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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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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

```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
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

```
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