ESPRESSO: design and analysis of Coudé-Train concepts for stable and efficient optical feeding

The first purpose of ESPRESSO is to develop a competitive, innovative high-resolution spectrograph to fully exploit the potentiality of the Very Large Telescope (VLT) of the European Southern Observatory and to allow new science. It is thus important to develop the VLT array concept bearing in mind the need to obtain the highest stability, while preserving an excellent efficiency. This high-resolution ultra-stable spectrograph will be installed at the VLT Combined Coudé Laboratory. A Coudé Train carries the light from the Nasmyth platforms to the Combined Coudé Laboratory, where it feeds the spectrograph. Several concepts can be envisaged for the Coudé Train depending on the use of mirrors, prisms and lenses or fibers or any of the possible combinations of these elements. Three concepts were selected for analysis, one based on purely optical components and two other using fibers (with different lengths). These concepts have different characteristics in terms of efficiency, stability, complexity, and cost. The selection of the baseline concept took into account all these issues. In this paper, we present for each concept the optical setups, their opto-mechanical implementation and analyze the expected throughput efficiency budget, and we also detail the current baseline concept.