Large ground-based telescopes with mirrors over 8 m in diameter¹ use azimuthal mounts to point the stars. When tracking a star, the Earth's rotation causes the observed field on the astrophysics detector to rotate, creating "spun" images. To correct this effect, the instruments mounted on these telescopes use a "field derotator", a mechanism whose main function is to rotate a set of mirrors at very low speed and with very high precision. For the METIS instrument, CEA-Irfu has developed one of the world's very first² derotators operating at -210°C. Entirely designed, developed and tested by the Irfu Systems Engineering Department, the METIS derotator drive achieves performance beyond the expected specifications. Currently in the testing phase, its qualification will be completed by the end of 2023 at ESO in Garching, Germany.

While the use of field derotation mechanisms is relatively common in ground-based astrophysics instruments, most scientific communities choose to place this mechanism upstream of the cryogenic instrument, so that it operates at ambient temperature. Doing this way, the required levels of accuracy (of the order of a few thousandths of a degree) are achieved using 'standard' components such as motorised stages, optical encoders, ball bearings and lubricated gearboxes.

To reduce the instrumental background noise, the METIS instrument of the future ELT will be entirely cooled down to temperatures around -210°C, so the field derotation mechanism will also have to operate at these temperatures in vacuum.