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    Re: A sextant calibrator
    From: Bill Morris
    Date: 2011 Feb 14, 15:06 -0800

    Thank you, Kermit, for your plaudits.

    I think the Royal Navy and the Royal Canadian Navy did issue sextants that had calibration curves rather than tables, at least from the 1960s onwards, but I suggest that such a curve in most instances would not be useful for interpolation, as the calibration interval varied from 30 degrees (Heath and Co. pre WWII) to 15 degrees, though Filotecnica Salmoirhagi did issue a certificate in 1948 at 10 degree intervals. Post WWII, manufacturers seem increasingly to have altered the micrometer vernier to reflect intervals in the Nautical Almanac and sight reduction tables and by 1967, Heath and Co issued certificates at 15 degree intervals, reading to the nearest 0.2 minutes, as the vernier was divided to this amount.

    Most German sextants seem to have certificates that simply say that the instrument is free from error for practical use, leaving the user to imagine what this might mean. I would suggest that errors of observation and in deciding on the value of index error would swamp most instrumental errors less than, say, 0.3 minutes. Perhaps for this reason the vernier was increasingly omitted from micrometers in the second half of C20

    During WWII and for long afterwards US Naval shipyards seem to have issued certificates to the nearest second, but this was quite pointless, given that micrometer errors, which I have never seen tabulated, were often quite large. I gave some results of micrometer calibration in a blog post that preceded the one on the calibrator.

    This morning, just for fun, I calibrated some more sextant micrometers, starting with that of the calibrator itself. The process uses the full range of the autocollimator (10 minutes), over which range the accuracy is +/- 2 seconds (over 1 minute it is +/- 0.5 seconds). The error of the calibrator micrometer varied from -3 to + 4 seconds, and that of an intact SNO-T sextant from -3 to + 2 seconds. However, a 1960's W Ludolph sextant in near-new condition had micrometer errors from -16 to + 9 seconds and the Filotecnica sextant mentioned above varied from -2 to + 9 seconds.

    The calibration certificate is perhaps best regarded as an indicator of goodness of manufacture, not to be relied upon for interpolation or at values that are not whole numbers of degrees unless you know something about the goodness of the micrometer.

    Arthur Hughes of Hughes and Son took a very close interest in the practice of navigation, perhaps especially in air navigation. Francis Chichester wss associated with the firm, and the chief designer, P F Everitt, seems to have had a talent for ergonomics. Here then was a close association between designers, manufacturers and end users.

    Maybe for these reasons, the Mark IX series of bubble sextants is a favourite with many who still take an interest in the subject. It is easy to hold and use, and all the knobs seem to come naturally to hand (or rather, finger). Scale and watch illumination is achieved with a single lamp, unlike some others that used up to three separate ones, and no juggling is required to make a note of readings. By contrast the AN5851-1 seems to have been designed by a large committee of people with three thumbs (I hope this view will not lead to too many enraged responses from the USA).

    Kind regards

    Bill Morris
    Pukenui
    New Zealand


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