Optical and Opto-mechanical Analysis and Design of the Telescope for the Ariel Mission

The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (Ariel) is the first space mission dedicated to measuring the chemical composition and thermal structures of thousands of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. Ariel was officially adopted in 2020 as the fourth medium size (M4) mission in the scope of ESA “Cosmic Vision” program, with launch expected in 2029. The mission will operate from the Sun-Earth Lagrangian point L2. The scientific payload consists of two instruments: a highresolution spectrometer covering the waveband 1.95–7.8 µm, and a multi-purpose fine guidance system / visible photometer / low resolution near-infrared spectrometer with wavelength coverage between 0.5 µm and 1.95 µm. The instruments are fed a collimated beam from an unobscured, off-axis Cassegrain telescope. Instruments and telescope will operate at a temperature below 50 K. The mirrors and supporting structures of the telescope will be realized in aerospace-grade aluminum. Given the large aperture of the primary mirror (0.6 m2), it is a choice of material that requires careful optical and opto-mechanical design, and technological advances in the three areas of mirror substrate thermal stabilization, optical surface polishing and optical coating. This thesis presents the work done by the author in these areas, as member of the team responsible for designing and manufacturing the telescope and mirrors. The dissertation starts with a systematic review of the optical and opto-mechanical requirements and design choices of the Ariel telescope, in the context of the previous development work and the scientific goals and requirements of the mission. The review then progresses with the opto-mechanical design, examining the most important choices in terms of structural and thermal design. This will serve as an introduction to a statistical analysis of the deformations of the optical surface of the telescope mirrors and of their alignment in terms of rigid body motions. The qualification work on thermal stabilization, polishing and coating is then presented. The three procedures have been set up and tested to demonstrate the readiness level of the technological processes employed to fabricate the mirrors. The first process, substrate thermal stabilization, is employed to minimize deformations of the optical surface during cool down of the telescope to the operating temperature below 50 K. Purpose of the process is to release internal stress in the substrate that can cause such shape deformations. Then a combined optical surface figuring/polishing process is applied to reduce residual surface shape errors and bring surface roughness to below 10 nm RMS. Polishing of large aluminum surfaces to optical quality is notoriously difficult due to softness of the material, so a dedicated polishing recipe was set up and tested. Finally, an optical coatingrecipe with protectedsilver was characterized in terms ofreflectivity and qualified for environmental stability, particularly at cryogenic temperatures, and for uniformity. Some of the coated samples are also being monitored and measured periodically for any sign of performance degradation while they age. All tests were performed on samples of the same aluminum alloy chosen as baseline for the mirror substrates and on a full-scale prototype of the Ariel primary mirror. Results from the coating characterization were also used to prepare an estimation of the various components contributing to the expected throughput of the telescope at the end of the scientific lifetime of the mission.