20th anniversary - H.E.S.S. Collaboration

November 2022

The first of the now five HESS telescopes was completed (telescope, mirrors, camera) in the first half on 2002 and commissioning started promptly. In September 2002 this telescope was inaugurated. The construction of the other three identical telescopes was in full swing at the time. The inauguration of the first telescope marked the beginning of operations of the HESS experiment. In its first phase the telescope(s) were used to validate the numerous technical novelties that replaced the solutions of the previous generation of imaging atmospheric Cerenkov telescopes (IACT). The acquisition of science data started with the launch of the second telescope and the opportunity to conduct stereoscopic observations.

**Fig. 1:** The initial HESS phase I telescope array in 2002 when the 1st telescope was inaugurated and operations started in September 2002.

The history of the experiment from the beginning, the start of construction, through the first decade has been recalled in earlier SOM reports.

The initial HESS phase I array of four 106 square-meter sized telescopes started operations in early 2004 and was formally inaugurated in September 2004. In 2012 another inauguration was hosted to launch the start of the largest IACT, the giant 600 square meter ‘CT5’ telescope of the HESS array. While the initial plans foresaw an operational period of 10 years with subsequent prolongations, HESS operations were formally extended for a first time in 2019 for an initial period of three years. Following the extraordinary successes of this first extension with the integration of a new camera for the CT5 telescope and much improved reliability of operations, a second extension was granted and the HESS collaboration celebrated the 20th anniversary.

**Fig. 2:** Celebrating the 20th anniversary, the making and the achievements of HESS were reviewed by the founding and the acting director. Representatives of the Namibian Government, the University of Namibia, representatives of founding agencies, members of the diplomatic corps and industry addressed the audience and put the anniversary into context.

The 20th anniversary started off with a scientific symposium near Windhoek, followed by the formal anniversary celebrations on Farm Goellschau, the site of the HESS telescope array. On the occasion, the HESS collaboration met for its third collaboration meeting in Namibia to discuss scientific studies and technical projects. In the end of a very busy week, the observatory opened its gates for a very well-attended HESS open-day, welcoming hundreds of Namibians to the site.

**Fig. 3:** On the open day, hundreds of cars from as far as 250 km and 5 buses from Windhoek brought large numbers of interested Namibians, including many students and families to the site. Tours to the telescopes, camera shelters, control buildings and workshops along with posters, many displays, science games, planetarium presentations offered insights and entertainment.

In its own celebrations, the collaboration looked back to more than 20.000 hours of observations, during which 47359 observation runs of about 2000 target positions were acquired. While a large number of these target positions were used for the largest single project of the HESS array, the HESS Galactic Plane Survey, and many Target-of-opportunity observations, more than 50 individual sources were observed for 100 to 1000 hours each. More than 10.000 and 7.000 hours were obtained in 4- and 5-telescope operations, respectively. with the large field-of-view about 1.5 million events were recorded in each ‘observing run’, which is a single pointed observation, lasting typically for 28 minutes. More than 51 billion of the ~ 70 billion total events have been recorded with 5 telescopes. The observations resulted in more than 250 publications, which, collectively, are cited more than 1200 times each year. Within the collaboration, which has grown to its current 240 members early career researchers made significant contributions that led to approximately 1000 thesis projects.