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    Re: Sextant accuracy (was : Plumb-line horizon vs. geocentric horizon)
    From: Bill B
    Date: 2005 Feb 20, 19:18 -0500

    I think the power-line is quite possible with a 1 inch or better cable, a
    good scope, and a dry high-pressure system.  I had been playing with sextant
    parallax and the thickness of the power line needed before your post.
    At 3 statute miles, a 1 inch power line would be >1" to the naked eye, and
    parallax error created by the distance between the center of the index and
    horizon mirror would be < 3".   If I figure my height of eye at 8 ft. on a
    sailboat, then the horizon is 3.8 statute miles away, then my parallax error
    is still >2" with a horizon that is probably harder to see sharply than a
    power line.
    I currently use closer power lines to test the precision (repeatability) of
    the the operator/instrument combination, and gear backlash.
    > The standard imaging resolution of the eye is just what you would expect from
    > diffraction limits and the spacing of the cones. Vernier acuity, which goes
    > waay beyond normal resolution, apparently involves only certain very specific
    > discrimination tasks. Do these have direct relevance to sextant use? Some
    > should, but it depends on how specific these tasks are. For example, it is
    > clear that the eye can distinguish discontinuities in straight lines at very
    > high angular resolution. So if I were to use a distant power line (or other
    > narrow straight feature) to get the index correction of my sextant, I should
    > be able to get very accurate results.

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