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It might be of interest to mention at this point how I do the photos
of the sunsets and how I do the scaling of the pixels.

Note that I do not put a filter in front of the lens to dim the sun,
however, depending on the height of the sun I reduce the exposure by 2
units. The reason for this is the following: my camera indicates the
time of the photo only in hours and minutes. In order to have the time
in seconds I hold in addition to the camera also a watch between the
fingers. When taking a timed photo the viewer and the watch have to be
observed alternatively in order to have the sun in the picture, the
horizon horizontal and the exact time. As a result of this my photos
of the sun in a clear sky give often the impression of being slightly
overexposed.

Not knowing how good the photos are for measuring, i.e. for finding
the scale (pixels per degree) I wondered how well they would reproduce
the annual change of the sun's diameter (about 4 percent).

For finding this scale the sun's diameter in degrees has been
calculated for each day where photos have been made. The diameter in
pixels was determined from the mean of 4 measurements of the right
limb minus the mean of 4 measurements of the left limb.

"First iteration":
For this I selected photos where the sun seemed to be sufficiently
dimmed by thin clouds, haze or smog, thus photos where I thought that
the picture should provide a good measure. At this stage I did not yet
filter any colour in the photo before measuring the pixels. This
"first iteration" was done with something over 70 photos from
different days over a year. The median of those measurements provided
a first estimation of the scale (pixels per degree).

Note that as from here the difference between measurement of the sun's
diameter and predicted size of the sun's diameter (in pixel) could be
calculated with the median of the measurements and the calculated
value of the sun's diameter in degrees.

The result showed that about 20 percent of the photos were outliers,
with mainly too large diameters.

"Second iteration"
The photos of the outliers were again measured and - if still too
large - removed from the set.

"Third iteration"
The photos have now been filtered measuring only either the green or
the blue image. That filter is selected where the difference to the
expected size is minimal. Applying the filters increased now the
useful photos to, at the moment, over hundred. (There are still much
more to be analysed.)

Statistics:
The statistics has now been arranged in such a manner that the median
is taken only from those measurements where the difference expected
minus measured is less than a given limiting input value. At the
moment I have this value set to 1 pixel (compared to the sun's
diameter, changing between 104 to 108 pixels within a year). I think
it might actually be better to increase this limit in future slightly
to may be 1.5 pixels

Result, Scale factor:
a) Limit: +/- 1 pixel
Number of meaurements: 121
Scale (Median): 198.2
Std. Dev: +/- 0.9
b) Limit: +/- 2 pixel
Number of meaurements: 128
Scale (Median): 198.2
Std. Dev: +/- 1.1

Correction for under exposure or overexposure:
The difference between expected sun diameter to diameter measured in
the photo is used for correcting the measurement of the upper limb
which I use for my purpose, i.e. for measuring the refraction.

Overexposure:
This analysis revealed that the severe overexposures are related to
special phenomena like ducting or omega sun which occur dominantly in
spring and autumn. In all such cases the sun is much brighter than
normal thus leading to photos with a sun diameter which is roughly up
to about 5 percent larger.

Camera:
Panasonic DMC-LZ5
Optics from Leica with optical zoom 6x
(Zoom is set always at maximum, i.e. at mechanical stop)
Selected internal scene menu: "Landscape"
Size of photos set at 1MB resulting in photos slightly less than 0.5MB
Exposure value is reduced during the setting event starting with 2
units when the sun is fully above the horizon and then gradually
reducing it to zero correction when the upper limb is near the
horizon.

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