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    Re: Using star-star distances
    From: George Huxtable
    Date: 2008 Sep 23, 20:36 +0100

    Frank wrote-
     In some of my recent sextant calibration tests, I've had the
    | pleasure of watching, in real time, the calibration of a plastic sextant
    | glide up two minutes of arc postive for a couple of minutes (of time) then
    | drift in the opposite direction until it was zero and then negative by a
    | couple of minutes of arc. Something like that simply cannot be corrected
    | live navigation.
    That's interesting, and rather a surprise to find such extreme
    bad-behaviour. Was the index error similarly unstable, or did the index
    error stay put while the calibration changed? What were the conditions of
    the test? Had the instrument been given any time to stabilise, in
    temperature? Presumably, the instrument had been checked-over to ensure that
    no mirrors or brackets were at all wobbly
    My own experience of plastic sextants has been confined to just one make;
    various models of the Ebbco design. And I must say that I've never been
    aware of such wild short-term changes in calibration as Frank describes.
    Such changes would have been quite apparent, in distorting a sequence of
    altitude observations. Nor have I ever been aware of measurable drifts in
    index-error, which I have always checked before and after such a sequence. I
    would never expect to measure angles to better than an arc-minute or so with
    an Ebbco, however.
    Frank's comments, applying to an unspecified "plastic sextant", tar all such
    sextants with the same brush. It would be fairer, and more useful to others,
    for him to actually "name names", and specify the type of plastic sextant
    which he found to be so faulty.
    contact George Huxtable, now at george@hux.me.uk
    (switched from 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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