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

TL;DR

The paper develops a forward-modeling framework to characterize spatial variations in the parameters of telecentric dual-etalon Fabry-Pérot interferometers, demonstrated on the CRISP2 instrument at the Swedish 1-m Solar Telescope. By combining a forward model of the FPI transmission with a template quiet-Sun spectrum, it recovers 2D maps of the high- and low-resolution etalon cavities and reflectivities, as well as a spatially varying prefilter curve, without changing the optical setup. Key findings show that including secondary peaks at ±1×FSR and accurately modeling the prefilter are essential for reliable reflectivity estimates, with cavity-separation maps exhibiting RMS variations below 1.9 nm and reflectivity variations below 0.4% for the HRE and 0.3% for the LRE at 617.3 nm. The work also analyzes model simplifications, field-dependent profile variations, and flat-field correction, providing practical guidance for characterizing current and future FPI instruments and contributing software to the community.

Abstract

Imaging Fabry-Pérot 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. 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, and the prefilter variation across the field-of-view. 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 spectra 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 times the Free Spectral Range and a detailed estimate of the prefilter curve is necessary to obtain accurate values of both etalon reflectivities. For narrow prefilters (relative to the FSR), the former requirement can be relaxed. Our results show that the cavity separation of CRISP2 is very flat, showing an RMS variation below 1.9 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 have assessed data and modeling requirements in order to derive accurate FPI parameters and minimize errors in the determination of etalon reflectivities.

Figures (15)

• Figure 1: CRISP2 transmission profile at 617.3 nm. Top: HRE, LRE and full transmission profiles, where the prefilter shape is also indicated. Bottom: transmission profile and transmission profile multiplied by the prefilter transmission, illustrating the attenuation of secondary lobes by the prefilter.
• Figure 2: Factory-measured reflectivities of the etalons (red) and the corresponding instrumental profile FWHM for the CRISP2 instrument (black). The solid red line depicts the reflectivity of the high-resolution etalon, whereas the dashed-red line corresponds to the reflectivity of the low-resolution etalon. The FWHM of the profile was calculated assuming perfect co-tuning of the profiles from the two etalons and the factory nominal reflectivities.
• Figure 3: Central lobe of the CRISP2 transmission profile calculated with three different recipes: perpendicular incidence (ray, black), telecentric beam without etalon tilt (conv, blue) and the full calculation including the tilt of the LRE (full, red). Top: Transmission profiles. Bottom: peak-normalized transmission profiles, where the blueshift induced by the angular integral has been compensated in the conv and full calculations for a better comparison of their FWHM and degree of asymmetry.
• Figure 4: Distribution of inclination angles across the (circular) pupil for a non-tilted case ($\alpha=0$, left) and a tilt of $\alpha=1/2F$ in the y-axis (right).
• Figure 5: Observed mean intensity (black) and the derived fit (red) in the 617.3 nm spectral window. The inferred prefilter curve is plotted with a dashed gray line. The light-gray spectrum represents the FTS atlas multiplied by the prefilter curve. The fitted curve is the result of the FTS spectrum multiplied with the prefilter curve and convolved with the nominal CRISP2 transmission profile.