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Under the subject "Lunars and pixels" the question was raised how to
correct the over-exposed size of the moon. It may in this context also
be of interest on how I correct over-exposed sizes of the sun.

For the sun this correction is somewhat easier than for the moon since
the visible diameter does not change (no phases as in the case of the
moon). Knowing the scale in pixels per degree and, for the given date,
also the diameter of the sun in degrees, provides the nominal
(horizontal) pixel-width of the correct exposed sun. For measuring the
height above the horizon the correction is applied to the upper and
lower limb. For measuring the UL and LL in the photo I filter the
image in a copy of it leaving, depending on the amount of
overexposure, only green (small amount) or blue (large amount) and,
for further reducing the size, also the brightness of the remaining
colour. The positions of UL and LL are measured, and the remaining
difference corrected, when the horizontal diameter of the sun is close
or equal to the expected value. In those cases where the sun is close
to the horizon, i.e. when UL, middle of the sun and LL are differently
exposed may require some further "manual" adjustments.

Marcel

   

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