Design and performances of the ELT-METIS cryogenic derotator drive

This paper describes the work performed by the CEA Paris-Saclay team to build and qualify a cryogenic derotator mechanism for the ELT-METIS instrument. As the Extremely Large Telescope is an azimuthal telescope, a field derotator device is mandatory to keep the sky image perfectly aligned on the detector frame during the observations. The METIS consortium has chosen to place the derotator mechanism inside the cryogenic vessel, operating at 70 K under vacuum to reduce the background noise at the lowest level. This article gives the main results of the achieved performances, including a positioning accuracy in the arc second range.