Analysis of the optical alignment of the Ariel space telescope

Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) is ESA’s M4 mission within the “Cosmic Vision” program, set to launch in 2029. Its goal is to survey the atmospheres of known exoplanets using transit spectroscopy. The mission employs a 1-meter-class telescope that is optimized for spectroscopy in the 1.95 to 7.8 μm wavelength range, operating at cryogenic temperatures between 40 and 50 K. The Ariel Telescope features an off-axis, unobscured Cassegrain configuration, incorporating a parabolic recollimating tertiary mirror and a flat folding mirror that directs the output beam parallel to the optical bench. Additionally, the secondary mirror is mounted on a roto-translating stage to allow for adjustments during the mission. All mirrors and supporting structures are made from an aerospace-grade aluminium alloy, 6061-T651, chosen for its ease of manufacturing and thermalization. However, the material’s low stiffness presents unique challenges for integration and alignment. A series of simulations were conducted to analyse the telescope's alignment, with a specific focus on mechanical tolerances and their impact on the instrument's optical performance. In this study, the simulation is expanded by incorporating the interferometric measurements of the as-built mirrors surfaces into the model to accurately represent the system's behaviour and to assess how they influence image quality and overall optical performance. The paper thoroughly describes the simulation setup, the methodology used to assess tolerance effects, and presents the resulting data, offering valuable guidance for optimizing telescope alignment and ensuring robust optical performance.