This very large telescope named H.E.S.S-II saw its first light on 26 July 2012, detecting its very first images of atmospheric particle cascades generated by cosmic gamma rays and by cosmic rays, marking the next big step in exploring the Southern sky at gamma-ray energies. The new telescope not only provides the largest mirror area among instruments of this type worldwide (600 m2), but also resolves the cascade images at unprecedented detail, with four times more pixels per sky area compared to the smaller telescopes. Gamma rays are believed to be produced by natural cosmic particle accelerators such as Active Galactic Nuclei (AGN), binary stars, pulsars, clusters of galaxies and supernova remnants. The universe is filled with these natural cosmic accelerators, impelling charged particles such as electrons and ions to energies far beyond what the particle accelerators built by mankind can reach. As high-energy gamma rays are secondary products of these cosmic acceleration processes, gamma ray telescopes allow us to study these high-energy sources. The most extreme gamma ray emitters at TeV energies -AGN- shine in gamma rays with an apparent energy output which is one hundred times the luminosity of the entire Milky Way, yet the radiation seems to emerge from a volume much smaller than that of our Solar System, and turns on and off in a matter of minutes, a strong signature of supermassive black holes. When gamma rays interact high up in the atmosphere, they generate an air shower of secondary particles that can be recorded by telescopes and their ultra-fast cameras on the ground, thanks to the emission known as Cherenkov radiation. The electronic camera of H.E.S.S-II will be able to record this very faint radiation with an "exposure time" of a few billionths of a second, almost a million times faster than a normal camera. Today, well over one hundred cosmic sources of very high-energy gamma rays are known. With H.E.S.S-II, processes in these objects can be investigated in superior detail, also anticipating many new sources, as well as new classes of sources. In particular, H.E.S.S. II will explore the gamma ray sky at energies in the range of tens of Giga-electronvolts -the poorly-explored transition regime between space-based instruments (such as Fermi-LAT) and current ground-based Cherenkov telescopes, with a huge discovery potential. The telescope structure of H.E.S.S.-II and its drive system were designed by engineers in Germany and South Africa, and produced in Namibia and Germany. The over 800 hexagonal mirror facets which make up the huge reflector were manufactured in Armenia. The camera, with its integrated electronics, was designed and built in France. The construction of the new H.E.S.S. II telescope was driven and financed largely by German and French institutions, with significant contributions from Austria, Poland, South Africa and Sweden. The Paris Observatory (LUTH, Pôle Instrumental) was responsible for the construction of the large instrumented shelter (size: 18m, width: 12m height: 6m), which allows to store the camera, with a diameter of more than 2m and a weight of 2 tons, for calibration and maintenance. At the time of "first light", researchers from the H.E.S.S. group at the LUTH were on shift at the site in Namibia and participated in the acquisition of the first events seen with five telescopes. H.E.S.S-II also paves the way to the realization of CTA -the Cherenkov Telescope Array- the next generation instrument ranked top priority by astro(particle) physicists and funding agencies in Europe (including CNRS/INSU). The Paris Observatory is strongly involved in the Preparatory Phase of CTA, with researchers from the APC, GEPI and LUTH contributing to its conception and prototyping.
