Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: sextant precision.
    From: Henry Halboth
    Date: 2005 Jun 19, 22:22 -0400

    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---.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.
    >
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site