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The MICADO first light imager for ELT: control concept for the derotator

The Multi-AO Imaging Camera for Deep Observations (MICADO) is one of the three first light instruments of the Extremely Large Telescope (ELT). Based on the Multi Conjugate Adaptive Optics (MCAO) modul MAORY MICADO offers diffraction-limited near-infrared imagery with a maximum field of view of 53 arcsec. In order to maintain diffraction-limited performance at the edge of the field, a precise image derotator is needed, which compensates the field rotation due to alt-azimuth mount of the telescope. In MICADO a four-point contact ball bearing is foreseen to rotate the cryostat for the compensation of the field rotation. Due to the heavy load and the high precision positioning of the ball bearing a control concept for the derotator is needed. The main challenge of positioning the ball bearing is the handling of friction effects. In this paper we present a control concept based on a velocity feedforward and a PID feedback control to rotate the bearing in the required position performance. At a scaled-down laboratory setup we demonstrate the position accuracy. To further improve the position accuracy we also study an additional friction compensation, which is based on a dynamical friction model.