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METIS is the 'Mid-infrared ELT Imager and Spectrograph' for the European Extremely Large Telescope. This E-ELT
instrument will cover the thermal/mid-infrared wavelength range from 3 to 14 μm and will require cryogenic cooling of
detectors and optics. We present a vibration-free cooling technology for this instrument based on sorption coolers
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 40K and optics at 77 K. The latter
temperature is established by a liquid nitrogen supply with adequate cooling power. The cooling powers required at the
lower three levels are 0.4 W, 1.1 W, and 1.4 W, respectively. The cryogenic cooling technology that we propose uses a
compressor based on the cyclic adsorption and desorption of a working gas on a sorber material such as activated carbon.
Under desorption, a high pressure can be established. When expanding the high-pressure fluid over a flow restriction,
cooling is obtained. The big advantage of this cooling technology is that, apart from passive valves, it contains no
moving parts and, therefore, generates no vibrations. This, obviously, is highly attractive in sensitive, high-performance
optical systems. A further advantage is the high temperature stability down to the mK level. In a Dutch national research
program we aim to develop a cooler demonstrator for METIS. In the paper we will describe our cooler technology and
discuss the developments towards the METIS cooler demonstrator.
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