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    Sextant calibration
    From: George Huxtable
    Date: 2006 May 14, 09:18 +0100

    This is in reply to GregR, who wrote, under threadname "Question about
    Davis Mk 25 sextant beam converger"
    George wrote:
    > First, in no way does such a zero adjustment get
    > any sort of "calibration" of the sextant,
    Just for my own further education (and not wanting to duke it out with
    you over semantics, it's a lot more fun to do that over a sextant
    auction... ;-)), what is the "proper" term for the initial accuracy
    checkout of a sextant when it's first acquired?
    I probably used "calibration" in the wrong context here (wasn't
    referring to an optical lab collimation at all), but what would you
    call the initial "shakeout cruise" testing - or is there even such a
    Response from George.-
    In the past, new sextants, when supplied, would often be accompanied
    by some sort of "test certificate", produced by a laboratory such as
    (in Britain) the National Physical Laboratory. This stated any
    deviations in the scale reading, usually at 15 degree intervals. In a
    good sextant these would be within +/- 10 or 20 arc seconds. That
    certificate would be tacked inside the box, and a careful observer
    would correct his observations accordingly. Those corrections being
    small, many navigators didn't bother to do so.
    Nowadays, many sextants come with a document which does not state such
    deviation, but just say something like "sufficiently accurate for the
    purposes of navigation". And that is usually the case. Usually,
    sextants retain that original accuracy. Owing to the way sextants are
    constructed (both micrometer and, in the past, Vernier types) it takes
    rather a lot of severe trauma to damage them in a way that seriously
    affects their scale-accuracy, even though they are such precise
    instruments. Any such damage will often be detectable by a stiffness
    somewhere in the travel of the arm.
    Many, I would say most navigators, simply accept that scale
    calibration and do nothing to check it. There are exceptions, who are
    particularly fussy about such matters. Alex Eremenko on this list is,
    for special reasons, an example. I will explain why later.
    Checking the calibration in a lab requires rather specialised
    equipment, but a user can do it for himself by measuring the angle in
    the sky between appropriate pairs of stars, with different
    separations. The job calls for considerable skill. Those angles do not
    change (measurably) with time, but refraction has to be allowed for.
    The reason that sextants are such precise instruments is partly
    historical. Most of the uncertainty, in measuring the altitude of a
    celestial body at sea, relates to problems at the horizon, not with
    the body itself. Even when the horizon is clear, sharp, and straight,
    it is subject to variations in actual dip, compared with the textbook
    value, due to changes in refraction conditions near the sea surface.
    These can easily reach an arc-minate, and occasionally several
    minutes, and there's no knowing when they occur. Such anomalous-dip is
    the main contributor to errors in observations from the high bridge of
    a large vessel. From a small craft, any wobble in the underfoot
    platform combines with the effect of waves on the horizon (which is
    much closer) and vertical heave of the vessel on the swell, to mean
    that even in calmish conditions, altitudes are meaningful to only a
    few arc-minutes. In the beginning (the early 1700s) simple wooden
    octants were regarded as appropriate to do that job. And indeed, they
    Later in the 1700s, measuring longitudes by lunar distance became
    possible, from the angle in the sky between the Moon and Sun or star.
    The horizon, with all its problems, was not involved, so in theory
    that measurement could be made very precisely. It needed to be,
    because an error of only 1 arc-minute in the lunar distance would
    result in a longitude error of about 30 minutes. There was a new need
    for high precision, to a fraction of an arc-minute, which was met by
    the development of the brass sextant. In the 1800s, chronometers
    gradually came in for longitude, supplanting the need for lunars, in
    which case the precision of a brass sextant was no longer relevant.
    However, navigators were reluctant to relinquish such beautiful
    precision, and sextants have been prized ever since, as the badge and
    totem of their profession. Today, more sextants are used as home
    decoration than taken to sea, but even so the call for precision
    remains, as a virtue in itself.
    Lunar distances, needing no horizon, can be taken from on land, even
    better than at sea, and many landbound observers today enjoy testing
    their skills that way. That's where Alex's obsession (hope he doesn't
    object to that word) with the precision of his instrument becomes
    relevant, in meeting that challenge.
    But for the purposes of up-from-the-horizon celestial observations for
    navigation within a few miles, any old sextant, even a plastic one, is
    good enough for that job. If you don't reach your destination, it
    won't be the sextant's fault. I have never read an account of any
    error in horizon-navigation which was attributed to faulty calibration
    of a sextant; it would be interesting to learn if any have occurred.
    Alhough GregR may hope to test out his new sextant in a "shakedown
    cruise", by comparing  observations with his known position, such a
    test will detect only gross errors in the sextant, way outside its
    normal precision.
    That was perhaps more than GregR was expecting, or needing, in answer
    to his question about calibration. Yes, there are other adjustments,
    as Frank Reed has pointed out. Sextant handbooks and textbooks always
    seem to make a meal of such matters, but in general once they have
    been got right once, they won't need adjusting again, though it does
    no harm to check occasionally. But one adjustment that does NOT  EVER
    need to be made is the zeroing of index error, whatever it may be.
    That plays no part in the accuracy or calibration of a sextant. A
    careful observer will check the zero error before and after making
    observations, and even within a sequence, in the unlikely event of
    finding that it changes. As Frank has said, it doesn't matter how big
    it is. It's the work of a moment to check it, and trivial to subtract
    off any offset. GregR, and others, should forget about adjusting out
    index error.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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