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Polarizing filter
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A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular polarizers.
Polarizing filters adjust the brightness of images to a better level for observing, but much less so than solar filters. With these types of filter, the range of transmission varies from 3% to 40%. They are usually used for the observation of the Moon, but may also be used for planetary observation. They consist of two polarizing layers in a rotating aluminum cell, which changes the amount of transmission of the filter by rotating them. This reduction in brightness and improvement in contrast can reveal the lunar surface features and details, especially when it is near full. Polarizing filters should not be used in place of solar filters designed specially for observing the sun.

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For glare reduction while observing the Moon, the Meade #905 Variable Polarizer performs well with two Polarizer filters mounted in a specially-machined cell. The #905 permits variable settings of the light transmission between 5% and 25%, to account for varying lunar surface brightness according to the phase of the Moon and the magnification and aperture of the telescope.
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Quality circular polarizer test for photographers

The effect of a polariser on a rainbow



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