Fabry-Pérot etalons in solar astronomy. A review

During the last decades, the use of Fabry-Pérot etalons as filtergraphs has become frequent in solar instruments. The main reason is that they stand out for offering quasi-monochromatic, two-dimensional fields of view much higher than those provided by conventional slit-based spectrographs in a given time interval. Unfortunately, they also present several drawbacks. The number of etalons employed, the chosen way to illuminate them and the material they are made of have a large influence on the performance of the instrument. In this work we review and discuss the main results obtained by some of the most relevant studies in the design of etalon-based instruments. We present the general properties of etalons and their particularities when employed in solar instruments. We examine the (common) use of systems of several etalons to increase the free spectral range and to narrow down the filter transmission width. We compare the advantages and drawbacks of the two most common configurations —collimated and telecentric— paying special attention to their limitations. Finally, we also inspect the properties of crystalline etalons and their use in solar astronomy.