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

Franziska Zeuner; Daniel Gisler; Michele Bianda; Renzo Ramelli; Svetlana V. Berdyugina

doi:10.1117/12.2629250

29 August 2022 Enhancing the accuracy of solar polarimetry by coalescing slow and fast modulation: method description and first performance tests

Franziska Zeuner, Daniel Gisler, Michele Bianda, Renzo Ramelli, Svetlana V. Berdyugina

Author Affiliations +

Proceedings Volume 12184, Ground-based and Airborne Instrumentation for Astronomy IX; 121840T (2022) https://doi.org/10.1117/12.2629250
Event: SPIE Astronomical Telescopes + Instrumentation, 2022, Montréal, Québec, Canada

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.

Conference Presentation

Citation Download Citation

Franziska Zeuner, Daniel Gisler, Michele Bianda, Renzo Ramelli, and Svetlana V. Berdyugina "Enhancing the accuracy of solar polarimetry by coalescing slow and fast modulation: method description and first performance tests", Proc. SPIE 12184, Ground-based and Airborne Instrumentation for Astronomy IX, 121840T (29 August 2022); https://doi.org/10.1117/12.2629250

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