Enhancing the accuracy of solar polarimetry by coalescing slow and fast modulation: method description and first performance tests

Abstract

The polarimetric zero-level accuracy of spectropolarimetric measurements with ground-based solar telescopes usually suffers from systematic telescopic and instrumental effects which are difficult to model, and therefore, cannot be easily removed during post-measurement data reduction. Here, a novel measurement method to enhance the zero-level accuracy to an unprecedented level of such compromised measurements is presented. The method is comprised of adding a slow polarization modulation (< 1 Hz) before any polarizing component of the telescope to a high-sensitivity polarimeter with fast modulation (> 1 kHz). This additional slow modulation significantly mitigates systematic instrumental polarization signals induced by the telescope and post-focus instruments such as polarimetric offsets or cross-talk between polarization states. We present the results and limitations learned from implementing the method at the 45 cm Gregory-Coudé telescope at IRSOL, Locarno. The slow modulation is performed with a low-cost zero-order retarder film mounted in front of the telescope and is combined with the fast modulating Z¨urich IMaging POLarimeter (ZIMPOL). We find that the ground zero of polarization normalized to the intensity is determined within a few 10⁻⁵. This level is consistently achieved over a wide wavelength range in the visible. An improvement of up to a few orders of magnitude for cases where the polarization offset induced by the telescope is as high as 10⁻² is achieved. This measurement technique allows for enhancing the zero-level accuracy of solar polarimetry, which is crucial for scattering polarization measurements and their theoretical interpretations.