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

Abstract: 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. Ariel was adopted in 2020 as the M4 mission in ESA "Cosmic Vision" program, with launch expected in 2029. The mission will operate from the Sun-Earth Lagrange Point L2. The scientific payload consists of two instruments: a high resolution spectrometer in the waveband 1.95-7.8 microns, and a fine guidance system / visible photometer / low resolution near-infrared spectrometer. 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. Telescope mirrors and supporting structures will be realized in aerospace-grade aluminum. Given the large aperture of the primary mirror (0.6 m$^2$), 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, starting with a systematic review of the optical and opto-mechanical requirements and design choices of the Ariel telescope, in the context of previous development work and scientific goals and requirements of the mission. The review then progresses with opto-mechanical design, examining the most important choices in terms of structural and thermal design, and with 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 details of the qualification work on thermal stabilization, polishing and coating are then presented.