The European Solar Telescope (EST) is a 4.2-m solar telescope, based on an aplanatic Gregorian configuration with an alt-azimuthal mount. EST has an open configuration for observations without dome to improve the local seeing around the telescope mount and the main optical elements, enhancing the natural air flushing through the structure. This approach minimizes the active thermal control and reduces the telescope´s installation and operating costs. In contrast, the wind will be an extra load that requires a stiff telescope structure, as well as an advanced adaptive optics and control system. This contribution presents the comprehensive wind tunnel testing campaign carried out for the preliminary design of the Telescope Structure, Enclosure and Pier. The objective of these tests is to compare the results obtained from Computational Fluid Dynamics (CFD) analysis and to consolidate the results obtained from the Finite Elements (FE) and the end-to-end control analysis. For this purpose, three models were built. The first one is a topography model with a 2 km radius area, the second one is a detailed proximity model including neighboring telescopes and the last one is a detailed telescope model including the axes and main optical elements. The results of the tests were compared with FE and CFD analysis, observing differences lower than 15% in representative indicators. This allowed for an empirical correlation of the models. Additionally, the wind load contribution to the pointing and tracking error budgets were replicated with the correlated models, bringing compliant results with the performance requirements.
The European Solar Telescope (EST) is a next generation large-aperture solar telescope, to be located in the Canary Islands. It will be optimized for studies of the magnetic coupling of the solar atmosphere. This will require diagnostics of the thermal, dynamic and magnetic properties of the plasma over many scale heights, by using multi-wavelength imaging, spectroscopy and spectropolarimetry. The optical design of the EST is based on an aplanatic Gregorian telescope, characterized by a 4.2-metre primary mirror, installed above the elevation axis with the aim of enhancing the natural air flushing. The EST works in open configuration, requiring an active/passive thermal control at telescope level to comply with the maximum temperature gradients of ±2°C. The telescope will be placed on the top of a tower to improve the local seeing conditions. The open configuration exposes the telescope to wind disturbances, higher than in other telescopes. The natural frequency of the global modes affecting the position servosystem bandwidth of the telescope are stablished in 12-15 Hz to ensure pointing and tracking accuracy of 2.3 arcsec and 0.8 arsec during 10 mins, respectively. Sophisticated end-to-end control analysis have been carried out to assess in detail the effects of the wind disturbances, but also the impact of non-linear friction, cogging and torque ripple, among others. CFD analyses and wind tunnel test campaigns have been performed to verify the performance of the telescope in operational conditions. The main axes of the telescope must be in parking position before the closure of the retractable enclosure in order to optimize its size. This requires a robust design, including redundancy in azimuth and elevation mechanisms to ensure the protection of the telescope in case of failure. The detailed maintenance strategy has been also established to ensure that every operation can be performed with the closed enclosure.
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