The making of robust and highly performing imaging spectropolarimeters for large solar telescopes

TL;DR

This work presents a comprehensive blueprint for robust, high-performance imaging spectropolarimeters based on dual Fabry-Perot interferometers operating in telecentric, lens-based reimaging chains. It shows how combining a high-reflectivity HR etalon with a low-reflectivity LR etalon mitigates cavity-error–driven spectral distortions, while focus compensation preserves image quality in telecentric configurations. The paper details calibration strategies (pixel-by-pixel cavity/error mapping, pinhole-based alignments) and an image-restoration pipeline (MOMFBD) that together deliver near-diffraction-limited, multi-line polarimetric data; it also presents simulations and parameter optimizations for seven EST-like FPI systems to guide future large-aperture solar telescopes. Collectively, the results demonstrate the practicality of compact, fold-free FPI instruments with wide spectral coverage, high throughput, and high polarimetric fidelity, shaping the design of next-generation solar instrumentation. The methods and designs here provide a concrete pathway for deploying robust spectropolarimetric capabilities on 4-meter-class facilities like EST, with implications for data quality, calibration, and processing in solar physics.

Abstract

We discuss the requirements, concepts, simulations, implementation, and calibration of two dual Fabry-Perot based imaging spectropolarimeters, CRISP and CHROMIS, at the Swedish 1-meter Solar Telescope, and CRISP2 that is under construction. These instruments use a combination of a high-resolution and a low-resolution etalon together with an order-sorting prefilter to define the bandpass. The overall design is made robust and stable by tailoring the low-resolution etalon reflectivity to accommodate expected cavity errors from both etalons, and by using a compact optical design that eliminates the need for folding mirrors. By using a telecentric design based on lenses rather than mirrors, image degradation by the FPI system is negligible, as shown in a previous publication, and the throughput of the system is maximised. Initial alignment, and maintaining that alignment over time, is greatly simplified. The telecentric design allows full calibration and/or modelling of essential system parameters to be carried out without interfering with the optical setup. We also discuss briefly the polarimeters developed for CRISP and CHROMIS. The high performance of CRISP and CHROMIS has been demonstrated in an earlier publication through measurements of the granulation contrast and comparisons with similar measurements simultaneously made through broadband continuum filters. Here, we focus on the aspects of the design that are central to enabling high performance and robustness, but also discuss the calibration and processing of the data, and use a few examples of processed data to demonstrate the achievable image and data quality. We put forward a proposal for a similar conceptual design for the European Solar Telescope and conclude by discussing potential problems of the proposed approach to designs of this type. Some aspects of these FPI systems may be of interest also outside the solar community.

Figures (8)

• Figure 1: Averaged scans of a spectral region of the solar Fe I lines and the telluric lines around 630.2 nm recorded with CRISP (dots) together with the scanned FTS spectrum of 1984SoPh...90..205N, degraded by the transmission profile of CRISP, as determined from fits of the reflectivities and cavity errors shown in Fig. \ref{['fig:etalons']}. The dashed curve corresponds to the transmission curve of the prefilter, as obtained from the same data.
• Figure 2: Cavity maps and reflectivity maps of the high-resolution (HR) and low-resolution (LR) etalons of CRISP, obtained by first recording images while scanning the two etalons together, then scanning the LR etalon across the passband of the HR etalon tuned to a continuum wavelength, and finally fitting the scanned profiles from the Fourier Transform Spectrometer at the McMath-Pierce Telescope 1984SoPh...90..205N.
• Figure 3: Collage of high resolution CRISP and CHROMIS images showing a plage (top) and parts of active regions (bottom). The 400 nm (clean continuum), line core Ca II K , and H$\beta$ images were recorded with CHROMIS, the Fe I 617.3 nm and Ca II 854.2 nm images are line core images recorded with CRISP. These images, selected from multi-wavelength scans through the corresponding spectral lines, illustrate the wide range of multi-line diagnostics possible by combining data from CRISP and CHROMIS, and the quality of the data.
• Figure 4: High resolution CRISP data connecting the solar chromosphere (top) and photosphere (bottom). The top two panels show a line core Ca II 854.2 nm image of the quiet Sun, and the LOS magnetic field obtained from a polarimetric scan through the same line. The inversion with the Ca II 854.2 nm data uses the weak-field approximation with spatio-temporal coupling 2024AA...685A..85D and the chromospheric LOS magnetic field map shown is clipped outside $\pm$50 G. The lower two panels show an image recorded in the continuum near the Fe I 617.3 nm line, and the LOS magnetic field obtained from a regularised Milne-Eddington inversion of a polarimetric scan through the same line. The photospheric LOS magnetic field map shown is clipped outside $\pm$25 G.
• Figure 5: High resolution CHROMIS (400 nm and Ca II K) and CRISP (617.3 nm and H$\alpha$ 656.3 nm) images displaying the photospheric and chromospheric fine structure of an active region. The lower four panels show the corresponding full spectral scans through the cores of the same spectral lines, and also that of the Ca II 854.2 nm lines at four spatial locations indicated with colours that correspond to those of the spectral scans. The black dashed line corresponds to the line profile averaged over the entire FOV. The data shown are from 2025AA...696A...3L.