Engineering the ELT’s reflective surfaces

Abstract

The ESO Extremely Large Telescope (ELT) requires a dedicated coating facility capable of processing all of its large mirrors—M2, M3, M4, and M5—within a single, highly flexible industrial-scale system. During telescope operations, its mirrors and coatings are exposed to open air and the dome environment, allowing dust, dirt, and other contaminants to accumulate on the optical surfaces, thereby degrading optical performance. Even when telescope operations involve regular in-situ cleaning, it is generally insufficient to restore the initial performance; therefore, the coatings must be renewed periodically to maintain the reflectivity and a low level of diffusion. The Large Coating Plant (LCP) has been designed to support mirrors up to 4.2 m in diameter and to accommodate convex, concave, and flat geometries in a single machine. The LCP will be used for the initial coating and regular recoating of the large mirrors at the ELT Technical Facility (ETF). A dedicated clean area is available within this ETF building, where the plant is installed. The ETF is located at the Paranal Observatory base camp at an altitude of 2360 m, approximately 30 km from the ELT telescope. While magnetron sputtering of a Protected Silver layer is the baseline for all ELT mirrors, a dedicated process is implemented for M4 due to embedded magnets on the rear side of its deformable mirror shells, which affect the homogeneity of the magnetron sputtering. Because of these interactions, M4 receives an evaporated aluminum coating using a purpose-configured evaporation system within the same LCP. The first part of this paper presents the capabilities and validation of the LCP, including an overview of acceptance testing and coating performance results. The second part of this paper presents a dedicated investigation aimed at enhancing the reflectivity of the aluminum layer within a specific band and tests to obtain an aluminum evaporation recipe for applying six consecutive coatings to the M4 petals under similar conditions, thereby achieving an even, segmented M4 reflectivity.