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    Re: sextant precision.
    From: George Huxtable
    Date: 2005 Jun 19, 00:19 +0100

    Henry Halboth wrote-
    >As respects sextant accuracy. Lecky goes into some detail on the subject,
    >and ultimately concludes the bugaboo of expansion and contraction,
    >especially when dissimilar metals are employed in the sextant's
    >construction, to be a consideration. This is, of course, of special
    >importance if the instrument is exposed to extremes of heat and cold,
    >however, any such exposure can easily result in a permanent set and a
    >consequent change in errors as previously observed. He also discusses the
    >matter or rigidity, as a function of construction and materials employed
    >- all in detail beyond inclusion in a posting to this List, and it is
    >probably worthy of note that most matters regarding sextants, as
    >discussed on this List, can be found in literature of the past.
    response from George-
    Yes, I've read that passage in Lecky. He is good at providing useful
    practical advice. In this case, he seems to be warning of permanent damage
    caused by extremes of heat and cold. He points out that when a scale is
    inlaid into a frame made of a different metal (as is usually the case), the
    difference in the expansion of the two metals can try to tear them apart,
    and result in a distorted scale.
    That may or may not be important advice. I'm not aware whether such scale
    distortion was a prevalent problem in sextants of Lecky's day, or indeed in
    modern sextants.
    But I wonder how a navigator can manage to follow Lecky's advice, and avoid
    exposing his sextant to extremes of heat and cold. If he was taking a noon
    sight in the tropics, or navigating a winter convoy to Murmansk, his
    sextant had to take the climatic conditions as they came. There was no
    choice in the matter.
    Lecky was referring to permanent distortion. The tiny discrepancies that
    have worried Alex (as a perfectionist) are presumably attributable to some
    minor short-term instability of his instrument, rather than to any
    permanent "set".
    >The matter of determining shade error is also relatively simple to
    >accomplish - assuming one's sextant to be of such quality to have been
    >supplied with a single telescope screen or set thereof.
    >Again quoting Lecky, "Make an accurate contact of the sun's limbs, on or
    >off the arc, as the case may be, using with the telescope one of the
    >coloured screens belonging to it. Then after discarding this screen from
    >the telescope, turn down suitable combinations of the index and horizon
    >screens, and see if the contact still remains perfect. If not, make it
    >so, and the difference between the first and last reading will be the
    >error of that pair of screens, and so on for the remainder." Imperfection
    >of the telescope eyepiece screen is of little consequence, as the direct
    >and reflected image are affected alike, and the between them remains
    >I do not know if modern sextants are sold with telescope screens or not -
    >both of mine are so provided, albeit with only one, intended for use with
    >the Sun, and I have always employed it when ascertaining errors by use of
    >the Sun. If not so provided, I am sure an adequate shade could be
    I've read Lecky's view, and he seems to be ducking the problem here. Yes, I
    can do just what he suggests. There are similar dense deep-blue shades
    available in both light-paths, via the horizon mirror and the index mirror.
    So I can put both those shades in place, then I can comfortably look at the
    sun in both views, so that the limbs kiss. Then I can take both shades
    away, and instead cobble another deep shade, borrowed from another sextant,
    in front of the telescope objective, and kiss again. Any offset in the
    setting, between those two readings, tells me what the error is of that
    COMBINATION of two shades, one for horizon view, one for index mirror
    view.. That's what Lecky explains, and it's fair enough, as far as it goes.
    But in a real-life altitude observation, it's common to be measuring a
    bright sun above a dull horizon. In which case, it's only the upper shade
    that's called for, and no shade at all in the horizon path. So what I need
    to know is the error caused by the upper, index-mirror shade, on its own,
    NOT the combination of two. I still don't see how to measure that. Am I
    missing something obvious?
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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