The Nautical Sextant

The United States Navy Bureau of Ships was founded in June 1940, tasked, among other things with the …”design, construction, procurement and maintenance …” of naval ships and their equipment. One item of equipment that was needed in large numbers was the nautical sextant. The BuShips Mark II seems to have been founded on a pre-Second World War model, which in turn owed much of its design to Brandis and Sons. The latter traded under various Brandis names from 1871. The Pioneer Instrument Company gained a controlling interest in 1922 and was itself bought out by the Bendix Aviation Corporation in 1928, though sextants continued to be made under the Brandis name until 1932.

At the onset of the War, contracts were let to at least three manufacturers to produce tens of thousands of sextants in a relatively short time, using modern mass-production methods. The Pioneer Instrument Division of the Bendix Aviation Corporation was probably the design leader, with the David White Instrument Company of Milwaukee and Ajax Engineering of Chicago also producing instruments that showed minor variations on the main design.

Figure 1 : Ajax Engineering Mark II Sextant

All had pressure die-cast aluminium alloy frames with bronze rack attached in a variety of ways: Ajax managed to cast the rack integrally with the frame, Pioneer-Bendix attached it with radial screws and dowels, necessitating a wider rack casting, while David White attached it with screws inserted from the back of the frame, a practice continued with the Mark III successor (see my post on Evolution of the Sextant Frame). There seems to be no special advantage to the adoption of a composite frame, as a bronze worm running in the alloy of the frame itself seems to have worn very well in post war sextants. There may have been production advantages, as an annular rack blank could have been mounted on a hobbing machine to cut the teeth and then cut into four racks, though this does not seem to have been the method used to produce racks for the Mark III, which were fitted to the frame before cutting the teeth. The alloy frame had the advantage of rapid production, high strength, similar to that of mild steel, good stability, cheapness and light weight.

The design of the index arm bearing is conventional, in the form of a tapered hard bronze shaft in a brass bearing. This self-centring bearing arrangement is unchanged from the form originated, probably by Jesse Ramsden, in the eighteenth century and used by nearly every sextant maker since. At the other end of the index arm, however, the micrometer worm has for some reason a left hand thread, so that when turned clockwise by the user, the reading descends (Figure 2). It is hard to see any advantage in this and cutting left hand threads is slightly more difficult to achieve.

Figure 2: Mk II Micrometer drum

Pioneer-Bendix and Ajax bury the complex little release catch mechanism between the index arm and the swing arm chassis, making its construction hard-to-fathom. For the un-initiated, I have covered the details in The Nautical Sextant. The axial pre-load spring is L shaped and the upright of the L is forked to surround the worm shaft and press on a shoulder in front of the worm. A conventional leaf-spring is used to keep the worm in contact with the rack (Figure 3). David White, by contrast, bury the latter spring between the index arm and the swing arm chassis, while the cam of the release catch is clear to see. Axial pre-load is applied by a leaf spring bearing on the end of the shaft and White introduced the complication of an adjustable split bearing for the swing arm chassis (Figure 4). White used two conventional keepers to keep the index arm in place while the other makers took the short cut of using a single shouldered screw.

Figure 3 : Pioneer-Bendix micrometer mechanism

Figure 4; Layout of David White micrometer mechanism

There are two striking  features of difference  when the sextant is compared to most of its contemporaries, particularly German ones and their Japanese followers:  the clear aperture of the telescope objective lens is a mere 15 mm, recalling the rather inadequate Galilean telescopes of eighteenth and early nineteeth century sextants, and the mirrors are small to match. The magnification was a nominal x 3.  The aperture of the silvered part of the horizon mirror is only18 x 16 mm or 288 square mm, exactly the same as its Brandis ancestor’s, compared to its Carl Plath contemporary,which had a silvered area more than four times as great. There is of course no point in having the light gathering power of large mirrors unless the telescope aperture is large to match.

As navigators use for the most part the sun, moon and navigational stars, only one of which has a magnitude greater than 3 (Acamar), light loss through the telescope may seem unimportant. However, accuracy of sights depends quite heavily on getting a clear view of the extended object of the  horizon, and in marginal conditions, light grasp is important. While the human eye-brain combination can only  just notice a halving in brightness of light, ability to perceive contrast is very much influenced by the relative light levels of the horizon and sky. Why then was the telescope designed as it was (Figure 5)?

