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Re: Camera sextant? was: Re: On The Water Trial of Digital Camera CN
From: Marcel Tschudin
Date: 2010 Jul 6, 10:13 +0300
From: Marcel Tschudin
Date: 2010 Jul 6, 10:13 +0300
George, Could the reason for this confusion be that in one case we have pixel POSITIONS and in the other pixel RANGES and as a consequence of this also the meaning of the origin (0,0)? Calibration function: Here we look at the scale (moa per pixel) as a function of pixel *positions*. This function shows to be about symmetrical about the centre of the picture at x about 1940 pixel. These measurements can't reasonably be approximated better than with a second order polynomial. If for this function the scale would be zero at zero pixel then something would definitely be wrong with the lens. Your reflections can't be valid for this function. Conversion function: Here we look at integrated angles as a function of pixel *ranges* derived from the calibration function around the centre at about x=1940 pixels. It is correct that for zero pixel range the angle has obviously also to be zero. However, in my opinion, the symmetry relative to this origin (0,0) doesn't apply the way how you imagine it since we are looking at pixel ranges and not at pixel positions. Greg's photos indicate that there could possibly be higher polynomial terms involved in his calibration derived directly from sun-horizon observations (but possibly containing additional errors from refraction). In the case of SAMT the conversion function is derived from the calibration function. The calibration function can just about be approximated with a second order polynomial. In this case I hesitate to use higher polynomial terms for the conversion function than what was used for the calibration function from where it was derived from. Do these explanations solve your concerns? Marcel