Characterization of telecentric dual-etalon Fabry-P\'erot systems from observational data. Properties of the CRISP2 instrument at the Swedish 1-m Solar Telescope

Abstract

Imaging Fabry-P\'erot Interferometer (FPI) observations are commonly used in solar physics to infer physical parameters in the photosphere and chromosphere through modeling of the observations. Such techniques require detailed knowledge of the spectral instrumental profile in order to produce accurate results. In this study we present a method to characterize the spatial variation of parameters of dual-etalon FPI instruments mounted in telecentric configuration: spatially-resolved cavity separation and reflectivities of both etalons, as well as the prefilter variation across the field-of-view. Here, we aim at characterizing the field-of-view dependence of the parameters of the new CRISP2 FPI. We have implemented a forward model of the FPI instrumental degradation combined with a template average quiet-Sun spectrum at disk center in order to model two sets of observational data. Our method does not require any change in the optical setup or the utilization of external sources of illumination. We assess the validity of several functional forms in the calculation of the FPI transmission profiles. Our results show that (generally) the inclusion of the secondary transmission peaks at 1 FSR and a detailed estimate of the prefilter curve is necessary to obtain accurate values of both etalon reflectivities. Our results show that the cavity separation of CRISP2 is very flat, with an RMS variation below 2 nm over the entire field-of-view for both etalons. Reflectivity RMS variations are 0.4% and 0.3% for the primary and secondary etalons at 617.3 nm. The methods described in this paper are relevant for the characterization of present and future FPI instruments and we have made them publicly available to the solar community.