Imaging spectropolarimetry is an important observational tool in solar physics because it provides fast-cadence spectral scans with high spectral resolution, large field-of-view, and inherent suitability for post facto image restoration. Fabry–Pérot etalons are the key optical elements of these instruments. Their optical quality critically defines the instrument’s performance. The two etalons of the GREGOR Fabry–Pérot interferometer were used for more than 10 years, raising questions about the potential deterioration of etalon coatings. We present an assessment of the etalons’ optical quality, describe the inspection method based on Zernike polynomials, discuss the field dependence of the finesse and its consequences for instrument design, and investigate the impact of the measurement technique to achieve plate parallelism. We find that extended exposure to sunlight affects the etalon coatings, i.e., lowering the peak transmission and leaving an imprint of the pupil of the GREGOR solar telescope on the etalon that is directly exposed to sunlight. The finesse of both etalons, however, remains high, so the impact on imaging spectropolarimetry is negligible.
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