The Multi Unit Spectroscopic Explorer (MUSE) is a second-generation VLT panoramic integral-field spectrograph currently in manufacturing, assembly and integration phase. MUSE has a field of 1x1 arcmin² sampled at 0.2x0.2 arcsec² and is assisted by the VLT ground layer adaptive optics ESO facility using four laser guide stars. The instrument is a large assembly of 24 identical high performance integral field units, each one composed of an advanced image slicer, a spectrograph and a 4kx4k detector. In this paper we review the progress of the manufacturing and report the performance achieved with the first integral field unit.

ABSTRACT. The research and development activity on image slicer systems for integral field spectroscopy has increased in recent years, allowing the simultaneous observation on the same detector matrix of twodimensional sky maps and the spectral decomposition of light. Nowadays, image slicers lead to possible applications on future instrumentation for ground-and space-based telescopes, covering a spectral domain ranging from UV to mid-IR wavelengths. Among such projects, MUSE (Multi Unit Spectroscopic Explorer) is an integral field spectrograph proposed by the European Southern Observatory for its second-generation Very Large Telescope instrumentation. MUSE combines a field of view, with a spectral resolution reaching 3000 and a spatial 1 # 1 sampling of 0Љ .2 provided by an adaptive optics system. MUSE operates in the visible and near-IR wavelength range (0.465-0.93 mm) and is composed of 24 identical integral field units; each one incorporates an advanced image slicer made up of a combination of mirrors and "minilens" arrays, a spectrograph, and a detector. This paper describes the optical design of the MUSE image slicer, which complies with top-level requirements at a reasonable cost. The optical manufacturing technique, based on an original method of assembly using molecular adhesion, is presented. Finally, we focus on the optical tests of the prototype both at a system level and at the level of individual optical elements.

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