Local seeing analysis for the European Solar Telescope

The European Solar Telescope (EST) will be built at the Observatorio del Roque de los Muchachos, a site known for its excellent atmospheric seeing conditions. The fundamental design of EST lies on two premises to improve the local seeing: placing the primary mirror as far as possible from the ground layer and using an open-air configuration. In this setting, the telescope will benefit from undisturbed wind allowing natural ventilation. These considerations aim to optimize the site conditions; however, the design itself may introduce local seeing. As the design evolves from conceptual to preliminary phase, the shape of the EST has profoundly changed, and the fundamental aspects shall be re-evaluated. In this article, strategic aspects of the current design are studied, including elements from the telescope structure, the pier, and the enclosure. These trade-offs considered the local seeing as a driving force seeking to optimize the optical performance and provide feedback to the current design of EST. This study uses transient Computational Fluid Dynamics (CFD) to estimate the air temperature and refractive index as it flows over the observatory and the telescope. The air refractive index inside the optical path is then analyzed using postprocessing scripts to obtain appropriate seeing metrics. Predominant wind directions and median wind speed at the site together with environmental conditions and different telescope orientations were used to thoroughly analyze the local seeing conditions. Optical results enabled the characterization of the spatial distribution of turbulence, assimilating it to a Kolmogorov law. These results are valuable input for the telescope’s error budget in terms of local seeing.