Figure 5: Mark II sextant telescope exterior

The specification may have called for a telescope giving an erect, wide field image of x3 powers and may not have emphasised brightness of image. A Galilean telescope can easily be designed to give a bright, erect image magnified three times, but the field of view is constrained by the diameter of the objective lens. Unprepared as the USA was for war, it may be that all suitable large diameter objective lenses were reserved for binocular production, even though here there were bottlenecks in the production of good quality prisms. In Germany and Japan at the time, Plath and Tamaya were producing sextants having 3 power Galilean telescopes with 40 mm diameter objectives. Even so, the usable field of view was only about 5.7°, compared to the 8.6° f.o.v. of the Mark II telescope. Inexperienced navigators sometimes struggle to find and keep in sight heavenly objects, particularly stars, when in any sort of seaway and the wide field of view may well have been regarded as a good trade for some brightness.

The lens plan shown in Figure 6, which gives approximate focal lengths and separations, illustrates that there was plenty of scope for light loss, as there were seven lenses, each with two air-glass interfaces at which light could be lost by reflection (about 8 percent per surface) and contrast lost by scattering of light.

Figure 6: Telescope lens plan

Figure 7 shows some of the elements of the ‘scope’s construction. A two-lens objective group screws into one end of the front tube and the reverting lens group into the other end. A thread on the other end of the reverting group joins the front and rear tubes. All the threads are locked by radial screws.

Figure 7 : General construction of Mark II sextant telescope

Would-be restorers of these venerable instruments may find that the focussing eyepiece is seized or has received the attention of water-pump pliers or a vice in an attempt to get it moving. In the first instance, I suggest that some releasing compound is used and left to work overnight in a warm place. If the eyepiece will still not rotate, forcing it is liable to shear the slender screw that passes through a clearance hole in the thimble and through a tapped hole in the slide into the eyepiece assembly (Figure 8). It is best to remove this screw if at all possible before donning rubber household gloves to enhance one’s grip and pulling while at the same time twisting. The eyepiece lenses may then be removed, if necessary, in order to clean them by wiping from side to side with lens paper moistened with a little alcohol.

See Update at end of post.

Figure 8: Mark II sextant telescope eyepiece assembly

A final quirk of the Mark II design is in the method of adjusting the horizon mirror for side and index error (Figure 9). Nearly every other maker had adopted the simple mechanism for adjusting mirrors devised, or, at least, first described by Peter Dollond in 1772, of applying adjustable screw pressure to the back of the mirror at two out of three  points, opposed by springs. Brandis, however had early adopted a relatively complicated method of levers adjusted by opposing screws, that was not only difficult to adjust, but also easy to knock out of adjustment. This method was carried over into the Mark II.

Figure 9 : Mark II horizon mirror adjustment.

The horizon mirror is carried on a bracket that pivots as a second class lever  in the region of the mirror’s base, while the adjusting force is provided by two screws, one of which, as it were, pushes the lever, while the other pulls. When both are tight, the mirror is locked, albeit uncertainly, in position, but just to make sure of it, a screw passes through the pivot into the mirror bracket.

The mirror bracket itself is carried on another bracket that I have labelled “index error bracket,” as this tilts the mirror bracket in the plane of the sextant frame. It uses the same potentially unstable lever system, except that the lever arm is four or five times longer and much less stable, again requiring a screw through the pivot . The whole is mounted on a cast bracket that is screwed to the front edge of the sextant frame and perhaps the best that can be said of the set up is that it simplifies the attachment of the index and horizon shades in the Pioneer Bendix model. In this sextant there is an upstand at each end of the base bracket for attachment of the shades, though the Ajax model discards half of even this slight advantage by having a standard bracket attaching directly to the sextant frame for the index shades.

In sum, the Mark II sextant design, compared to German and Japanese instruments of the same period, was far from the pinnacle of perfection. Perhaps, like the camel, it was designed by a committee.

Addendum In response to a request by a reader I have added some pictures of the cases. Only Ajax used solid wood. The others used plywood, which has not stood the test of time, since it was not of marine grade. The White case has almost disintegrated and the bottom of the Bendix case has had to be replaced.

From top to bottom, cases by Bendix, Ajax and White, front view

From top to bottom cases by Bendix, Ajax and White, three-quarter view

Update 16 July 2017: There are at least two different approaches to the focusing assembly of the telescope, though both have a sliding piece in a part helix. In one pattern of telescope, the second lens from the left replaces an achromatic lens with a single, symmetrical biconvex lens. There were a couple of errors in the original drawing, with two of the lenses back to front. I have now corrected these.