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Polarimetry is the measurement and analysis of the polarization of electromagnetic radiation (EMR). Polarization can indicate and quantify gravitational lensing, magnetism (such as the magnetic fields of the Sun and stars, and their fields' changes), and other phenomena. It is of particular interest in the study of cosmic microwave background (CMB) and cosmology. An instrument detecting polarity is termed a polarimeter.
Dust polarimetry, i.e., using polarization of dust clouds, is useful for detecting and mapping their magnetic fields, e.g., for clouds, and disks such as protoplanetary disks.
Polarimeters used in astrophysics generally use optics that eliminates all but one polarization (or more accurately, allows just a narrow range of polarizations), that are adjustable so that by multiple imaging, the incoming EMR with various polarizations can be determined, to gather various characteristics of the incoming polarized light.
(Note that a common type of laboratory polarimeter is designed differently, to measure the polarizing effects of some sample material, by providing a light source, passing the light through such optics to filter it down to a limited polarization, then passing that light through the sample, then measuring how much has been transmitted, repeating all this with filtering for varied polarizations.)