Cryogenic cooling systems for the ELT instruments

Abstract

The European Southern Observatory (ESO) is building the Extremely Large Telescope (ELT), a 40-m class telescope to be installed on top of the 3046 m high mountain Cerro Armazones in the central part of Chile’s Atacama Desert. Once operational the ELT will be the largest optical/near-infrared telescope in the world. Powerful facility instruments that can deliver the science cases for the ELT are under development. The instrument roadmap lists more than six scientific instruments, each of them in the 15-35 tons range. While the telescope optics operate at ambient temperature, the instrument optics structure and in particular the detectors will be cooled to cryogenic temperatures down to as low as 4 Kelvin. ESO is aiming to implement proven technologies and commercial off-the-shelf components to build the cryogenic infrastructure for the ELT instruments. A combination of open loop Liquid Nitrogen cooling and low-vibration mechanical cryo-coolers will be installed to provide the required temperature levels and cooling capacities. ESO’s vacuum and cryogenic standards required major updates in order to match with the needs and challenges of this new class of huge instruments, each of them coming with up to 50 m 3 vessel volume and more than 5 tons cold mass. The paper outlines the instruments vacuum and cryogenic requirements, gives a brief overview of the ESO vacuum and cryogenic standards, and of the ELT cryogenic infrastructure baseline concept. The current testing approach for selected standard components such as low-vibration cryo-coolers and vibration damping systems will be presented.