On Fabry–Pérot Etalon-based Instruments. III. Instrument Applications

The spectral, imaging, and polarimetric behavior of Fabry-P\'erot etalons have an influence on imaging vector magnetograph instruments based on these devices. The impact depends, among others, on the optical configuration (collimated or telecentric), on the relative position of the etalon with respect to the polarimeter, on the type of etalon (air-gapped or crystalline), and even on the polarimetric technique to be used (single-beam or dual-beam). In this paper we evaluate the artificial line-of-sight velocities and magnetic field strengths that arise in etalon-based instruments attending to the mentioned factors. We differentiate between signals that are implicit to telecentric mounts due to the wavelength dependence of the point-spread function and those emerging in both collimated and telecentric setups from the polarimetric response of birefringent etalons. For the anisotropic case we consider two possible locations of the etalon, between the modulator and the analyzer or after it, and we include the effect on different channels when dual-beam polarimetry is employed. We also evaluate the impact of the loss of symmetry produced in telecentric mounts due to imperfections in the illumination and/or to a tilt of the etalon relative to the incident beam.