The optical configuration of the telescope for the ARIEL ESA mission

The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) has been recently selected as the next ESA medium-class mission (M4) with a foreseen launch in 2028. During its 3.5 years of scientific operations, ARIEL will observe spectroscopically in the infrared (IR) a large population of known transiting planets in the neighbourhood of the Solar System. ARIEL aims to give a breakthrough in the observation of exoplanet atmospheres and understanding of the physics and chemistry of these far-away worlds. ARIEL is based on a 1-m class telescope feeding a collimated beam into two separate instrument modules: a spectrometer module covering the waveband between 1.95 μm and 7.80 μm; and a combined fine guidance system/visible photometer/NIR spectrometer. The primary payload is the spectrometer, whose scientific observations are supported by the fine guidance system and photometer, which is monitoring the photometric stability of the target and allowing, at the same time, the target to be properly pointed. The telescope configuration is a classic Cassegrain layout used with an eccentric pupil and coupled to a tertiary off-axis paraboloidal mirror; the design has been conceived to satisfy all the mission requirements, and it guarantees the requested “as-built” diffraction limited performance. To constrain the thermo-mechanically induced optical aberrations, the primary mirror (M1) temperature will be monitored and finely tuned using an active thermal control system based on thermistors and heaters. They will be switched on and off to maintain the M1 temperature within ±1 K by the Telescope Control Unit (TCU). The TCU is a payload electronics subsystem also responsible for the thermal control of the main spectrometer detectors as well as the secondary mirror (M2) mechanism and IR calibration source management. The TCU, being a slave subsystem of the Instrument Control Unit (ICU), will collect the housekeeping data from the monitored subsystems and will forward them to the master unit. The latter will run the application software, devoted to the main spectrometer management and to the scientific data on-board processing.