Study and realization of a prototype of the primary off-axis 1-M diameter aluminium mirror for the esa ariel mission

ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) has been selected by ESA as the next mediumclass science mission (M4) to be launched in 2028. The aim of the ARIEL mission is to study the atmospheres of a selected sample of exoplanets. The payload is based on a 1-m class telescope ahead of a suite of instruments: two spectrometric channels covering the band 1.95 to 7.80 ?m and four photometric channels working in the range 0.5 to 1.9 ?m. The production of the primary mirror (M1) is one of the main technical challenges of the mission. A trade-off on the material to be used for manufacturing the 1-m diameter M1 was carried out, and aluminium alloys have been selected as the baseline materials both for the telescope mirrors and structure. Aluminium alloys have demonstrated excellent performances both for IR small size mirrors and structural components, but the manufacturing and thermo-mechanical stability of large metallic optics still have to be demonstrated especially at cryogenic temperatures. The ARIEL telescope will be realized on-ground (1 g and room temperature), but it shall operate in space at about 50 K. For this reason a detailed tolerance analysis was performed to assess the telescope expected performance. M1 is an off-axis section of a paraboloidal mirror and will be machined from a single blank as a stand-alone part. To prove the feasibility of such a large aluminium mirror, a pathfinder mirror program has been started. The prototype has been realized and tested, so far at room temperature, by Media Lario S.r.l. Cryogenic testing of the prototype will be performed during Phase B1.