An overview of the preliminary optical design of the European Solar Telescope (EST)

The European Solar Telescope (EST) is a 4-m class-solar telescope that will become part of the next generation of groundbased facilities. Located at the “Observatorio del Roque de los Muchachos” in La Palma (Spain), it will be aimed to study the magnetic connectivity of the solar atmosphere with high spatial and temporal resolution. The EST optical design has been optimized during the preliminary design phase to maximize throughput, balance the instrumental polarization and to reduce the image rotation due to the change in orientation during operation. The optical system consists of a 4.2m active primary mirror located above the elevation axis to ensure natural air flushing and minimize local seeing degradation and a secondary mirror assembled as an Adaptive Secondary Mirror (ASM). Both arranged in an on-axis Gregorian configuration to deliver an aplanatic secondary focal plane. These are followed by four fold mirrors, which will be upgraded to deformable mirrors and are conjugated to different layers of the atmosphere. These, together with the ASM, M7 and two wavefront sensors, will make up the Multi-Conjugate Adaptive Optics system. Finally, a dioptric system, housed in a vacuum vessel, transfers the light to the science focus, which will be delivered to the Scientific Instrumentation by a dedicated distribution system. EST ultimately provides a diffraction-limited telecentric F/50 science focal plane covering a FOV of 90×90 arcsec² over a wavelength range from 380nm to 2300nm. Along the contribution, details about the preliminary optical design of EST and its subassemblies will be presented. The expected performance is also discussed.