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    Re: sextant precision.
    From: Robert Eno
    Date: 2005 Jun 19, 22:51 -0400

    I have been monitoring this discussion quietly as I have been loathe to
    question the veracity of the issue of shade error, lest I have gotten it all
    wrong for so many years. I have owned two high quality sextants -- C.Plath
    and Cassens and Plath -- and a lesser quality sextant -- a Frieberger -- and
    have taken hundreds of observations at sea and with artificial horizons. I
    have never detected any significant errors that might have been caused by
    the shades. At sea, of course, it is a crap shoot at times with all of the
    possible errors created by this generally hostile environment. On land, with
    an artificial horizon, one can and should expect to be able to zero in on
    one's position to within tenths of a nautical mile. In most cases, I have
    been able to do this. That I can not pin my position down to the nearest
    foot, is likely due to operator, rather than shade error.
    I read somewhere that if the shades are ground perfectly planar, no errors
    should exist. On a Plath, this should be automatically assumed. I cannot
    attest to the quality of the other brands, not having used all of them.
    So I agree with Henry that agonizing over shade error, for practical
    purposes, is overkill.
    ----- Original Message -----
    From: "Henry C. Halboth" 
    Sent: Sunday, June 19, 2005 10:22 PM
    Subject: Re: sextant precision.
    >I have never been exposed to a quantification of the error potential due
    > to improperly ground shade glasses in a quality instrument, however, such
    > references as are available to me, specifically Wharton & Field, tend to
    > indicate any such error existent to be inconsequential to observations
    > made on the sea horizon, which appears to be George's concern. The same
    > reference differentiates as respects the artificial horizon, and
    > recommends either use of the telescope colored eyepiece or a
    > determination of shade errors in such observations. As respects practical
    > navigation utilizing the sea horizon, as opposed to survey quality work
    > with an artificial horizon, any shade error appears lost in the myriad of
    > other potential errors encountered. Outside of text books, I have never
    > heard shade error mentioned by practical navigators, although my Plath
    > sextant certificate does provide space for recording errors due to both
    > the index and horizon mirror shades - no error is indicated.
    > In a later post, Frank suggests use of a colored glass to achieve
    > contact, then removing the same and making a comparison by employing the
    > dark shade glass of George's instrument. This does not appear any
    > different than utilizing the provided telescope screen and removing it to
    > substitute the dark glass - I am a bit fuzzy here and will have to give
    > this a practical try; aren't we still dealing with a direct and reflected
    > image, one of which is unscreened?
    > An ultimate solution might well be, simultaneous observations by two
    > observers, one using George's instrument and the other an error
    > determined instrument, or, if only one observer be available, a series of
    > Latitudes ascertained by meridian transit in a place of accurately known
    > position would disclose any gross error in the shade - all other
    > possibilities having of course been ruled out and centering error at the
    > altitudes used known. But then again, based on my initial comments
    > regarding the potential magnitude of shade error, all of this may be
    > essentially overkill.
    > Henry
    > On Sun, 19 Jun 2005 18:14:33 +0100 george huxtable
    >  writes:
    >> >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
    >> >using
    >> >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.
    >> 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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