Overhaul of Mk V/ AN5851 sextant bubble chamber

20 12 2008

This post is preceded by “Bubble Illumination of Mk V and AN 5851 Bubble Sextants” and “Refilling Mark V and AN5851 Bubble  Chambers”

The preceding post dealt only with refilling a bubble chamber that was otherwise clean and tidy. This post covers dismantling for cleaning and refitting the glasses. You will need an electric iron, a pencil, a screwdriver, some flake shellac (from a decorating shop or French polisher), some xylene, a syringe (preferably glass) and needle or a fine pipette.

Start by removing the bubble chamber assembly from the sextant as in the previous post. Remove the taper plug. Remove the air chamber (4 screws from underneath), carefully preserving the gasket if you can. The control unit unscrews by using a wrench on the octagonal metal rim. At this point, it is wise to be wearing an apron with its skirt clipped to your bench or table, as there is a ball bearing with a hole pierced through its centre just  waiting to fall onto the floor and be lost for ever. Dont let this happen to you. It is difficult to drill holes through small balls. Empty the control unit into the bubble chamber by repeated pressing and shake out as much xylene as you can. You should now have reached the state shown in the following photograph:


Also remove the four tiny countersunk screws on the underside and carefully prise the illuminating annulus free. The paint beneath is radium paint and, while it no longer glows in the dark, the radioactivity from the contained radium is still present and is best removed, before replacing it with modern photo-luminescent paint.

See https://sextantbook.com/2008/12/09/bubble-illumination-of-mk-v-and-an-5851-bubble-sextants/  for some advice on how to do this.


Meanwhile, your electric iron, carefully secured in some way with the sole upermost, can be warming up. The maximum setting seems to be just the right temperature. For those lucky people with laboratory hotplates, the temperature of my iron cycles between 160 and 182 celsius. Place the stripped down bubble chamber on the hot iron and do something else for five minute while it heats up and softens the shellac that is holding everything in place. You can check whether it is hot enough by touching the chamber somewhere with a flake of shellac to see whether it melts.

When the right temperature  has been reached, hold the chamber in one hand with a thick towel or heavy leather glove and unscrew the bottom retaining ring. For this you can use the correct tool, a pin wrench, or a piece of 1.5 mm metal strip previously filed to fit the slots. Then rap the chamber sharply on a wooden bench and with luck the bottom glass will fall out. When your hand has stopped hurting from holding a very hot object,  turn over the chamber and unscrew the other retaining ring. While everything is still hot, push out the top lens with a pencil from underneath.

Allow everything to cool down and then soak the glasses and retaining rings in alcohol while you chip away as much old shellac as you can from the chamber body. If you are patient, you could instead soak all the parts overnight in alcohol and rinse in the morning with clean alcohol. Everything should now look something like this:


There is a very tiny passageway between the air and bubble chambers and you should make sure that this is perfectly clear of shellac, as it enter the bubble chamber partly under the bottom glass and can easily be completely blocked by stray shellac.

Return the bubble chamber to the hot iron with the top of the chamber uppermost and when it is hot enough, carefully apply a ring of shellac to the flat lens seat. Do this by picking up flakes about 5 to 10 mm long in a pair of fine forceps and smear as you dab. When the ring is complete, align the etched line of the top glass with the fore and aft line of the chamber (the bubble control is aft and the air chamber to the left) and drop the lens into place. You will get perhaps one chance at realigning it if you get it wrong, before the glass gets too hot to touch. The lens goes in convex side up. The concave side has a flat annulus ground on it and this goes downwards.

The official manual instructs one now to apply pressure to the lens with a jig until the shellac has cooled somewhat, when the retaining ring is screwed into place. If you have the means, you can improvise a jig like mine, shown in the next photograph, or use a Mark I pencil as shown in the one after,  to apply a little downward pressure to make sure there is a complete ring of shellac beneath the glass. Having tried both on a variety of bubble chambers, I now use a pencil and screw down the retaining ring while everything is still nice and hot, and I bet the WW II technicians did too.



Allow the chamber to cool down and check the inside for loose flakes or little teardrops of shellac on the underside of the top glass. The latter can be chipped off easily and the job completed with a wipe of alcohol. The cleaner the inside is, the better the bubble will move and the clearer the view, so do it now, as you won’t be able to after the next step, which is to fit the bottom glass, following the same procedure as for the top. When applying the bottom ring of shellac, take special care to keep it away from the mouth of the capillary passage to the air chamber, but make sure there is some more peripherally to complete the seal.

When everything has cooled down again, chip away shellac from the outside of the glasses, clean up and refit the control and air chamber, the reverse of dis-assembly. Take care that the hole through the ball lines up with the passage into the bubble chamber from the control.

The last photograph actually shows an A10 vapour pressure chamber being resealed. The original seals were lead washers and don’t work for me when re-used. You can mess about with O rings that cause alterations in the optical path length and sometimes dissolve in xylene, or search for the ideal material with which to replace the lead washers, or you can make a really good job of it with shellac.

