Optical coatings are one of the key elements of the VLT’s second generation instrument MUSE. The Multi Unit Spectroscopic Explorer is developed for the European Southern Observatory (ESO) and will be installed in 2013 at the VLT (Very Large Telescope). MUSE is a panoramic integral field spectrograph (1x1arcmin² Field of View) operating in the visible wavelength range (465 nm - 930 nm). The throughput, which strongly depends on the optical coatings, is one of the most important parameters of the MUSE instrument, which aims at observing very faint objects. This article focuses on the different refractions and reflections required by the optical design of MUSE. Between the output of the VLT and the final detectors of MUSE, photons are typically reflected 7 times by mirrors and transmitted 26 times through antireflective coatings. A comparison between metallic and multi-dielectric coatings is presented here in order to explain the best compromise that has been chosen for MUSE purpose. High reflective multi-dielectric coatings of large bandwidth are rather thick and induce significant stress on the substrate which may bend the substrate. This deformation of mirrors is simulated and compared to measurements on MUSE optics. Finally, systematic optical coating tests have been conducted, so as to check the durability under severe conditions such as humidity, temperature change, abrasion. In the end, the choice of high quality optical coatings should allow MUSE to reach a global throughput higher than 40%.
