Design and testing of the MOONS fibre positioning units

Abstract

MOONS (Multi-Object Optical and Near-infrared Spectrograph) is a third-generation visible and near-infrared spectrograph for the ESO Very Large Telescope currently under construction. The instrument’s spectroscopic capabilities are multiplexed via a fibre positioning module (FPM) which configures the location of 1001 fibres. The fibre positioning units (FPUs) have been jointly developed by the UK Astronomy Technology Centre (UKATC) and MPS Microsystems (MPS) to optimise instrument efficiency by providing excellent transmission and an open-loop positioning strategy, allowing a tightly packed focal plane to be rapidly reconfigured. The mechanism geometry enables all positions in the focal plane to be observed in conjunction with a companion sky fibre at close separation. A description of the as manufactured design and production process of the FPUs is presented, along with a discussion of the performance proven to date, including achievement of the critical pupil alignment and positional repeatability requirements. An overview of the custom testing rig built to automate the characterisation and calibration process is also presented.