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Thanks to Greg Rudzinsky for letting us in on his celestial camera 
techniques. They leave me keen to learn more detail.

The following may just show my ignorance about modern digital cameras, 
fitted with interchangeable lenses. Greg refers to a 50mm lens, and a 24 
degree field. Is 24 degrees the angle subtended right across the frame's 
diagonal, corner-to-corner? It must, presumably, depend on the dimensions 
of the sensor array, which I would expect to be somewhat smaller than the 
36mm x 24mm of standard 35mm film. With a 50mm lens, I would expect such a 
film frame to cover nearly 47 degrees across the diagonals.

Greg's technique should work well. If he clamps the camera firmly to look 
at the sky, in such a direction that the Sun will pass close to the centre 
of the frame as it transits roughly across the diagonal, at carefully timed 
regular intervals, that should tell him everything he needs to know about 
the central calibration and the radial distortion. What's more, it should 
also tell him if his deduced Sun diameters really do correspond well with 
the Sun's true diameter. He will see a string of Sun images that, at 
2-minute spacings, will just slightly overlap at the limbs, which should 
allow for precise measurement in terms of pixels. It's the along-track 
motion that is most relevant, and it should be kept, as closely as 
possible. along a radial line passing through the centre of the frame.

However, that was based on the assumption that with a camera, the 
distortion function must be symmetrical about the centre point of the 
frame. All very well for a film camera, if it's been assembled 
well-aligned, on centre. But there are other possibilities with a digital 
camera that may well need checking. After all, the array itself is not the 
same at all angles, but has defined x and y directions. Does the 
pixel-array have the same pitch spacing across its two dimensions, or 
alternatively a precisely-known ratio between them? Are the rows and 
columns precisely at right-angles? Is there scatter in the pixel 
positioning? I would expect that the answer to these questions is that 
there's nothing to worry about, and the pixel placement in the array is 
highly precise, but still, it might well be worthwhile asking such 
questions. They would be difficult to answer by observation.

Greg refers to "lens distortion and tangent geometry", which is exactly the 
concept we have been struggling with in recent postings. I would be 
interested to learn what the results of his observation and analysis have 
been, in his widest-angle configuration, in providing a correcting function 
to allow for these effects.

Greg refers to a simple formula, "(.371 x pixels + 12.35 = MOA for a fixed 
Pentax 50mm lens). ", and this leaves me wondering exactly what is being 
plotted against what.

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: Tuesday, June 15, 2010 12:18 AM
Subject: [NavList] Re: NG's "Midnight Fun"


I would like to explain how I have been effectively performing CN with a 
SLR 10mp digital camera using 50mm, 100mm, and 200mm lenses. Lens 
distortion and tangent geometry are corrected for together for each 
individual fixed lens by observing the Sun or Moon every two minutes then 
graphing the minutes of arc per pixel derived from reduced observation at a 
known GPS position. This allows the entire field of view along a central 
axis to be usable (24� for a 50mm lens). Marcel helped convert my graphed 
data to a simple formula (.371 x pixels + 12.35 = MOA for a fixed Pentax 
50mm lens). This formula works great at +/- one MOA through the entire 24� 
field. The trick for consistency is in how the camera settings and 
polarizers are used. All my images are shot at f22 and infinite focus (any 
deviation causes problems). The polarizers are held out in front of the 
lens at arms length. This mysteriously improves results. I strongly 
encourage others to try this neat way of generating an LOP from the Sun or 
Moon during the day.

Greg Rudzinski
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