instrumenttipolarisaation

Instrumenttipolarisaation, or instrumental polarization, is the spurious polarization signal that an optical or infrared instrument can imprint on light, even when the source is unpolarized. It arises from reflections, birefringence, coatings, asymmetric optical paths, and detector or modulator responses within the instrument or telescope.

The effect depends on several factors, including wavelength, position in the field of view, and the exact configuration of the instrument. It can vary across the detector and with telescope orientation, introducing changes in both the polarization fraction and the polarization angle. In spectropolarimetry, instrumental polarization can be wavelength dependent due to coatings and birefringent elements.

Measurement and representation are typically handled with the Stokes formalism. Instrumental polarization is described using Stokes parameters I, Q, U, and V, and is often modeled with a Mueller matrix that characterizes how the instrument transforms incoming polarization. Calibration uses unpolarized standard stars to determine leakage from I into Q/U/V and polarized standards to fix the polarization angle and scale.

Mitigation and correction involve both instrument design and data processing. Design approaches aim to minimize induced polarization by reducing asymmetries and reflections and by using rotating modulators or symmetric optical paths. Data correction uses calibration observations to construct a Mueller matrix and apply inverse transformations to science data, removing or reducing the instrumental contribution. Corrections may also include rotating the reference frame to standard celestial coordinates and accounting for field- or wavelength-dependent variations.

Instrumental polarization is a key consideration in astronomical polarimetry, solar physics, and remote sensing, where achieving high polarimetric accuracy requires careful calibration and correction of the instrument’s own polarization signatures.