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X-shooter is a wide band (U to K) intermediate resolution (4000-14000) single object three-arms spectrograph for the VLT. Currently in the last phase of integration, X-shooter will see the first light at ESO Paranal as the first of the VLT second generation instruments in the last quarter of 2008. We describe in this paper the final steps in the integration and testing phase of the central Backbone with its key functions (including the active flexure compensation mirrors) and of the two UV-Blue and Visible spectroscopic arms. We report on the stability results of the preslit optics and of the spectrographs and on the remarkable efficiency which is derived from the measurements of the optical components of the instrument.

X-shooter is a second generation VLT instrument currently under construction by a Consortium of Institutes from Denmark, Italy, The Netherlands, France and ESO. X-shooter is designed to acquire intermediate (5000-10000) resolution spectra of single objects in an unprecedented wide wavelength coverage (320-2500 nm). In order to maximize efficiency the beam is divided into 3 arms (UV, VIS and NIR) by a system of dichroics. X-shooter is designed for the Cassegrain focus of one VLT unit. The mechanical assembly has to provide specific solutions to maintain 3 arms within the strict tolerances required by the intermediate resolution, during the typical motions of the Cassegrain focal station. It must as well ensure the permanent co-alignment of the 3 slits and the stability of the spectral format on the focal plane of each arm, allowing long intervals between calibration exposures. The above requirements have been met via an innovative mechanical design merging passive stiffness and active control to obtain a light, accessible and functional assembly. This paper gives a description of the X-shooter mechanical assembly with main emphasis on the common “backbone” structure and the UV- and VIS spectrograph arms.