I hope to have an e-book overhaul manual for the A10/10A ready by the end of February, 2009. Meanwhile, you could enjoy reading The Naked Nautical Sextant and its Intimate Anatomy (but first you have to buy it).

Refilling Mark V / AN5851 bubble chambers

18 12 2008

This post is preceded by “Bubble Illumination of Mk V and AN 5851 Bubble Sextants”

Mark V is the US Navy designation for the AN5851 sextant. Early models were fitted with vapour pressure bubbles, which are hard, but not impossible to refill. This post will deal with only the somewhat easier air bubble models, which are distinguished by having a cylindrical air chamber projecting from the top left hand side of the chamber and, because I want at least to start to answer a request for details before Christmas intervenes, I’m sticking to the simplest case. This is where there is a bubble of some sort present and the fluid and glasses appear clear. If there are particles floating around in the fluid, or it has completely dried up leaving a powdery film inside the chamber, a full overhaul will be needed.

At the front of the chamber, you will see projecting a peg about 2 mm in diameter. This is a tapered pin that occupies a tapered hole through which the chamber was filled. To remove it, remove the chamber and eye lens assembly as described in the previous post. Then grasp the end of the pin in a pair of pliers and pull directly in line with it while twisting. It should come out without too much trouble, unless you pull and twist out of line, when it may break off and leave you in deep trouble, the solution for which will be covered in a future post.


Once the pin is out, you can refill the chamber with xylene using a hypodermic syringe and needle, if you can locate a glass syringe. You can use a disposable plastic one, but the xylene attacks the plastic and the syringe rapidly becomes very stiff to operate. An alternative is to use a glass pipette with the end drawn out to a fine tube of about 1 mm diameter. This is what I now use. Some decorating shops stock xylene in bulk and may be prepared to sell you 500 ml or so. It will last you a long time. An alternative suggested, with which I have no experience, is Zippo or Ronsonol cigarette lighter fluid. You should not use alcohol, as the glasses  have been sealed with shellac, which is soluble in alcohol.

When the chamber is full, temporarily replace the taper pin with a pushing and screwing motion,  hold the chamber with the air chamber downwards and press repeatedly on the control knob until no more air comes into the bubble chamber. This displaces air from the bellows into the bubble chamber. Then refill the bubble chamber . At this point you can  replace the taper pin using a firm pushing and screwing motion. Do not tap it in with a hammer. It may be necessary to experiment a little to get the right amount of fluid. It should be possible to remove the bubble altogether, so that sights using the natural horizon can be taken. If the bubble cannot be entirely removed, remove the taper pin and add a little more xylene.

To empty the air out of the bubble chamber to reduce the size of the bubble, hold with the air chamber upwards while pressing repeatedly on the bubble control knob until you have a bubble of the desired size. Pressing forces air into the air chamber through a tiny capillary tube and when you release, xylene passes into the bubble chamber. To get a larger bubble, hold with the air chamber downwards while operating the control. 

If you cannot get a bubble at all, the air chamber may be completely full of xylene and you will have to remove a little, the capillary passage between the bubble and air chambers may be blocked by debris( clear with a piece of fine wire), or the hole in the ball that seals the bellows unit to the bubble chamber may be misaligned (see next post).

Do let me know if you find this post useful and ask if something is unclear. Consider, too,  buying yourself a gift of The Nautical Sextant…

Bubble illumination of Mk V and AN 5851 bubble sextants

9 12 2008

A recent posting to the Sextants@yahoogroups.com asked about renewing the illumination of  the US Navy Mk V bubble chamber. I think I am correct in saying that this is identical to the AN 5851 chamber illumination. A little oddly, there was no attempt to have electrical illumination. Instead, self-luminous radium paint was used. Now radium has a half life of nearly 1600 years, so plainly it is not the radium that has decayed, but the luminescent compound mixed with it. Replacing the paint with a modern photoluminescent paint is not difficult, but  some minimal commonsense precautions should be taken to avoid swallowing or breathing in the old paint.

The bubble chamber, incidentally, is one of the easier ones to service, as the manufacturer sensibly provided a plug in the form of a taper pin to allow refilling. The top and bottom of the inside of chamber need to be scrupulously clean before refilling and this is not so simple.

Let us assume, however, that you have a bubble in a chamber that is free from debris. The first step is to remove the chamber together with the eyepiece prism by removing the three screws shown in the following photograph:


Removal of the eyepiece prism is completed by inverting the bubble chamber and removing two more screws, as shown in the next photograph:


The radium paint is contained beneath the black washer-like disc that is secured by four tiny countersunk screws. Removing these allows it to be carefully prised from its seat. The paint can then be removed from the underside of the washer and from the underside of the bubble chamber bottom glass by softening  and dissolving the paint with acetone. Certainly you should take every precaution not to inhale or swallow any. How you dispose of the paint is up to you. The level of radioactivity is low and there is only a small quantity, so disposing in the sea is unlikely to do any harm to anyone.  A local university physics department or the radio therapy unit of a hospital might be prepared to advise.

