Test of Protected Silver Coating on Aluminum Samples of ARIEL Main Telescope Mirror Substrate Material

Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) has been adopted as the M4 mission for ESA “Cosmic Vision” program. Launch is scheduled for 2029. ARIEL will study exoplanet atmospheres through transit spectroscopy with a 1 m class telescope optimized in the waveband between 1.95 and 7.8 μm and operating in cryogenic conditions in the temperature range 40-50 K. Aluminum alloy 6061, in the T651 temper, was chosen as baseline material for telescope mirror substrates and supporting structures, following a trade-off study. To improve mirrors reflectivity within the operating waveband and to protect the aluminum surface from oxidation, a protected silver coating with space heritage was selected and underwent a qualification campaign during Phase B1 of the mission, with the goal of demonstrating a sufficient level of technology maturity. The qualification campaign consisted of two phases: a first set of durability and environmental tests conducted on a first batch of coated aluminum samples, followed by a set of verification tests performed on a second batch of samples coated alongside a full-size demonstrator of Ariel telescope primary mirror. This study presents the results of the verification tests, consisting of environmental (humidity and temperature cycling) tests and chemical/mechanical (abrasion, adhesion, cleaning) tests performed on the samples, and abrasion tests performed on the demonstrator, by means of visual inspections and reflectivity measurements.