The telescope assembly of the Ariel space mission: an updated overview

Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) is the adopted M4 mission in the framework of the ESA "Cosmic Vision" program. Its purpose is to survey the atmospheres of known exoplanets through transit spectroscopy. The launch is scheduled for 2029. The scientific payload consists of an off-axis, unobscured Cassegrain telescope feeding a set of photometers and spectrometers in the waveband 0.5-7.8 um and operating at cryogenic temperatures (55 K). The Telescope Assembly is based on an innovative fully aluminum design to tolerate thermal variations to avoid impacts on the optical performance; it consists of a primary parabolic mirror with an elliptical aperture of 1.1 m (major axis), followed by a hyperbolic secondary that is mounted on a refocusing system, a parabolic recollimating tertiary and a flat folding mirror directing the output beam parallel to the optical bench. An innovative mounting system based on three flexure hinges supports the primary mirror on one of the optical bench sides. The instrument bay on the other side of the optical bench houses the Ariel IR Spectrometer (AIRS) and the Fine Guidance System / MR Spectrometer (FGS/NIRSpec). The Telescope Assembly is in phase C towards the Critical Design Review; the fabrication of the structural and engineering models is in progress; some components, i.e., the primary mirror and the flexure hinges, are undergoing further qualification activities. This paper aims to update the scientific community on the progress concerning the development, manufacturing, and qualification activity of the ARIEL Telescope Assembly.