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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: On The Water Trial of Digital Camera CN
From: George Huxtable
Date: 2010 Jun 22, 00:48 +0100
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 ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------