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    Re: On The Water Trial of Digital Camera CN
    From: George Huxtable
    Date: 2010 Jun 22, 00:48 +0100

    I would like to add a few more cautionary words to those from Marcel about 
    the claims for precision "close to that of a metal sextant" made by Greg.
    
    That observation relies on the comparison with a calibration, made at a 
    different time and under different circumstances.
    
    What about thermal expansion, for example? The CCD array will expand with 
    the coefficient of Silicon. The lens-array spacing will expand according to 
    the combination of metals and plastics that it's constructed from. The 
    ratio between them will determine the pitch of the cells, in arc-minutes. 
    That will vary, if the temperature at calibration differs from the 
    temparature at observation. I do not know if the effect will be a 
    significant one, but it needs to be checked, before making claims for a 
    precision that approaches that of a sextant. A sextant has its component 
    materials chosen with some care, to ensure stability of calibration as the 
    temperature changes.
    
    Marcel asked about the horizon. How was the level of the horizon defined? I 
    presume that the upper edge of the horizontal thin streak of light 
    (actually, two streaks, with a darker zone between) has been taken to be 
    the horizon. But that upper streak is several pixels wide, with a somewhat 
    indeterminate upper edge. And can we be sure that this upper edge 
    represents the true horizon, and is not simply a ripple-line as sunlight 
    dances on the water? The problem here is that the horizon image is 
    necessarily very underexposed.
    
    The algorithm for correcting distortion has assumed that the angle being 
    measured has been equally split, above and below the centre point of the 
    array. I imagine that the correction is rather tolerant and forgiving about 
    small deviations, but how was that equal split obtained? Presumably, just 
    by eye-estimation in the viewframe of that SLR camera. It can, of course, 
    be checked after-the-event, and perhaps some correction might then be made 
    later, if found necessary.
    
    Measurements are very reliant on the optical quality (no prismacity) of the 
    polaroid filter pair. But if it was arranged that the orientation of these 
    filters had been marked, and then set up the same when calibrating and 
    observing, that could cancel out
    
    It would be interesting to deduce the Sun diameter, both vertically and 
    horizontally, to see how it compares with its almanac value, having made 
    allowances for tangent distortion, different in the two directions. If 
    there's agreement, it would provide confidence that it's the true edge of 
    the Sun disc that is being detected, and not one that's been expanded by 
    overexposure.
    
    Finally, the accuracy of the spacing of the elements of the CCD array is 
    being taken on trust, presuming a very high degree of precision. Are 
    specifications available to confirm that these elements are exactly where 
    they are supposed to be, with no position scatter or distortion? Perhaps 
    they are indeed placed to sufficient accuracy, but before relying on that, 
    perhaps some tests should be called for.
    
    All these matters should be at least considered, and weighed up, before 
    making claims about comparison with a sextant.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    ----- Original Message ----- 
    From: "Greg Rudzinski" 
    To: 
    Sent: Saturday, June 19, 2010 5:22 AM
    Subject: [NavList] On The Water Trial of Digital Camera CN
    
    
    See the link to a morning Sun image taken while at anchor in Smugglers 
    Cove, Santa Cruz Island.
    
    Camera: 10 MP Canon Rebel SLR
          Lens: 50mm SMC Pentax 1:2
    Polarizers: Vivitar linear
    
    Settings: ISO 200, F22, 1/4000s, Infinite Focus, Manual Mode
    Date: 6/18/2010
    Time: UT 14:30:23
    Temp: 62°F
    Press: 1013 mb
    H.E.  : 7 ft.
    GPS La:  34° 01.1'N
    GPS Lo:119° 32.4'W
    
    Pixels: 3093 Horizon to the upper limb
    Hs by graph: 3093 x .3749=1159.6'=19°19.6' (int 0.4'T)
    Hs Tschudin Formula: 3093x.371+12.4=1159.9'=19°19.9' (int 0.7'T)
    Hs Huxtable Formula: 120xARCTAN3093/18161.8=1159.8'=19°19.8' (int 0.6'T)
    Az: 74.0°
    
    Digital camera CN summary
    
    Pros
    1. Accuracy close to that of a metal sextant.
    2. Automatic UT time stamp of image.
    3. Very quick observation time.
    4. Light weight and compact.
    5. Observations saved to laptop.
    
    Cons
    1. Not waterproof.
    2. Only good for day time Sun and Moon observations up to 24°.
    3. Precision 0.4' vs 0.1' of a sextant.
    4. Requires editing on a laptop.
    
    Conclusion
    
    Digital camera CN is an intensely interesting way to obtain a Sun or Moon 
    LOP with the bonus of improving photo editing skills.
    
    Greg Rudzinski
    
    
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