A cryogenic actuator for the image derotator on ELT-METIS

The Mid-infrared Imager and Spectrograph (METIS) is one of the first light instruments of the European Extremely Large Telescope (ELT). METIS optical design contains a first stage called common fore optics, which operates at cryogenic temperatures. Because of the azimuthal motions of the telescope, it integrates a field derotator, which has to run under vacuum and at 67 K. The French Alternative Energies and Atomic Energy Commission (CEA) is in charge of developing the cryogenic rotation stage, which actuates this field derotator. This is a kind of technological breakthrough as all the existing derotator systems operate outside the cryogenic vessels. This paper gives an overview of the derotator actuator design with the different trades that have been studied and some preliminary tests results.