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    Re: sextant precision.
    From: George Huxtable
    Date: 2005 Jun 19, 18:14 +0100

    >I wrote, about determining shade error-
    >"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."
    And Frank responded-
    >Yes, I know what you're talking about. What you need is  something brighter
    >than the Moon and fainter than the Sun. That's a big range:  around 14
    >magnitudes or a factor of 400,000 in luminosity, so lots
    >or  possibilities... I have
    >used the setting sun which is faint enough to look at  comfortably with a
    >medium shade and bright enough to see through a very dense  shade --there
    >is *some*
    >altitude near the horizon where this should be possible.  I've also used
    >metal rooftops that are reflecting almost direct sunlight. You  get an IC
    >the medium shade (whose shade error you've already determined  with the Moon
    >perhaps) and then you get an independent IC with the dense shade.  A little
    >arithmetic yields the shade error of the dense  shade.
    It's the large factor between the brightness of the sun and the brightness
    of terrestrial objects that's the reason why the sun shade has to be so
    very dense, and that's why there's a problem.
    Yes, it should be possible to deduce a shade error by building it up from
    measurements of the errors of less-dense shades, but that also builds up
    the errors in those multiple measurements. So, if it's possible, I would
    prefer to measure the error of that darkest sun shade (which is the most
    important one to know about) in a single measurement. But is that possible?
    Perhaps I could watch a fading setting-sun until it got dim enough that I
    could tolerate looking at it through the sextant's telescope with no shade
    at all (a procedure which carries risk and would need to be done with some
    care).  And then, if I swung that dense shade into place, would the sun
    then be so dim that I couldn't see it at all? I suspect so, but can't be
    sure until I've tried it.
    Does any textbook address this problem more thoroughly than Lecky does?
    Lecky points out that in the best instruments the shades are arranged so
    that they can be rotated through 180 degrees (top to bottom, that is), and
    taking the mean of observations made with those alternative positions will
    null out shade error (and the difference would determine that error). Do
    any modern sextants have their shades constructed that way?
    Perhaps the best way for me to assess the error of that darkest shade might
    be to detach it from its normal mounting (which is rather easy) and instead
    cobble it back into roughly the same spot using insulting tape, in such a
    way that it can be inverted, top to bottom. If that shade is indeed found
    to be prismatic, it could then be oriented to such an angle  that it gave
    rise only to a small sideways displacement of the image, and didn't affect
    sextant readings.
    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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