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A discrepancy of 8% is hard to accept. Something is wrong, as Marcel
suspects. And such a big discrepancy should be easy to check for.

I suspect the angular calibration of his camera Perhaps he should check that
more simply, in terms of the angle subtended by an object a few metres away,
rather than by relying on distant topography. Near enough to that he can
check dimensions with a tape, yet large enough so that the camera focus
doesn't need to change from infinity, and also so that uncertainty in the
position of the effective optical centre of the lens is unimportant (within
much better than 8%).

Alternatively, even though Marcel considers that the Sun image is not
overexposed, perhaps it is, and he is looking at a bright halo of light
around it.

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: "Marcel Tschudin" 
To: 
Sent: Friday, August 21, 2009 12:49 PM
Subject: [NavList 9583] Which diameter of the sun in digital photos ?


|
| For investigating the refraction near the horizon I perform systematic
| observations by photographing sunsets. So far I used the horizontal
| diameter of the sun as a reference scale (pixel per degree) for
| measuring the sun's position above the apparent horizon.
|
| Recently I compared this scale to one derived from pictures of
| landscapes and found to my surprise that they differ by around 8%; the
| sun is systematically slightly larger.
|
| The scale in landscape photos has been measured with distinct features
| and the distances measured with Google Earth. The scale for (not
| overexposed) sun photos has been calculated with the angular size of
| the sun for the corresponding days; this size is based on the sun's
| diameter (of the photosphere) and the earth's distance.
|
| The difference is not related to refraction since the measurements are
| made in horizontal direction.  Any idea, why those two scales differ?
| It is known that some CCDs have a leak in the IR, also the one in my
| camera. Could this be responsible for it? Does anyone know more on
| this?
|
| Marcel
|
| |


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