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    Sextant precision.
    From: George Huxtable
    Date: 2005 Jun 18, 09:53 +0100

    Discussion about the ultimate precision that a sextant can achieve provides
    regular fodder for this list. Not that I can claim much first-hand
    experience along those lines, as all I own is a plastic Ebbco, though I've
    used several other instruments. So discussion of discrepancies of a
    fraction of an arc-minute are not directly relevant to me.
    Recently, Alex has been discussing some small discrepancies in sextant
    readings, between star-to-star observations and others, at the
    fraction-of-a minute level. I wonder if this might perhaps be related to a
    senstivity of the sextant to tilt? It's a question I've asked before, some
    years ago, but since then the list membership has changed somewhat.
    I raised the matter, then, in terms of comparing the measured elevation of
    a distant fixed object, with the sextant its normal way up, and then
    measuring again with the sextant inverted.
    It's not just a question of whether the frame of the sextand distorts, ever
    so slightly, under its own weight, as it's tilted. I would suspect that at
    the sub-minute level, a sextant's readings are most sensitive to the
    tiniest flexure in the mounting arrangements of the two mirrors. If there's
    the tiniest droop in the angling of those mirrors, under their own weight,
    as the sextant is tilted in different directions, that will upset the
    precision. Their fixings have to be very rigid indeed, even though the
    mirrors are so light in weight.
      Alex, straining for maximum precision,  has had the calibration of his
    sextant checked against a lab. instrument when mounted in a particular
    orientation. I ask, will it maintain that calibration, over its whole
    range, to sub-minute levels, as it is tilted about to measure a lunar or an
    inter-star angle? Is tilt-sensitivity a possible source of error? What do
    others think? Have any such checks on sextants been reported?
    On a different tack, the question arises of any error from refraction by
    the sextant's shades. Frank has explained how to check and allow for this.
    However, one such shade on my sextant quite defeats any such attempts. It's
    the very darkest shade, a very deep-blue one in my case, that's required
    for viewing the sun. When I look through that shade, it's so dark that the
    only object I can see is the sun. Without that shade, I can't safely look
    at the sun at all. So how do I compare two measured angles, observed with
    and without that shade? What do I look at, to do that job? Others must have
    met that same problem. Suggestions, please.
    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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