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    Re: buying a Mark IX A bubble sextant
    From: Douglas Denny
    Date: 2010 May 15, 13:36 -0700

    The Mk IX designed by Mr. Everitt of Hughes and Sons Ltd London in the 1940s is, in my opinion, the best bubble sextant of them all in general terms for ease of use, consistency in results and the best possible accuracy.

    The bubble chamber system is the best for producing a bubble of the size you want simply and reliably (some sextants have an unreliable pressure diaphragm method of bubble control). The MkIX is very robust, and although not the very highest quality for scientific instruments, as they were made in wartime presumably to a reasonable standard to a specification required, are very well made.

    The 'A' does not have the clockwork averager, and this is not needed unless it is going to be used in anger in an aircraft which is what it was designed for - with unknown accelerations affecting the bubble. The 'B' version is the clockwork mechanism type. 'M' means it has the x2 telescope as well as x1 eyepiece if I remember rightly.

    Considering they are well over 70 years old they are usually in quite good condition, but like an old clock, there are usually lubrication issues in the gearing, perhaps with some sticking, the lenses and mirror are always dirty, and the mirror might have some de-silvering patches.

    They are not at all difficult to overhaul. The case splits into two halves with bubble mechanism on one side and the gearing / mirror assembly on the other.
    If the mirror is intact, and the bubble 'working', just a clean and re-lubrication is all that is needed.

    If the bubble chamber is not functional (no bubble) they can be overhauled too. Again it is not difficult if you are familiar with say, model engineering, and have those kind of facilities available.

    Calibration is not difficult for a single altitude and the gearing quality/accuracy which is good, determines the overall scaling accuracy.

    There are various people who overhaul them; there is one chap I know up in the Midlands who does them, Paul Brewer; and he also makes and sells new rubber eyeguards and Led lamps.
    There are also people who sell booklets on maintaining them. Bill Morris in New Zealand in probably the world expert on sextants now and sells manuals for restoration, and Paul Brewer has a booklet too I believe.

    I have resurrected several of them now and once you have 'done' one they are not at all scary to work on. I cannot offer my 'services' however, as I have too many things going on in my life to take on such work.

    Mirrors can be re-silvered easily too with Brashear's method, or you can obtain new mirrors from specialist suppliers or re-silver is done by some optical manufacturers with sputter-coating equipment for tints/ silvering of glass lenses.

    If you can find a relatively 'good' one and have it restored they are a very good sextant indeed.

    Herewith are a couple of pictures of a MKIX BM with the case opened into the two halves.

    Douglas Denny.
    Chichester. England.
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