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.