The Nautical Sextant

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.

UPDATE

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.