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Chromaticity in solar adaptive optics: a case study for the European Solar Telescope

The effects of atmospheric dispersion on the performance of astronomical Adaptive Optics (AO) were already described in the 70’s, and they are generally regarded irrelevant to nighttime AO, except for the very specific case of Extreme AO for the next generation of 30 and 40-m class large ground-based telescopes. However, the situation changes when evaluating the impact of chromatic aspects for solar AO observations, due mostly to three factors. First, the usually more turbulent ground-layer conditions experienced during daytime observations. Second, solar telescopes operating at mountain sites need to be designed to be efficient at low elevation angles, when the atmospheric turbulence is usually the best. Last but not least, as in the case of the European Solar Telescope (EST), the solar AO simultaneously feeds several instruments probing the light spectrum from the Near-Ultraviolet (NUV) to the Near-Infrared (NIR). In this contribution, we review the literature to assess the impact of atmospheric dispersion in the context of a 4.2-m solar telescope as planned for the EST and how the chromatic AO errors can impact the wavefront sensing and correction architectures.