Most telescope mirrors of the European ELT shall have protected silver coatings that are highly reflective across all astronomic wavelength bands from the UV (370 nm) to the far infrared. Another aspect is that the coatings shall be frequently cleaned in situ, so they must be highly durable against abrasion and chemical agents such as sulfur and salt. However, they also must be easy to strip with a non-toxic etchant before each recoating. We identify trades and optimize silver coating recipes with different protection layer materials and thicknesses to meet these partly contradictory objectives. A test coating campaign is ongoing.
For the ELT, a total of 931 M1 Segment Assemblies will be manufactured. These will be of 133 different types, 7 copies each, with different optical and mechanical properties. The manufacturing of the segment support, the glass blank and the polishing will be done by industrial partners. ESO will be responsible for the shipment of the Segment Assemblies to Chile, for the integration of the edge sensors and their electronics, and for the cleaning and coating. After performing several health- and quality-checks, the Segment Assemblies will be temporarily stored in the warehouse, before being installed at the telescope and eventually recoated around every 2 years. The telescopes and instruments for optical astronomy are usually prototypes, while a new approach is required to manage such a series production of crucial components, which differ in small but significant aspects. In this paper, we will present the processes we have developed to manage the series production of M1 Segment Assemblies for the ELT, starting from the reception of the Segment Assemblies in Chile, inspection, installation of sub-components, health-checks, storage, and installation at the telescope.
Several telescopes like VISTA or the ELT are using or will use silver coatings, replacing aluminum (Al). The advantage of silver is a higher overall reflectivity, in particular around 825 nm. Yet, silver must be protected by covering layer(s), which lowering the reflectivity in the blue/UV region. Therefore, ESO completed a 2-year development with Fraunhofer IOF under the objective of extending the wavelength range of high reflectivity to shorter wavelengths without decreasing the coating durability. The developed coatings have been examined by standardized tests like scratching sensitivity, peeling, salt mist and H2S gas. The quasi-standard in silver coatings for telescope mirrors was developed for the Gemini observatory and it uses nickel chromium nitride (NiCrNx) as interlayer between silver and the protective top layer(s), finished by a hard silicon nitride (Si3N4) layer of ~ 15 nm thickness. We replaced the NiCrNx by aluminium oxide (AlOx) and it led to higher reflectivity but low durability towards the salt mist test. This test is relevant for many large telescopes because of their proximity to oceans, causing salty airborne dust. By replacing the NiCrNx by a ruthenium-based layer and by optimizing the protection, the objective of extending the wavelength range of high reflectivity to shorter wavelengths without decreasing the coating durability could be achieved.
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