The afocal telescope optical design and tolerance analysis for the ESA ARIEL Mission

ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is one of the three present candidates for the next ESA medium-class science mission (M4) to be launched in 2026. During its 3.5 years of scientific operations from L2 orbit, this mission will observe spectroscopically in the infrared (IR) a large population of known transiting planets in the neighbourhood of the Solar System. The aim is to enable a deep understanding of the physics and chemistry of these exoplanets. ARIEL is based on a 1-m class telescope ahead of a suite of instruments: two spectrometer channels covering the band 1.95 to 7.80 µm and four photometric channels (two wide and two narrow band) in the range 0.5 to 1.9 μm. The ARIEL optical design is conceived as a fore-module common afocal telescope that will feed the spectrometer and photometric channels. The telescope optical design is based on an eccentric pupil two-mirror classic Cassegrain configuration coupled to a tertiary paraboloidal mirror. An all-aluminum structure has been considered for the telescope layout, and a detailed tolerance analysis has been conducted to assess the telescope feasibility. This analysis has been done including the different parts of the realization and life of the instrument, from integration on-ground to in-flight stability during the scientific acquisitions. The primary mirror (M1) temperature will be monitored and finely tuned via an active thermal control system based on thermistors and heaters. The heaters will be switched on and off to maintain the M1 temperature within ±1K thanks to a proportional–integral–derivative (PID) controller.