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    Re: sextant precision.
    From: Henry Halboth
    Date: 2005 Jun 18, 16:02 -0400

    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.
    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
    On Sat, 18 Jun 2005 09:53:03 +0100 george huxtable
    > 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.
    > George.
    > ===============================================================
    > 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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