This paper describes the preliminary design of the Adaptive Secondary Mirror (ASM) for the European Solar Telescope (EST), as designed by TNO. This ASM will contain 1950 actuators over an optical aperture of Ø80cm. The mirror-shell of this ASM is concave with a radius of 2.156m and has a thickness of 2.5mm. To cope with the high optical heat-load, the mirror shell will be cooled actively by means of conduction through a small (0.3mm) airgap between the mirror shell and the cooled SiC supporting structure. One of the unique characteristics of this ASM are the highly efficient actuators by TNO, delivering 30N Peak-to-Valley of linear force range within a packaging of Ø15x40mm. These actuators are based on the proven electromagnetic actuation principle by TNO but have been significantly redesigned to improve manufacturability and integration. The actuators are interfaced to the mirror shell via a stiff carbon fiber strut assembly and are designed to be replaceable during off-telescope maintenance. For overall alignment of the ASM a hexapod is used, while a faster tip-tilt stage is used to compensate for tip-tilt disturbances up to 20Hz bandwidth. The optical performance of the ASM under various environmental conditions has been extensively studied with Finite-Element-Analysis. To verify the performances of the actuator and the cooling systems, an actuator and thermal- breadboard are being constructed.
The European Solar Telescope (EST) is a 4.2-m telescope which has been redesigned with a fully integrated Multi-Conjugate Adaptive Optics (MCAO) into the optical path right after the EST primary mirror. The current baseline configuration considers four altitude Deformable Mirrors (DM) conjugated to 5, 9, 12 and 20 km above the telescope entrance pupil and an Adaptive Secondary Mirror (ASM) conjugated to the entrance pupil. The wavefront sensing will be performed by a set of correlation-based Shack Hartmann wavefront sensors (WFS) combining an on-axis High-Order WFS (HOWFS) to be used either in Single Conjugate AO (SCAO) to drive the ASM as well as operating simultaneously with a Multi-Directional WFS (MDWFS) to drive the MCAO. Beyond the current baseline configuration, different alternatives are currently being investigated both in the wavefront sensing strategy by evolving from a HOWFS+MDWFS into possibly a single High Order Multi Directional WFS (HOMDWFS) and/or wavefront sensors operating at different observing bands.
The European Solar Telescope (EST) aims to become the most ambitious ground-based solar telescope in Europe. Its roots lie in the knowledge and expertise gained from building and running previous infrastructures like, among others, the Vacuum Tower Telescope, Swedish Solar Telescope, or the GREGOR telescope. They are installed in the Canary Islands observatories, the selected EST site. Furthermore, the telescope has a novel optical design, including an adaptive secondary mirror (ASM) that allows reducing the number of optical surfaces to 6 mirrors (plus two lenses) before the instruments’ focal plane. The latter, combined with a configuration of mirrors that are located orthogonally oriented to compensate for the instrumental polarisation induced by each surface, makes EST a reference telescope in terms of throughput and polarimetric accuracy. In its main core design, EST also includes a Multi-Conjugated Adaptive Optics (MCAO) system where the ASM compensates for the ground layer turbulence. The rest of the mirrors on the optical train correct for the atmospheric turbulence at different layers of the atmosphere. The MCAO guarantees that the large theoretical spatial resolution of the 4-metre EST primary mirror is achieved over a circular FOV of 60 arcsec. Those main elements, combined with a set of instruments with capabilities for spectropolarimetry, make EST the next frontier in solar ground-based astronomy. In this contribution, we will cover the main properties and status of all the mentioned sub-systems and the following steps that will lead to the construction phase.
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