MOONS, the next ESO VLT’s multi-object spectrograph: the field corrector and the rotating front end

Abstract

MOONS will be the next Multi-Object Optical and Near-infrared Spectrograph for the Very Large Telescope, able to simultaneously observe 1000 targets, feeding a set of optical fibres which can be placed at user-specified locations on the Nasmyth focal plane using individual robotic positioners. The sub-fields thus selected are then driven by the fibres into two identical cryogenic spectrographs mounted on the Nasmyth platform of one of the ESO VLT 8 m telescopes. The instrument will provide both medium and high-resolution spectral coverage across the wavelength range of 0.65 μm to 1.8 μm. In this paper we will describe the two components that interface with the telescope: the MOONS Field Corrector (FC) and the Rotating Front End (RFE) Assemblies. The FC optics will correct the off-axis aberrations of the telescope, as well as determining the shape of the focal surface and the pupil location. The RFE assembly consists of a rotating part, which will be mounted on the VLT Nasmyth Rotator, and be connected to the two static Spectrographs via fibre assemblies, and all the sub-systems that give support to the fibre positioning, metrology and calibration units.