brings pads of very fine brown felt into play to secure instrument and accessories securely in place. A finely crafted stand receives the sextant, location being ensured by large wooden spigots that fit into the circles of the frame, while the telescopes fit into well-thought-out pockets (Figure 3).    

Figure 3 : Contents of case

  The mirrors, shades, the index arm and its bearing are all conventional. The horizon mirror is fully silvered so that the horizon is viewed viewed directly, without passing through an unsilvered part of the mirror. This would be a disadvantage with a Galilean (so-called “star”) telescope, in which each half sees its own field of view, so that without reflection from the clear glass of a half silvered mirror there is little overlap of the direct and reflected images. However, this sextant is provided only with a 6 x 30 mm prismatic monocular and a 10 x 17 mm inverting telescope, both Keplerian in principle, so that there is very adequate overlap of the images. There is a sighting tube for use when surveying or taking sights in heavy weather. In what follows, I will describe only what is unusual or unconventional.    

Figure 4 ; Frame of sextant.

The bronze frame (Figure 4) is of about 162 mm radius and is of three-circle pattern. It resembles superficially the three-circle instruments of Hughes and Son (see my post of 18 Feb 2010) and of C Plath (see my post of  24 Jan 2010) but there are differences in detail. It is heavier than other frames of its radius, weighing in at a hefty 750 G compared to the Hughes frame of 600 G. Sailors who like a bit of “heft” in their sextants will not be disappointed by the Filotecnica as, equipped for viewing with the monocular, it weights 1750 G (3lb 14oz) versus the Hughes at 1500 G (3lb 5 oz). Both are equally and very resistant to flexing. With the exception of the arc, which is chromium plated, the face of the frame is painted semi-gloss black, while the rest is painted in a black wrinkle finish. The index arm bearing is attached with screws from the back.    

In a general view of the instrument (Figure 5), one is immediately struck by the length of the micrometer shaft. Unlike almost every other sextant of this pattern, the micrometer worm is not tapered. While it is easier to make a cylindrical worm, the penalty is that either the micrometer drum has to be smaller to fit beneath the curve of the limb or the shaft has to be longer and therefore more vulnerable to damage.    

Figure 5 : General arrangement, front view.

 The worm shaft runs in plain parallel bearings mounted on a swing arm chassis, which pivots around the ends of two cone-pointed screws attached to the back of the index arm expansion (Figure 6). These latter screws can be adjusted to remove all end float and then locked in place. In this instrument, the release catch swings the worm at right angles to  the plane of the arc (Figure 7), in contrast to most other sextants, which swing it out of engagment parallel to the plane of the arc. There is no  adjustment for end float of the micrometer shaft, however, and absence of float, equally as undesirable here as in the swing arm chassis, depends entirely on skilled fitting, rather than using a preload spring like most other instruments .

Figure 6 : Micrometer mechanism


Figure 7 : Swing arm detail

Figure 8 shows how thrust in either direction is received on collars on the shaft and the inner ends of the plain bearings. Upon testing by directing an autocollimator on to the index mirror and approaching zero with the micrometer reading increasing and  decreasing, I could detect no difference in the autocollimator readout, indicating a complete absence of measurable backlash (unwanted longitudinal shaft movement) with a precision of better than 1 arcsecond. This absence of backlash, besides indicating immaculate fitting of the shaft and bearings, is of course necessary to make meaningful the non-adjustable error readings shown in Figure 9. Almost needless to say, these are quite exceptional results for a micrometer sextant of 1948 date. The pitch of the worm is 1.4 mm giving a pitch circle radius for the rack of (1.4 x 720)/2 pi = 160.43 mm.

Figure 8 : Micrometer worm shaft bearings.

Figure 9 : Chart of non-adjustable errors

The method chosen to mate the telescope and sighting tube to the sextant seems somewhat over-engineered. The tubes are fitted with  heavy brass collars that have  coarse interrupted threads on the outside. The rising piece has the usual arrangement of a tiltable bush for collimating the telescope, with the addition of a large captive nut having a female interrupted thread on the inside. A stout pin projects from the face of the bush to locate in a hole on the telescope collar, so that the thread starts correctly and attaches the ‘scope to the rising piece with one sixth of a turn. Figures 10 I hope make this clearer.

Figure 10 : Telescope attachment

But there is further complication in the telescope bracket. It has a female vee into which the vee on the rising piece fork fits and a thumb screw to lock it into place, but there is also a captive thumbscrew whose thread passes through a short rectangular nut, to which is attached a leaf spring. In the end of the leaf spring is a hole that engages with a short nib on the rising piece. The whole is intended to act as a slow motion adjustment for the position of the rising piece (Figure 11). I have seen a somewhat similar slow motion adjustment on a 1940s Tamaya that is an almost exact copy of a pre-war C Plath instrument and may even be a C Plath, sold to have a Tamaya trade mark added. Most post WW II sextants of course managed very well without this complication.

Figure 11 : Exploded telescope mounting

The whole arrangement strikes me as being at the same time clumsy and yet an elegant solution to a problem that did not really exist. It must surely have been cheaper simply to provide each scope and tube with its own rising piece, manufactured so that no collimation adjustment was necessary, as was done for the prismatic monocular (Figure 12). This could surely have posed few problems for a factory capable of achieving the almost impossibly precise fit of the micrometer shaft bearings.

Figure 12 : Monocular and its rising piece.

 The battery handle also shows an elegant and complex solution to a relatively simple problem (Figure 13). The positive pole of the single AA cell makes contact with a screw head to which is soldered the wire to the lamp. The negative pole makes contact with the lid of the compartment via a spring. The hinge of the lid is attached via a tongue let into the side of the handle and the tongue is prolonged as a piece of brass rod with a switch contact on the end. A second piece of brass rod (shown dotted in brown) with a similar contact passes through a hole from above and a single screw that attaches the upper pillar passes through a threaded hole in the rod. When the switch button is pressed, the contacts are bridged and current flows from the negative pole to the frame of the sextant, completing the circuit to the lamp. A second screw , which does not form part of the circuit, passes through the handle into a threaded hole in the upper pillar base and the head of the screw forms the third point of rest when the sextant is set down with its face up. How the very narrow slot for the tongue that attaches the lid was made still eludes me.

Figure 13 : Battery handle

The over-all impression is of a very finely made instrument having some unnecessary complications. It cannot have been cheap to produce, but professional mariners continued to pay a premium for perceived quality even when cheaper instruments able to do the job just as well became available.

If you enjoyed reading this post, you will very likely enjoy reading my book, which goes into similar detail about the structure of The Nautical Sextant.