Measuring the etalon quality of the GREGOR Fabry–Pérot interferometer

Abstract

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-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 (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.

Improved High-resolution Fast Imager

“…With a new Fabry-Pérot interferometer on the horizon, covering an almost identical wavelength range as the GREGOR Fabry–Pérot Interferometer (GFPI) 30 , 43 , the decision to discontinue the GFPI became imminent. Currently, the option is explored to adapt the Fabry-Pérot etalons 44 for imaging spectropolarimetry in the blue part of the solar spectrum, building on an existing design for the blue imaging solar spectrometer (BLISS) 45 . As a result, the two Imager M-lite 2M CMOS cameras of the GFPI and the two Imager sCMOS cameras of the original HiFI 46 , 47 became available.…”