Replace the paint on the underside of the washer with, first, a layer of white paint and then a layer of modern photoluminescent paint. A layer of the latter (but not the white paint) can also be applied to the bottom glass beneath the washer – the thicker the layer of the luminescent paint, the better the illumination. Re-assembly is the reverse of dis-assembly.

To charge the photoluminescent paint, shine a bright light, say, a 40 watt bulb, onto the index prism for 5 minutes or so. How long the light lasts depends on how long you charge the paint for and on the grade of the paint.

You might wonder whether overpainting the radium paint with a paint containing copper-doped zinc sulphide would renew the radioluminescence. The answer seems to be “No” because the alpha particles emitted by the radium penetrate poorly and the ZnS.Cu needs to be mixed intimately with the radium.

Post me a comment if you would like me to deal with cleaning and refilling the bubble chamber. It is not as simple as replacing the paint, but needs no very special tools, notwithstanding the instructions given in the original overhaul manual.


2 12 2008

Nearly all sextants were made right handed, that is to say, the instrument was held in the right hand and the adjustments to the index arm made with the left hand which, for ninety percent of us, is the non-dominant hand. Furthermore, for some strange reason, the legs were usually placed on the same face of the instrument as the handle, so that to set it down safely on its legs, the frame had first to be grasped in the left hand. As far as I know, only Brandis and Sons, Inc. made a left handed sextant, and only that firm and the French firm of Le Petit-Poulin fitted the legs on the opposite face to the handle.

The Brandis Aeronautical Octant Mark I, Models 3 and 4, were sextants (strictly speaking, octants) that were left handed and had the legs on opposite sides to the handle, so that the instrument could safely be set down without juggling it from one hand to the other. Moreover, the ivorine note pad on the handle was accessible to right-handers. Hughes and Son sometimes also provided  note pads on the handles of their higher-class sextants , but they were of course only of use to left-handers.

A Brandis Aeronautical octant recently came into my hands. It is the 125th example of Brandis’s Model 206C. The Smithsonian Museum has two, one of which has the higher serial number of 134, but it is unlikely that more than two hundred were made. It also has a Bureau of Aeronautics number of 33-31, dating it to 1931. It is essentially a small left-handed nautical micrometer octant of 130 mm radius fitted with a Willson bubble telescope and also provided with a plain sighting tube and eye shades.


The expanded ends of the legs slide into fittings in the floor of the case and a wooden piece is slid across to prevent the top  leg from sliding out. Another interesting feature of the case is that it has a loose sliding lid which was held closed by two hooks and eyes and a lock. No doubt this was an annoyance to the aviator, who would have to find somewhere to put the lid while extracting the instrument.

Although C Plath had started to make micrometer sextants in about 1906, it took other manufacturers until the mid 1920s to follow suit. The micrometer screw is short, having only five threads and is of relatively large diameter, while the rack is rather narrow. The micrometer drum reads to single minutes and has no vernier. The release catch operates a cam that presses on the swing arm chassis to disengage the micrometer screw from the rack. The index arm keeper is a slender piece of bent brass that would certainly suffer if the sextant were to have been picked up by the index arm.


The mirrors and shades are, unusually for Brandis, of conventional design, while the collimation adjustment of the rising piece follows Brandis’s standard pattern. The handle has a battery compartment and a switch that supplies current to a lamp for illuminating the scales. The articulated arm carrying the lamp is an adaptation of the arm used to carry the magnifier in earlier, vernier sextants by Brandis. There are also two jack sockets, one of which provides unswitched current to the Willson telescope and the other of which was presumably to receive power from the aircraft or an auxiliary supply, as inserting a jack plug breaks connection to the battery in the handle. The handle easily accomodates a single AA battery, provided it is steadied by packing pieces each side. I have added the bottom brass plate to replace a badly corroded contact

MkI handle

Of interest perhaps only to air navigation enthusiasts is the Willson bubble telescope. It is a two-power inverting telescope of 18 mm clear aperture, with a Ramsden eyepiece. Between the field and eye lenses, a partially reflecting mirror provides a view of the bubble in the chamber lying above it. The chamber can be adjusted up or down to focus on the bubble and it is illuminated from above by a frosted lamp shining through an iris diaphragm and diffuser. A rheostat and on-off switch is built into the lamp housing. It is far from easy to remove the lighting assembly, so I am forced to the conclusion that no provision was made for illuminating the bubble by daylight. When used as a bubble sextant, the telescope would be used close to the frame, to intercept nearly all the light from the index mirror, but there is sufficient travel of the rising piece for the telescope also to function well in the normal, nautical mode.


If you would like more details of this instrument do contact me. You will find much more about the nautical sextant in general in my book “The Nautical Sextant.

Update 6 February 2017: At least one early Coutinho-pattern aeronautical sextant was made left handed. It is in the Museu Marinha in Lisbon.