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17 December 2020 HARMONI - first light spectroscopy for the ELT: broadband reflective coatings
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Abstract
HARMONI is the first light visible and near-infrared integral field spectrograph for the Extremely Large Telescope. It covers a broad spectral range from 450 nm to 2450 nm with resolving powers from R (≡λ/Δλ)3500 to 18000 and spatial sampling from 60 mas to 4 mas. The HARMONI optical design is predominantly reflective, with approximately 25 mirrors in the science path. We, therefore, want to maximise the instrument transmission by procuring the best reflective coatings for our optics. Reflective dielectric coatings offer better performance than metallic coatings but are challenging for our wide wavelength range. We discussed with several companies and purchased from Asahi Spectra and Optics Balzers. On average, the theoretical reflectance of the provided coatings is higher than 99% with features. To ensure the applicability of the coatings for HARMONI, we tested the coating survivability of the procured samples considering their adherence, cryogenic cycling endurance, and cleaning ease. We report reflectance measurements for one of our AOI design ranges. We also discuss which of HARMONI's many reflective surfaces we could coat considering their optics size and surface shape.
© (2020) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Andrea Hidalgo and Fraser Clarke "HARMONI - first light spectroscopy for the ELT: broadband reflective coatings", Proc. SPIE 11451, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation IV, 114515V (17 December 2020); https://doi.org/10.1117/12.2560329
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