The Primary Mirror of the ARIEL Mission: Study of Thermal, Figuring and Finishing Treatments and Optical Characterization of Al 6061 Samples Mirrors

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 mu m and 7.8 mu m (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.