Henry Hughes and Son's Mk IX Sextant

The RAF took into service in 1938 the latest sextant designed by P.F.Everitt of Henry Hughes and Son. They were already makers of the RAF Mk VIII and the new model was given the title Mk IX.

Mk IX Types

When originally introduced the Mark IX had no means to suspend it and take the weight off the navigators arms, as late as 1941 they were still unsuspendable but a modification was introduced to attach a spring hook that would engage with a ring in the astrodome. From the A model onwards these clips were a designed feature and the bracket addition was no longer required. The clip was later replaced by a "pigs tail", presumably for ease of use with gloved hands and incidentally to save on costs.

Suspension Bracket

The Mk IX with it's very quirky shape is immediately recognisable and can be found at almost any time on Ebay selling from as low as £30 to as much as £120. There are several points to watch out for when buying one. The bubble chamber fluid is a Hexane Paraffin mixture and is highly volatile and it may well have been in situ for 60 years. From my experience, as many as 60% of those sold on Ebay have leaking or empty chambers. Most sellers will say they don't know how they work so can't say if there is a bubble or not. Assume no bubble unless the sellers states it is OK in the listing. The other major problem with most of the wartime sextant designs, not just the Mk IX, is stiff bearings, sometimes to the point of being siezed solid. It should be possible to rotate the index mirror knob with one finger throughout its range. It is worth asking about both these points before buying, as although fairly straightforward to rectify it is a long job that could effect the calibration.

Stiff Index Mirror Bearing

There are a couple of other common problems associated with the age of the instruments and the unstable materials used during the war. The rubber eye guard will often have perished and become mis shapen. The clear cover for the IX's prism will have turned a variety of shades of orange and the Ivorine Tablet used for logging sights will have discoloured from white to a murky orange, shrunk and probably torn its screw holes. Lamps used in the Mk IX and some of the American sextants were not patented till 1938 I discovered to my surprise and I believe they are still available via Aircraft part suppliers. There are at least two American firms that stock a bulb that will fit mechanically. However, they are not bright enough for use in these sextants for bubble illumination and are intended for instrument panel use. Suitable new bulbs for the wartime sextants are effectively unobtainable.

Here are the electrical circuits for the Mk IX range of sextants.

I have been a collector of aircraft sextants, mostly military, because of their abundance and hence low cost, for some years now and have been forced to investigate repairs and spares. The first job I attempted was to re fill a bubble chamber. I had seen on the web various descriptions of how people go about a re fill and I started out to follow suit. I quickly realised there is in fact an easier way to do the job and that the original drawings of chamber construction are in fact wrong on one minor but important point. I have also noticed that a completely empty chamber will usually have a grey deposit on the inside of the glasses. I have tried all sorts of methods to clean this out but always end up having to remove the glass and polish it. After working on a sextant one needs to check and if neccessary adjust the index arm. There are many ways to do this and I have tried several. They all work with varying degrees of simplicity. Myself, I now use a surveyed "range" across a local valley where I have painted two targets that are level and easily visible. At night a cycle lamp makes an ideal "star". I have over thirty of the Mk IXs and most were in need of Eye Guards. They are unobtainable and I investigated getting some manufactured. The cost was prohibitive, so I gave up for a while, then tentatively tried a DIY approach. I quite surprised myself with the quality of what I have been able to turn out. The scarcity of bulbs led me to look into making replacements out of LEDs. I was spured into action by a fellow collector and restorer in Portugal, Ricardo. He sent me a bulb he had made from scratch. Frankly indistinguishable from an original. Unfortunately, it was not bright enough. This started the Lumens war and before too long I was able to make lamps that were several orders brighter than the originals and yet only consumed a fraction of the battery power. A local collector approached me for some lamps and after sorting him out it dawned on me that there was really no need to be carrying around a couple of C cells with the weight and potential damage from a leak, not to mention the cost implications they involved. Initially I made a replacement battery using a pair of AG 13 button cells, I had plenty about the place for other applications and it seemed an obvious choice, so I made a simple adaptor for a trial. This is still on trial, but I have moved on to a second version which uses a Lithium cell. I think these are superior and should prove to be cheaper and more long lasting than the AG13s and certainly the C cells. I have in any event, made about 50 adaptors now and have just about mastered their production. My next jobs are to build a collimator (that works) and locate suitable material for the logging tablets, I have made some using a printing technique and they appear to be quite functional.

As they say on the TV program Blue Peter,

Meanwhile here are some things I made earlier

Whilst researching the various sextants I have come across some material which whilst not exactly earth shattering is in fact interesting to people working with, or collecting, old aircraft sextants. Where possible I have taken the originals and re typed them as near to the original in appearance as I can and then printed and bound them as A5 booklets, Others are good enough to scan in to my computer then clean them up. My collection is as can be found here CATALOGUE.

Something I have noticed whilst reading the various snippits of information that I have come across, is that although most of these instruments are not more than 60 years old and much of the information is probably available in original form somewhere, there are many errors creeping into the "Published" or "Public" data. Some of it in the most surprising places. If you look at some of the Museum illustrations, you will see dates and locations that just don't add up, as well as type descriptions which are incorrect. I have come across several pieces of information which I know, because I have seen original material, are just plain wrong! I might myself be guilty of passing on and giving further credance to errors of fact. If I am it is un intentional and if you see anything here you wish to take issue with, please mail me, I am always open to being corrected. I would be delighted to hear from anyone who has used one of these, the Queen of instruments, in anger, or just for fun. I am using as a "master" a Mk IX of 1940 vintage. I had to replace the discoloured cover over the bubble illumination prism but have done nothing else to it and am regularly getting a "fix" of around 1 mile. Whilst my sights are made under ideal conditions this is not bad going in my opinion.

Many people have a working sextant but a damaged averager. My advice is to ignore that. If you want the instrument to navigate with, provided you are not going to use it in the air then the Averager is an encumberance. I would remove it completely. It is easy to do. You are left either with a small gear wheel sticking out of the front or you can go a step further and remove that. Either by partial stripping of the main chassis or saw off the shaft with a small hacksaw or better a Dremel tool.





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