Where will the world’s next generation ground-based γ-ray detector, the Cherenkov Telescope Array (CTA), be built? No one yet knows. But a panel of funders have narrowed the field slightly, following a meeting in Munich, Germany, this week.

Scientists had originally hoped to select two sites — a large one in the Southern Hemisphere and a smaller one in the North — by the end of 2013. But the selection process for the €200-million ($276-million) project has taken longer than originally foreseen.

At a meeting on 10 April, representatives from 12 government ministries narrowed the potential southern sites from five to two: Aar, a site in Southern Namibia; and Armazones, in Chile’s Atacama desert. They also picked a reserve site in Argentina.

The committee, a panel of representatives from Argentina, Austria, Brazil, France, Germany, Italy, Namibia, Poland, Spain, South Africa, Switzerland and the United Kingdom, decided that all four possible northern sites — in Mexico, Spain and the United States — needed further analysis. A statement from the board said that a final site decision will happen “as soon as possible”.

If the CTA is built, its two sites will contain around 120 telescopes, which will look for the faint blue light emitted when very-high-energy photons slam into Earth’s atmosphere and create cascades of particles. By triangulating the data from various detectors, astrophysicists hope to piece together the energy and path of such photons. This should help them not only identify the sources of the γ-rays — extreme environments such as supermassive black holes — but also answer fundamental questions about dark matter and quantum gravity.

Like many astronomy projects, the best site for the CTA would be a high-altitude, remote location with clear skies. But the site decision must also take into account environmental risks, such as earthquakes and high winds, and projected operational costs. How much each host country would be prepared to contribute is also a factor.

Last year, an evaluation by representatives of the CTA’s 1,000-strong consortium rated Aar in Southern Namibia as the best southern site, which would contain 99 telescopes spread out over 10 square kilometres. Two sites tied for second: another Namibian site, which already hosts the High Energy Stereoscopic System (HESS) γ-ray telescope; and Armazones, where the European Southern Observatory already has a base and plans to build the European Extremely Large Telescope. The group equally ranked the four contenders for the northern site, which would be a 19-telescope array spread out over one square kilometre. Mexico is already building the High-Altitude Water Cherenkov Observatory (HAWC), a γ-ray observatory of different type.

Although the consortium’s ranking was based largely on the science case and observing conditions, the latest decisions follows the report of an external site selection committee, which also took into account political and financial factors. Further decisions will rest on detailed negotiations, including host country contributions and tax exemptions at the various sites.

The CTA now aims to pick a final southern site by the end of the year. Board chair Beatrix Vierkorn-Rudolph, of Germany’s Federal Ministry of Education and Research, told Nature it was not yet clear whether the same will be possible for the northern site.