ESPRESSO front end: modular opto-mechanical integration for astronomical instrumentation

The opto-mechanical conceptual design for the Front-End unit and the calibration unit of the ESPRESSO Spectrograph is described in this paper. The front end system exploits a modular concept. Each FEU receive the beam directly from the relative Telescope Coudé Train and the calibration light from the calibration unit. On the other side the FEU feeds the fibers that carry the light to the spectrograph, corresponding in number and size to the scientific observing modes conceived for Espresso. The selection is made through a Toggling Unit. Purpose of the Front/End is to provide the needed connection between the input signal, i.e. Object light, Sky light, Calibration light, and the given output fiber in any of the foreseen observing modes.