Résumé
The HESS experiment is now fully operational with the four telescopes installed by the end of December, 2003. Many galactic and extragalactic objects have been observed since operation began and the detection of various sources has proven the performance of the detector and validated the technical options chosen. The collaboration is currently studying the next phase of the HESS project. The detector system currently in operation has a threshold around 100 GeV. Many sources such as pulsars, micro-quasars, or neutralino annihilation are expected to emit gamma radiation at lower energy. The second phase of the HESS experiment consists of an additional larger telescope positioned in the centre of the existing four-telescope array. The new system may reach a threshold as low as 10–20 GeV in single telescope mode and about 50 GeV in coincidence with the four other telescopes. It will also improve the sensitivity of the existing system above 100 GeV. The construction should start next year and the installation is expected to take place in 2008, less than one year after the launch of the GLAST satellite. After a brief overview of the HESS phase I experiment, we will describe the upgraded parameters of the HESS camera. Then the set-up and expected performance are presented.