Re: [ATM] Polarizing filters versus neutral density filters

[Mark Holm <mdholm@telerama.com>]

The polarizing filter for Lunar observations is probably two polarizing
filters mounted together, with one of them rotatable.  When the
polarization axes of the two filters are aligned, the theoretical
transmission of the filter for non-polarized light is 50%.  When the
axes are at 90 degrees, the theoretical transmission is 0.  No filter is
perfect, so the numbers won't be exactly 50 and 0.  The point is that
the moon does not produce strong polarization, and human eyes are poor
polarization detectors, so the variable angle crossed polars filter ends
up acting like a variable neutral density filter, a handy trick!

Normal photographic polarizing filters typically have only one
polarizing element which is also often rotatable.  The point is, that
some common landscape elements, e.g. sky and low angle reflections, do
produce significant polarization. Thus, the clever photographer can use
a polarizing filter to  selectively darken or lighten sky or reflections
in landscape scenes.

-- 
Mark Holm
mdholm@telerama.com

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