MOSAIC, the ELT multiobject spectrograph: positioners with integrated atmospheric dispersion correction

MOSAIC is the multi-object fibre spectrograph for ESO's 39-m Extremely Large Telescope (ELT). It offers unique capabilities for a gigantic telescope in terms of multiplexing, wavelength coverage and spectral resolution. MOSAIC covers the visible (0.39-0.95 microns) and NIR (0.95-1.8 microns) wavelength domains. It has been designed to cover the largest possible area (~40 arcmin²) on the ELT focal plane. This paper focuses on the optical realisation of the robotic arms designed to pick the light from the curved, non-telecentric focal surface while providing individual atmospheric dispersion correction for each of the 261 probes. Further challenges of the design are the large wavelength range and the tight tolerances necessary for an efficient optical fibre injection. An optical setup to test prototypes and to characterize the batch produced positioners will be introduced, allowing each individual pickoff arm to reach the tight tolerances for pointing accuracy and pupil stability.