Sorption-based vibration-free cooler for the METIS instrument on E-ELT

METIS is the 'Mid-infrared ELT Imager and Spectrograph' for the European Extremely Large Telescope (E-ELT) that will cover the thermal/mid-infrared wavelength range from 3 - 14 micron, and requires cryogenic cooling of detectors and optics. A vibration-free cooling technology for this instrument based on sorption coolers is developed at the University of Twente in collaboration with Dutch Space. In the baseline design, the instrument has four temperature levels: N-band: detector at 8 K and optics at 25 K; L/M-band: detector at 40 K and optics at 70 K. The latter temperature level is established by a pumped-liquid nitrogen line. The cooling powers required at the lower three levels are 0.4 W, 1.1 W, and 1.4 W, respectively. We propose a vibration-free sorption-based cooler with three cascaded Joule-Thomson (JT) coolers of which the sorption compressors are all heat sunk at the 70 K platform. A helium-operated cooler is used to obtain the 8 K level with a cooling power of 0.4 W. Here, three pre-cooling stages are used at 40 K, 25 K and 15 K. The latter two levels are provided by a hydrogen-based cooler, whereas the 40 K level is realized by a neon-based sorption cooler. In the paper, we present the preliminary design of this threestage cooler and we discuss the developments towards a demonstrator version of this METIS cooler