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    Re: sextant precision.
    From: Renee Mattie
    Date: 2005 Jun 21, 09:23 -0400

    Removing the shade and reattaching it in some jury-rigged way seems
    problematic, as
    it might no longer be perpendicular to the ray of light.  Wouldn't that also
    cause a
    I attempted the following experiment with my Freiberger:
    * I set my sextant near 0, took it into the closet, and laid it on its back
    on the carpet.
    * I shone a laser pointer through the scope and observed the spot on the
      Actually, I observed two half-spots unless I made a suitable index
      But I extinguished the horizon half-spot with the entire stack of shades
    so it would
      not distract me from the index-mirror half-spot.
    * When I swung the densest index-mirror shade in front of the mirror, it
    extinguished the half-spot.
    * Therefore, I could not observe any deflection of the spot caused by the
    darkest shade,
    did not repeat the experiment on a more stable platform, and did not attempt
    to observe
    deflection caused by the other 3 shades.
    The laser pointer is 7 or 10 years old, and there is no way to replace the
    I wonder if a brand new one would be bright enough to shine through even the
    darkest shade?
    Renee Mattie
    -----Original Message-----
    From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]
    On Behalf Of george huxtable
    Sent: Monday, June 20, 2005 3:57 PM
    Subject: Re: [NAV-L] sextant precision.
    Geoffrey Kolbe wrote-:
    >>At the point where you have set aside the welder's glass, and swung in
    >>the dense shade in the index-mirror view, you still have the full
    >>brilliance of the Sun in the horizon-mirror view. Unless you introduce
    >>a similarly dense shade into that direct view, your eye will be quite
    >>unable to take it. But if you do that, you are measuring, not the
    >>error of the upper shade, but the combined error of two shades (just
    >>as Lecky described), which was NOT the object of the exercise.
    >George, are you able to remove the index mirror shade and rotate it  -
    >even using plasticene or some other jury rig to hold it in place while
    >you see if the IC has shifted....?
    Yes, that was rather what I had in mind when I wrote yesterday-
    "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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