# On Fabry P\'erot Etalon based Instruments. I. The Isotropic Case

## TL;DR

This paper evaluates the spectral and imaging performance of Fabry-Pérot etalons in solar magnetographs, comparing collimated and telecentric configurations, and analyzing how imperfections affect their effectiveness.

## Contribution

It provides a detailed analysis of how real-world imperfections impact the spectral and imaging properties of Fabry-Pérot etalons in different configurations.

## Key findings

- Collimated configuration offers better PSF performance but varies across the field.
- Telecentric configuration produces constant PSF but introduces artificial signals.
- Imperfections in telecentric setups reduce photon flux and distort spectral profiles.

## Abstract

Here we assess the spectral and imaging properties of Fabry P\'erot etalons when located in solar magnetographs. We discuss the chosen configuration (collimated or telecentric) for both ideal and real cases. For the real cases, we focus on the effects caused by the polychromatic illumination of the filter by the irregularities in the optical thickness of the etalon and by deviations from the ideal illumination in both setups. We first review the general properties of Fabry P\'erots and we then address the different sources of degradation of the spectral transmission profile. We review and extend the general treatment of defects followed by different authors. We discuss the differences between the point spread functions (PSFs) of the collimated and telecentric configurations for both monochromatic and (real) quasi-monochromatic illumination of the etalon. The PSF corresponding to collimated mounts is shown to have a better performance, although it varies from point to point due to an apodization of the image inherent to this configuration. This is in contrast to the (perfect) telecentric case, where the PSF remains constant but produces artificial velocities and magnetic field signals because of its strong spectral dependence. We find that the unavoidable presence of imperfections in the telecentrism produces a decrease of flux of photons and a shift, a broadening and a loss of symmetrization of both the spectral and PSF profiles over the field of view, thus compromising their advantages over the collimated configuration. We evaluate these effects for different apertures of the incident beam.