The Nautical Sextant

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” ,  “Overhaul of MkV/An5851 bubble chamber” ,  “AN5851-1 : jammed shades carrousel” ,  “A Byrd sextant restored” and “Update on Byrd Aircraft Sextant”

A little while ago on e-bay I saw an adaptation of an A10-A bubble unit to a nautical sextant fail to reach its reserve at over $300, even though it was offered with a copy of my overhaul manual for the A10 series aircraft sextant. I recalled that a couple of months previously, I had made a very similar adaptation for a friend who lives in Paris, where natural horizons are not easily to be found. Since my means are relatively limited, I am always looking for ways of paying for my addiction to nautical sextants, so I decided to make another and this time to offer it for sale on the internet.

Most aircraft bubble units are of Second World War vintage, and after sixty five years, the fluid has leaked out of nearly all of them. The exceptions in my experience are the British Mark IX series, which were sealed with shellac and solder. US instruments sometimes sealed the glasses with shellac, but closed the filling hole with a taper pin or, as in the case of the A12, a ball bearing forced down upon its seat with a grub screw. Others used seals of lead or plastic and almost without exception, they leaked sooner or later. In the case of the A10A bubble unit, there were no fewer than six places where it could leak: two holes sealed with taper pins, one for filling and the other to allow a passage to be drilled btween the bubble and reservoir chambers, the top and bottom glasses, the joint between the diaphragm and the body of the unit and the joint between the reservoir and the body of the unit.

It is not possible to re-seal the A10-A units with shellac without damaging or destroying the Lucite illuminating ring. O rings had been patented by Niels Christensen in 1937 and during WWII the patent was taken over by the government in the national interest, but, curiously, did not find their way into sextant bubble units. It may be that, as most of them were filled with xylene, the elastomeres of the day were not equal to the task, but the A10-A units were, according to the official overhaul handbook, filled with relatively benign alcohol, just like the units in the German SOLD sextants and the later Russian copies of the SOLD. Although I have resealed units using home-made lead washers, it is much easier to remove the old seals and replace them with standard O rings if re-filling with alcohol or with Viton (fluorocarbon) O rings if using xylene.

So, having cleaned a bubble chamber and  resealed it with O rings I addressed the matter of attaching it and its optical attachments to a nautical sextant. Figure 1 shows the light path.

Figure 1 Light path through unit

The bubble lies at the focus of a spherical mirror, so that the rays that make up the image of the bubble reflected from the mirror are parallel and the bubble appears to be at infinity. These rays are intercepted by a partially reflecting surface or beam splitter and diverted into the eye. The eye also sees the image of the heavenly body, whose light rays, also apparently at infinity, pass straight through the beam splitter, so the images of the bubble and the object can be superimposed by adjusting the sextant. In daylight, the bubble is illuminated by the light from the sky and at night by a lamp that conducts the light through a Lucite strip that surrounds the top glass. Providing that the reflected rays from the spherical mirror are at right angles to the plane that contains the bubble, a line of sight through the centre of the bubble will always be horizontal. The mirror mounting allows it to be adjusted to this condition, and I give full details in my restoration manual. Providing it is collimated in this way (from the Latin collimare, which would have meant “to put in line” if a medieval scribe had not mis-copied collinare) it can be mounted on the nautical sextant without further adjustment.  A small index error may remain and have to be determined by observations from a known position.

The unit is attached to the sextant by a rising piece that I make using a shaping machine, the machine tool par excellence for cutting one-off vee ways. Rather than drill more holes into the unit, I removed the shouldered screw that held the shades and the top of the two  screws that limited their movement. I cut off the bottom screw short and used it to blank off the hole. I drilled out the holes and tapped them 4 BA. It is as well to dismantle the unit completely to avoid damage to internal parts when doing this. Instructions for dismantling are again given in my manual.

In day time the bubble is illuminated from above via a ground-glass diffuser screen that can be moved aside to view the bubble when adjusting its size. At night, a tiny bulb throws light onto the ends of a Lucite (UK : Perspex) strip that surrounds the top glass and the light is conducted around by total internal reflections. These bulbs are becoming hard to find nowadays, so I have experimented with using  a high-intensity red light emitting diode instead and it works quite well. The main difficulty with the adaptation is in reducing the diameter of the LED to fit the existing fitting. It is relatively simple to solder the LED to the base of a defunct bulb. The brightness of the lamps, incandescent or LED, is controlled by a potentiometer in the battery box. Incidentally, the Lucite strip does not seem to make a lot of difference to the quality of the lighting if for some reason it disintegrates or has to be dispensed with.

Here is another view of a bubble unit, from the rear of the sextant:

Figure 2 Rear view of unit