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Imaging spectropolarimetry is an important observational tool in solarphysics because of fast-cadence spectral scans with high-spectral resolution,large field-of-view, and its inherent suitability for post-facto imagerestoration. Fabry-P\'erot etalons are the key optical elements of theseinstruments. Their optical quality critically defines the instrument'sperformance. The two etalons of the GREGOR Fabry-P\'erot Interferometer (GFPI)were used for more than 10~years, raising questions about the potentialdeterioration of etalons coatings. We present an assessment of the etalonsoptical quality, describe the inspection method based on Zernike polynomials,discuss the field dependence of the finesse and its consequences for instrumentdesign, and investigate the impact of the measurement technique to achieveplate parallelism. We find that extended exposure to sunlight affects theetalon coatings, i.e., lowering the peak transmission and leaving an imprint ofthe pupil of the GREGOR solar telescope on the etalon that is directly exposedto sunlight. The finesse of both etalons, however, remains high so that theimpact on imaging spectropolarimetry is negligible.

Measuring the Etalon Quality of the GREGOR Fabry-P\'erot Interferometer