  Imaging spectropolarimetry is an important observational tool in solar
physics because of fast-cadence spectral scans with high-spectral resolution,
large field-of-view, and its inherent suitability for post-facto image
restoration. Fabry-P\'erot 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-P\'erot Interferometer (GFPI)
were used for more than 10~years, raising questions about the potential
deterioration of etalons 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 that the
impact on imaging spectropolarimetry is negligible.
