MOONS is the Multi-Object Optical and Near-IR Spectrograph for ESO’s Very Large Telescope. The instrument will use ∼1000 optical fibres which can be individually aligned to on-sky targets across a field of view of 500 square arcmin. Each fibre is positioned using a dual arm theta-phi fibre positioning unit (FPU). The MOONS metrology system must be able to simultaneously measure the position of each fibre to a high accuracy (∼15 micrometres) as well as measuring the orientation of the FPU arms. In this paper, we present a description of photogrammetry-based metrology system design and its implementation in the instrument. We also report on the integration, testing, and performance of the system within the instrument.
MOONS is the Multi-Object Optical and Near-IR Spectrograph to be mounted at a Nasmyth focus at the Very Large Telescope. The instrument is equipped with 1000 fibres configured over a field of view of ~500 square arcmin using theta-phi fibre positioning units (FPUs). The MOONS metrology system must accurately determine the position of the fibres in the focal plate, providing fast feedback to the instrument control software during operations. The returned fibre positions can be used for calibrations of the FPUs or fast system recovery after a power loss. If required, the system can also be used for calculating fine adjustments of the fibre positions during acquisition. In this paper, a description of the system design, implementation, and testing in the MOONS focal plate are provided. The presented system has high potential for adaptation to a variety of astronomical instrument applications during integration, testing, and operation stages.
Accurate positioning of opto-mechanical elements in the focal plane of large telescopes is a challenging requirements for many state of the art observational scientific applications. In particular high multiplexing multi object spectroscopy requires precise metrology tools for performing efficient observations and calibrations of the instruments. We have developed a metrology system based on modified commercial off-the-shelf components to reach high performances with a cost effective solution. Our system is based on the photogrammetry technique and on a number of fixed off-axis cameras. The cameras acquire images of the focal plane where metrology targets and references are located. The acquisition is based on Odroid-XU4, a single-board computer running on GNU/Linux. No moving parts in the setup ensures an extremely fast acquisition of the data. The calibration and metrology data processing is based on the computer vision library OpenCV. We present a prototype system and results of the camera calibrations and metrology tests obtained in our laboratory.
The Multi Object Optical and Near-infrared Spectrograph (MOONS) instrument is the next generation multi-object spectrograph for the Very Large Telescope (VLT). The instrument combines the high multiplexing capability offered by 1000 optical fibres deployed by individual robotic positioners with a novel spectrograph able to provide both low- and high-resolution spectroscopy simultaneously across the wavelength range 0.64μm - 1.8μm. Powered by the collecting area of the 8-m VLT, MOONS will provide the astronomical community with a world-leading facility able to serve a wide range of Galactic, Extragalactic and Cosmological studies. This paper provides an updated overview of the instrument and its construction progress, reporting on the ongoing integration phase.
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