The installation and ongoing commissioning of the MATISSE mid-infrared interferometer at the ESO Very Large Telescope Observatory

MATISSE is the second-generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L and M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 μm, exploring angular scales as small as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with the four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range from R 30 to R 5000. Here, we remind the concept, the instrumental design, and the main features of MATISSE. We also describe the last months of preparation, the status of the instrument, which was shipped to Cerro Paranal on the site of the ESO Very Large Telescope in October 2017, and the expected schedule for the opening to the community. The instrument is currently in its Commissioning phase. A complementary dedicated article details the Commissioning results, which include the first performance estimates on sky.