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The main way a dew shield works is by limiting the solid angle of sky 'seen' by any element of an optic surface (lens or corrector plate). Without a dew shield the optic sees a hemisphere (2Pi steradians) of cold sky. With a dew shield this area is reduced to a much smaller area of cold sky. This can then lower supercooling but not eliminate it.
By gently heating the dew shield, radiation from the dew shield to the optic can then compensate for heat loss from the optic to the sky thus eliminating supercooling and as a result no condensation.
Bert
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