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    Re: Which diameter of the sun in digital photos ?
    From: Greg Rudzinski
    Date: 2009 Aug 21, 11:49 -0700

    As a check do a statistical pixels per degree measurement of the Sun
    when it is high in the sky. For a terrestrial check try photographing
    distant horizontal angles and comparing them with horizontal sextant
    angles. I found that each of my 50mm lenses have different pixels per
    degree calibrations and I suspect even a same make and brand would not
    have identical calibrations.
    
    On Aug 21, 11:26�am, Greg Rudzinski  wrote:
    > Marcel,
    >
    > I agree with George's comments and would like to add that my Canon
    > DSLR Rebel camera produces reliable images when way under exposed in
    > black and white. Try using no filtering with settings of f22, 1/4000,
    > ISO 100 and then increase the amount of light incrementally until the
    > horizon is just visible as a reference. Also try a pair of opposing
    > circular polarizers held out in front of the camera lens and adjusted
    > to produce a faint image of the sun with a bright horizon beneath it.
    > Experiment to find the best combinations for your particular camera
    > and lenses.
    >
    > Greg
    >
    > On Aug 21, 4:49�am, Marcel Tschudin 
    > wrote:
    >
    > > 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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