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    Re: Sextant accuracy (was : Plumb-line horizon vs. geocentric horizon)
    From: Frank Reed CT
    Date: 2005 Feb 23, 14:44 EST
    Ken M you wrote:
    "In video microscopy it's common to go beyond the diffraction limit and
    pick out features (but not "detail") that should be too small to see. This
    works because of motion (video microscopy provides 30 frames/sec).
    Although on a still photo one would not be able to distinguish features
    from artifacts, persistance and motion give additional clues that allow
    one to discriminate. Artifacts don't obey Newton's laws, for example."
    Are you going beyond the diffraction limit? Or beyond the noise limit? Is this just the vision system's ability to average out noise in time-varying inputs? I suppose everyone is familiar with the fact that a single frame of  video can be very grainy and look completely smooth when displayed as a video stream. Capture a single frame of tyical television and the effect is dramatic. From a sextant user's point of view, maybe this means we should be happy our hands shake.
    "Somehow the brain puts all this detail into
    the moving image that's not really there (except that it is there--i.e. if
    you're familiar with histological sections of the same tissue, the MRI
    detail matches the genuine anatomical detail). I have no idea how this
    works, but it is amazing to witness."
    Fascinating. You're talking about viewing the "slices" like a fast slideshow, right? Does familiarity with the tissue structure help here, or are you saying that it confirms what you see whether you're familiar or not?
    "Before I had a scope I was able to get an index error consistently within
    0.2' by overlaying stars using just a sight tube. But I'm pretty sure I
    couldn't see any change in the image over a much larger range. Rather I
    would note an approximately equal separation on both sides and then just
    try to center the tangent screw in the middle of these positions. This
    worked because my sextant has very little backlash. Once I got a
    telescope, the index error remained the same (but was easier to test)."
    That would be consistent with this process being an example of hyperacuity or "vernier acuity". In my experience, I can't do much better than this with a telescope (and we should be able to in proportion to the magnification), but maybe that's because we're bumping against the limits of the instrument.
    42.0N 87.7W, or 41.4N 72.1W.
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