Design of a Fabry-Perot interferometer for the SO/PHI instrument on Solar Orbiter

We present our work on the spectral analyser of the Polarimetric and Helioseismic Imager (PHI) instrument to be flown aboard ESA's Solar Orbiter mission. We detail the choices that were made to determine the concept of the spectral analyser, a Lithium Niobate Fabry-Perot interferometer, and its characteristics, as to fulfil both scientific needs and technical requirements. We will present the first experimental results - including stability, repeatability, parallelism, spectral homogeneity and imaging capability - on an air-spaced piezoelectric-tunable etalon, which is the backup solution for PHI.