The manufacturing development of the 1.1 meter aluminium mirror for the Ariel mission

Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) is the fourth medium-sized mission (M4) of ESA "Cosmic Vision" program. Ariel optical payload consists of an off-axis, unobscured Cassegrain telescope feeding a set of photometers and spectrometers in the waveband between 0.5 and 7.8 μm, operating at cryogenic temperatures (50K). The Ariel telescope consists of a primary parabolic mirror (M1), followed by a hyperbolic secondary (M2), a parabolic recollimating tertiary (M3) and a flat folding mirror (M4). Media Lario is the industrial enabler in charge of the manufacturing development of the M1 mirror; made of aluminium 6061-T651, the M1 is an off-axis parabola with a big elliptical aperture having about 1.1 m major axis and 0.75 m minor axis. Among the manufacturing processes, diamond turning is the baseline process to produce the off-axis shape, followed by deterministic polishing in Media Lario and finally coated with protected silver coating by CILAS (France). Two development models have been manufactured at LT Ultra Precision (LTU, Germany) with a new diamond turning machine able to work mirrors up to 1.2 m diameter. The deterministic polishing was performed in the Media Lario laboratories with Zeeko Intelligent Robot Polisher IRP-1200, which offers a deterministic figuring and polishing process, allowing to work on aspheric, off-axis, or freeform optical designs, including the parabolic design of Ariel M1. Guided by interferometric metrology data, a figuring and polishing routine is created to control all the physical and geometrical parameters and the polishing bonnet dwell time across the surface. The results of the development models of the M1 mirror are presented and discussed in this paper. © 2025 ESA and CNES.