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    Re: Introduction
    From: Jean-Philippe Planas
    Date: 2008 May 3, 02:36 -0700
    Hello Bill,

    When you say Shellac is soluble in alcohol, do you liquid (hot) Shellac or dried Shellac?
    In this case is it a good idea to clean bubble sextants optics with alcohol?
    JPP

    engineer <engineer@clear.net.nz> wrote:

    Welcome, Bruce.

    You wrote:
    "1)Sextants are expensive precise instruments, but they are still made
    of brass and aluminum. Why does no one use invar to make them more
    resistant to thermal expansion?"

    Firstly, as George Huxtable has pointed out, there is no need to,
    provided that errors are not invited by leaving the instrument in the
    sun. Different thicknesses of metal will heat and expand at different
    rates, leading to distortion. It is not just a matter of part being in
    the sun and part in the shade. Russian sextant handbooks for instance
    recommend that the sextant box be opened for 20 to 30 minutes at the
    place of observation "to expose the sextant to the environment
    temperature."

    I don't know whether this is an actual or just a perceived source of
    error. When the weather improves at my place, I will experiment and
    report back.

    Secondly, Invar is an expensive metal that is very difficult to
    machine, though it can be cast. It is hard to get a good finish and in
    my experience of machining it, limited to clock pendulum rods, it is
    very prone to tearing. While relatively free-machining varieties are
    now available, they were not during most of the nautical sextant's
    heyday.

    "3) I have looked, and bid on, several A-12 air sextants on e-bay.
    Has
    anyone ever had a good result with an e-bay A-12? Are they fairly
    rugged? I hate buying a pig in a poke, but celestaire is not cheap
    (like me)."

    Around the time of WW II, the US produced a wide variety of aircraft
    sextants of more-or-less complexity. The navy seemed to have had a
    requirement that they could be used with the natural horizon as well
    as with a bubble horizon; and this further added to the complexity.
    Prisms seem to have been favoured over mirrors, also adding to the
    cost. The A 12("A" for "Army") was produced in order to provide a
    simple, rugged sextant with a precision appropriate to its use.
    Compared to the AN 5851 and the AN 5854-1("AN" for "Army and Navy"),
    they were lighter, cheaper and much easier to work on. The anti-
    backlash springs need a bit of manual dexterity to fix. The optics are
    simple, consisting of a bubble chamber, a plane mirror, a collimating
    lens, a plain glass index mirror and a couple of shades. Compare this
    with the bubble chamber, 4 prisms and four lenses of the AN5854-1, a
    sextant that cost as much as a small family home. For use on land, we
    can ignore the averager devices of the various instruments.

    I have worked on nearly all the aircraft sextant types that were
    produced(barring rare museum specimens). My favourite is the British
    MkIX A series, for which a restoration manual is available at modest
    cost. Unfortunately, neither it nor the A12 can be used with a natural
    horizon. Probably the smallest and neatest that can be so used is the
    A7, but they do not often come on to the market.

    Sooner or later, if you catch the navigation bug, you will want to own
    a proper nautical sextant, so why not start there? You can use a pan
    of old sump oil as an artificial horizon for bodies up to 60 degrees,
    more with some sextants(quintants?). Though the Russian naval sextants
    probably represent best value for money, the usually cheaper US Navy
    Mk II, despite some irritating features, served me well when I first
    succumbed to the addiction(the arc also goes up to 145 degrees).

    JPP wrote:
    "However the ring nut features two opposite grooves similar to the
    A10s bubble assemblies. Therefore I guess a wristwatch wrench or a
    specific tool (made of aluminium or steel) machined on a milling
    machine should be convenient.
    I also suppose the ring nut is likely to be stuck on a dried shellac
    seal bed that will have to be reconstituted when sealing and putting
    everything back into place."

    No complex machining is required to make a tool. One can be made from
    a piece of 3 x 20 mm steel, about 50 mm long, with the end filed to
    fit the grooves. If the locking ring resists, a wrench can be applied
    to the tool. Removing the rings is the least of the problem, as the
    lenses(one of plane glass) are embedded in lead washers which have to
    be picked out with a sharp implement. I have never had to do this, but
    a friend reports having done so. I think if I did, I would replace the
    washer by running shellac around the lens. This is not as difficult as
    it sounds. I heat bubble chambers on an up-ended domestic iron until
    flake shellac melts and can be fed in bit by bit around the part.
    Clean up with alcohol, in which shellac is soluble.

    Bill Morris





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