Mechanical based alignment of large optical instruments: ESPRESSO as an example

When long term instrument stability is required, traditional alignment techniques based on bulky and/or flexible mountings can not be used due to their reduced stiffness. Mechanical alignment of optical systems is nowadays possible thanks to different 3D Coordinate Measuring Machines, as the Laser Tracker, the Articulated and Cartesian Arms. In this paper we describe the methods we considered for the integration and alignment of ESPRESSO, the very high resolution visible spectrograph for the ESO VLT, now under commissioning phase at Paranal Observatory. Different examples of the Front End (FE), the Anamorphic Pupil Slicer Unit (APSU), and the spectrograph itself will be provided, to demonstrate that it is possible to align an optical system with mechanical methods with minimal optical feedbacks, reaching in an almost ‘blind’ way the best optical performances.