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    Re: Sextant vs. Digital Camera
    From: George Huxtable
    Date: 2008 Aug 9, 00:09 +0100

    Thanks to Greg Rudzinski for posting his interleaved sextant and camera
    observations of a setting sun; just the thing we need to assess the accuracy
    of one against the other.
    His stated conclusion was this-
    The results demonstrate the ability of a 7 mega pixel camera at optical zoom
    4 power  to accurately function  as a sextant "
    However, I think we need to examine the details first, before agreeing with
    that conclusion.
    Because this also is stated-
    "Sextant observations  observed with CASSENS & PLATH using sight tube and
    whole horizon mirror."
    And when we compare the results of the camera and a sextant USED WITH A
    SIGHT TUBE (i.e., without a telescope) we see that the camera gives an
    average difference between calculated and observed altitudes of 3.5',
    against 2.1' for that sextant,  the scatter being about the same, in the two
    cases,. of 2.2' or 2.1' between extreme values. Not a wonderful performance,
    for an observation made from firm ground in what are described as ideal
    conditions. Neither were the sextant observations anything to be proud of;
    but that may be due to the difficuly of working without a telescope,
    combined with the problems inherent in correcting for refraction at such low
    altitudes, and perhaps an unusual value (unknown) for local dip.
    In which case I would rephrase that conclusion to state only that the camera
    gave a mediocre performance which was significantly worse than that of a
    sextant without a telescope.
    I wonder if Greg Rudzinski has retained any of the images from that set; in
    particular, the image(s) which were used to obtain the Sun diameter, and
    from that the overall calibration factor. I am interested to see how sharp
    and well-defined the Sun limbs were, and whether there was any leeway in
    assessing the Sun's diameter, at 108 pixels, by choosing a different
    brightness contour for the fit to the disc. After all, changing that contour
    by just one pixel, from 108 to 107 pixels, would cause a difference of
    nearly 1% to the calibration factor, and shift the first of the camera
    observations in the set by more than 3 arc-minutes
    Finally, I wonder if Greg has made any assessment for non-linearity of the
    angular scale that was used, and made any correction based on that
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Navigation List archive: www.fer3.com/arc
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