A New Technique for Correcting Polarization Cross-Talk in Spectropolarimetry

The symmetry and anti-symmetry properties of the polarized signal from Zeeman-split spectral lines can be used to infer and correct the polarization cross-talk present in spectropolarimetric observations of sunspots. Previous such ad hoc techniques could correct small levels of cross-talk based on the assumption that the optical system was weakly polarizing, or if a small level residual polarization cross-talk remained after correction by other techniques. Our new technique is capable of accurately treating arbitrary levels of polarization cross-talk in optical systems which are well-described as non-depolarizing. By modeling the system polarization as the combined Mueller matrices for an elliptical diattenuator and elliptical retarder, it is possible to accurately correct the data and retrieve the system Mueller matrix at the time of the observation using an optimization method. This model-based approach can be applied more generally to calibration of specropolarimetric data by choosing a suitable minimization criteria.