The polarimetric calibration and alignment assembly of the European Solar Telescope

Abstract

The European Solar Telescope (EST) is a next-generation 4.2 m solar facility designed to achieve high-sensitivity spectropolarimetric observations of the solar atmosphere. Meeting its scientific objectives requires a precise characterization of the telescope Mueller matrix, which imposes stringent requirements on the design of the polarimetric calibration tools. This work presents the preliminary design of the Polarimetric Calibration and Alignment (PCA) assembly, located around the Gregorian focal plane (F2). The PCA is implemented as a modular system organized into four optomechanically independent levels distributed along the converging beam at F2, enabling functional separation between polarimetric calibration, adaptive optics calibration, alignment and diagnostic tasks. The design is driven by the combined constraints of broadband operation (380–1600 nm), high thermal load at F2, and stringent polarimetric stability requirements. The baseline polarization calibration concept employs a wire-grid polarizer combined with compound MgF₂ zero-order retarders to generate controlled input polarization states for system characterization. A detailed modelling of the calibration optics is presented, including the impact of interference fringes, thermal variations, and clocking errors. Finite element thermal simulations under worst-case solar radiation are used to constrain operational stability and define calibration time windows. Finally, prototype development activities for both polarizer and retarder units are introduced as part of the validation strategy for the final design.