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    Re: sextant precision.
    From: Fred Hebard
    Date: 2005 Jun 20, 00:00 -0400

    One area where shades might be an issue is with lunars, which I am sure
    we would all agree are not part of practical navigation.  Nevertheless,
    there are those of us who would like to become proficient in the
    technique.  My experience has suggested a systematic error with sun
    lunars, but not other types.  If I could narrow this down to shades,
    that would be helpful.
    There is also the issue of sextant accuracy: for instance, how would
    one ascertain at sea whether the sextant is still accurate enough for
    navigation after it has been dropped?  Star-to-star sights appear to be
    the answer here: how accurate can these be?  That would set a limit on
    determinations of the sextant's accuracy.
    There finally is the issue of achieving proficiency with the sextant,
    where Alex noted a difference in index error measured with the sun
    versus the stars.  It would be useful to be able to eliminate potential
    sources of error, such as shades.
    On Jun 19, 2005, at 10:51 PM, Robert Eno wrote:
    > 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.
    > Robert
    > ----- Original Message -----
    > From: "Henry C. Halboth" 
    > To: 
    > 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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