X-shooter Near-IR Spectrograph Arm Realisation

X-shooter is a new high-efficiency spectrograph observing the complete spectral range of 300-2500 nm in a single exposure, with a spectral resolving power R>5000. The instrument will be located at the Cassegrain focus of one of the VLT UTs and consists of three spectrographs: UV, VIS and Near-IR. This paper addresses the design, hardware realization and performance of the Near-IR spectrograph of the X-Shooter instrument and its components. Various optical, mechanical and cryogenic manufacturing and verification techniques are discussed. The cryogenic performance of replicated light weight gratings is presented. Bare aluminium mirrors are produced and polished to optical quality to preserve high shape accuracy at cryogenic conditions. Their manufacturing techniques and performance are both discussed. The cryogenic collimator and dispersion boxes, on which the optical components are mounted, feature integrated baffles for improved stiffness and integrated leaf springs to reduce tension on optical components, thereby challenging 5 axis simultaneous CNC milling capabilities. ASTRON Extreme Light Weighting is used for a key component to reduce the flexure of the cryogenic system; some key numbers and unique manufacturing experience for this component are presented. The method of integrated system design at cryogenic working temperatures and the resulting alignment-free integration are evaluated. Finally some key lab test results for the complete NIR spectrograph are presented.