September 2020
The H.E.S.S. experiment is the biggest and longest-operating among all currently active facilities in ground-based gamma-ray astronomy, exploring the Very High Energy (VHE) radiation from the Universe. The collaboration that planned, built and now operates the array was inspired from the first generation of ground-based gamma-ray instruments that discovered cosmic gamma-ray sources at these high energies, notably the Whipple-10m telescope and the first stereoscopic array HEGRA. Looking back towards the beginnings of H.E.S.S., it is difficult to point to a clear birthday – given a rather large number of subsequent milestones that marked the early days. Groundbreaking at its site, the Farm Goellschau in the Khomas Highland, was certainly one of the first achievements “visible” in Namibia. While celebration ceremonies in 2020 mostly fall victim to measures containing the Covid-19 pandemic, we look back in September 2020 to the 20th anniversary of groundbreaking, when the beginnings of ground work marked the beginnings of efficient constructions and subsequent operations of the H.E.S.S. telescopes.
While everyone active in setting up the new experiment might have been impatient at the time, in hindsight it is remarkable, how quickly this infrastructure has been set up. In 1997, a letter of intent was issued, which provided the science case and technical solutions for telescopes and cameras but did not even mention the site that was ultimately chosen as the home for the facility. In early 1998 the H.E.S.S. collaboration was set-up (including eight institutions which remained active until now). By the end of 1998, negotiations with land-owners in Namibia had started and the Founding President of Namibia had expressed his support for the project. In 1999 final decisions on many important components were taken, the University of Namibia (UNAM) became a partner, and a first batch of test mirrors were exposed to Namibian climate.
2000
In 2000 Namibia celebrated its 10th anniversary of independence as the youngest state in Africa and the H.E.S.S. project became its first international research infrastructure. The future facility was introduced with a beautiful stamp issued in the year of groundbreaking.
In April 2000 a lease for the site was signed, an exchange of notes between the governments of Namibia and Germany had been concluded, and a contract for mount and dish steel structures had been awarded to NEC, Namibia. With these essential prerequisites in place, construction was ready to start.
While planning, construction and testing of many components was in full swing in the various European institutes that built and provided hardware for the array, in Namibia the preparations for site development and the construction of telescopes began. A small “construction office” (Figure 3 top left ) was the first building on the Goellschau farm that was dedicated to the H.E.S.S. array. In parallel, water pipes, leading to the construction site were installed in the middle of Namibian winter. The first excavations for telescope foundations started and concrete was poured (all Figure 3).
By the beginning of September 2000 ground-breaking had been completed, the footprint was in place, but this was merely the beginning. The progress in constructing further foundations was covered by the national press (figure 4), and was complemented by many other, equally fast developments in September 2000, including: – the welding of the dish radial beams started at NEC, Namibia), – the first container with two azimuthal bearings and many mirrors leaving MPIK Heidelberg, Germany, for Goellschau, Namibia – tests of the final version of drawers for the first generation of H.E.S.S. cameras starting in Paris, France.
Completing construction of H.E.S.S. phase 1
Nine months after groundbreaking, the steel structure of the first telescope had been erected, within 18 months the structures of all four telescopes were on site and most of the buildings had been completed, and two years after groundbreaking the first inauguration ceremony saw the complete and operating first telescope. After the third year, in September 2003, all four telescopes were erected, the first results, presented at the International Cosmic Ray Conference in Kashiwa, Japan, in August 2003 had been published, and the integration of the four-telescope array of the HESS phase 1 was close to completion. The first full year of normal science operation commenced in January 2004.
A grown-up facility
Since the start of observations, H.E.S.S. has yielded amazing discoveries. More than 100 sources of highly energetic gamma-rays have been detected and studied in detail. One of the scientific highlights in the last year was the discovery of the first Gamma-Ray Burst afterglow – presented as Source of the Month in November 2019.
The capabilities of the telescope array have been improved continuously. The largest reflecting telescope in the world (the 28m “CT5 telescope”) was inaugurated as addition to the original four telescopes in 2012. The cameras and mirrors of the original telescopes were upgraded and in 2019 a new-technology camera, designed for the future generation of Cerenkov telescopes, has been mounted on the giant CT5 telescope.
Working with the facility is a fascinating experience for young researchers and hundreds of students have obtained their master, diploma, or doctoral degrees in the participating universities and institutions through research with the H.E.S.S. telescopes, many of whom now lead research and engineering projects.
Just last year H.E.S.S. completed its originally anticipated lifetime and remains the highly overbooked facility it has been since its beginnings – it has now entered its first extension phase and looks ahead to new important astrophysics results, pioneers new analysis methods, and incorporates new instrumental developments to advance the forefront of science and technology.
