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

Fabry-Pérot interferometer (FPI) observations are commonly used in solar physics to infer physical parameters in the photosphere and chromosphere using observational modeling. Such techniques require detailed knowledge of the spectral instrumental profile to produce accurate results. We present a method to characterize the spatial variation of the parameters of dual-etalon FPI instruments mounted in a telecentric configuration: spatially resolved cavity separation, the reflectivities of both etalons, and the prefilter variation across the field of view. We aim to characterize the field-of-view dependence of the parameters of the new CRISP2 FPI. We implemented a forward model of the FPI instrumental degradation combined with a template average quiet-Sun spectrum at disk center to model two sets of observational data. Our method does not require any changes to the optical setup or the use of external illumination sources. We assessed the validity of several functional forms to calculate the FPI transmission profiles. Our results show that, in general, including the secondary transmission peaks at ± 1 times the free spectral range (FSR) and a detailed estimate of the prefilter curve are necessary to obtain accurate values of both etalon reflectivities. For very narrow prefilters relative to the FSR, the former requirement can be relaxed. Our results show that the CRISP2 cavity separation 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. We assessed the data and modeling requirements needed to derive accurate dual-etalon telecentric FPI parameters and minimize errors in determining etalon reflectivities. 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 .