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Atmospheric Remote-Sensing Infrared Exoplanet Large Survey (Ariel) has been adopted as ESA “Cosmic Vision” M4 mission, with launch scheduled for 2029. Ariel is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 and 7.8 μm, operating in cryogenic conditions in the range 40–50 K. Aluminum has been chosen as baseline material for the telescope mirrors substrate, with a metallic coating to enhance reflectivity and protect from oxidation and corrosion. As part of Phase B1, leading to SRR and eventually mission adoption, a protected silver coating with space heritage has been selected and will undergo a qualification process. A fundamental part of this process is assuring the integrity of the coating layer and performance compliance in terms of reflectivity at the telescope operating temperature. To this purpose, a set of flat sample disks have been cut and polished from the same baseline aluminum alloy as the telescope mirror substrates, and the selected protected silver coating has been applied to them by magnetron sputtering. The disks have then been subjected to a series of cryogenic temperature cycles to assess coating performance stability. This study presents the results of visual inspection, reflectivity measurements and atomic force microscopy (AFM) on the sample disks before and after the cryogenic cycles.

Atmospheric Remote-Sensing Infrared Exoplanet Large Survey (ARIEL) is the M4 ESA mission to launch in 2028. ARIEL is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 μm and 7.8 μm (main instrument), operating in cryogenic conditions in the range 50 - 60 K. For the main mirror substrate, the Aluminum 6061 alloy has been chosen as baseline material after a trade- off. The large size of the mirror however (0.6 square meters) presents specific production challenges concerning opto-mechanical stability in cryogenic applications. To minimize risk, the machining, polishing, thermal treatments and coating processes will first be tested on flat samples of 150 mm of diameter and then applied to a full-size demonstrator mirror, before finalizing the design and producing the flight mirror. This study, following a review of existing literature on fabrication of Al 6061 mirrors for spaceborne IR applications will characterize the optical properties of the samples after each phase of thermal treatment with the goal of determining an optimal process for material stress release, figuring and surface finishing and final optical stability in the operating cryogenic environment.

In the framework of an assessment of optical polishing techniques, ESA has signed a contract with Media Lario to deliver two 250 mm mirrors with a common optical design to be polished down to very tight surface requirements. NOPT mirrors are respectively made of Zerodur and AlSi alloy with electroless nickel and will they be polished by means of bonnet polishing. Mirrors are light-weighted up to 20kg/m2; the mechanical and optical design is proven to be scalable up to 1m CA surfaces. This paper reviews the mirrors opto-mechanical design, introduce the polishing and metrology strategy while highlighting the differences and the common point in fabricating such mirror in Zerodur and metal.