The Multi-Object Optical and Near-infrared Spectrograph (MOONS) will cover the Very Large Telescope's (VLT) field of view with 1000 fibres. The fibres will be mounted on fibre positioning units (FPU) implemented as two-DOF robot arms to ensure a homogeneous coverage of the 500 square arcmin field of view. To accurately and fast determine the position of the 1000 fibres a metrology system has been designed. This paper presents the hardware and software design and performance of the metrology system. The metrology system is based on the analysis of images taken by a circular array of 12 cameras located close to the VLTs derotator ring around the Nasmyth focus. The system includes 24 individually adjustable lamps. The fibre positions are measured through dedicated metrology targets mounted on top of the FPUs and fiducial markers connected to the FPU support plate which are imaged at the same time. A flexible pipeline based on VLT standards is used to process the images. The position accuracy was determined to ~5 μm in the central region of the images. Including the outer regions the overall positioning accuracy is ~25 μm. The MOONS metrology system is fully set up with a working prototype. The results in parts of the images are already excellent. By using upcoming hardware and improving the calibration it is expected to fulfil the accuracy requirement over the complete field of view for all metrology cameras.
The Multi-Object Optical and Near-infrared Spectrograph (MOONS) is a new fiber-fed spectrograph for the VLT. MOONS will exploit the full 500 square arcmin field of view offered by the Nasmyth focus of VLT and will be equipped with two dual-arm spectrographs covering the wavelength range 0.8 µm-1.8 μm, with a possible extension down to 0.5 μm. Each double-arm spectrograph will produce spectra for 250 targets simultaneously, each with its own dedicated sky fiber for optimal sky subtraction. The system will have both a medium resolution (R 3000 - 5000) mode and a high resolution (R 20000) mode to allow detailed dynamical and chemical studies. To ensure the accurate positioning of the 500 fiber pairs over the focal plane that has 880 mm in diameter, a metrology system has been designed to provide position measurements within 7.5 μm considering that the final positioning accuracy of each fiber with respect to the target object must be less than 15 μm. The metrology system is composed by a circular array of 12 cameras located at VLT’s de-rotator ring around the Nasmyth focus. The paper presents the design of the metrology system and discusses the proposed methodology to align multiple the views of the focal plane array.