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Mirrors for the Large-Sized Telescope on La Palma

The mirrors for the largest telescope in the planned CTA observatory are now largely installed. In recent weeks, most of the 198 mirrors have been mounted. The telescope will be completed in the summer and commence operations in October 2018.

Installation of mirrors has been nearly completed: The prototype of the Large-Sized Telescope (LST) with mirrors wrapped in protective covers. (Photo: T. Dettlaff/MPP)

Completion of the first of a total of four Large-Size Telescopes (LST) on La Palma is advancing in leaps and bounds. Scientists and technicians involved in telescope construction have now installed 181 individual mirrors on the telescope. The telescope has a diameter of 23 meters; the entire area of the mirrors is approximately 390 square meters.

An eye for high-energy objects in the cosmos

The task of the telescope mirror is to capture and bundle radiation from the atmosphere. The light is recorded by a camera and electronically analyzed. The light is what is known as Cherenkov radiation, which is generated when high-energy gamma rays from space hit the atmosphere. This method allows high-energy celestial objects to be investigated.

Examples of such celestial bodies include black holes in galactic centers, supernova remnants or pulsars, a special type of neutron star. They emit gamma radiation, sometimes for periods of only a few hours. So-called gamma-ray bursts, the cosmic sources of which remain unknown to this day, often flicker for only a few minutes or even seconds.

Fast reaction times, optimal light yield

In order to observe and classify the routes taken by gamma emissions, scientists need measuring instruments that respond quickly and flexibly – capturing as much as possible of the light that can be analyzed.

The LST is optimally designed to meet these demands: the hexagonal, approximately 1.5-meter wide and 6.6-centimeter thick, individual mirrors weigh only 45 kilograms. Thanks to the lightweight design of the mirrors, the gross weight of the movable telescope dish is a mere 28 tons. This allows the telescope, despite its size, to be rotated and tipped into any position in less than 20 seconds.

The mirrors reflect 94 percent of the incident light – traditional mirrors around 90 percent. To increase light yield even further, the mirrors can be exactly aligned with the angle of incidence of the light source. This is done by a complex control system, allowing each mirror to be moved individually.

The telescope and the mirrors are exposed to extreme weather conditions. A special coating material, consisting of five layers, makes the mirrors especially durable.

The CTA Observatory

The LST prototype is the first of four telescopes of this kind, which will be built over the next few years at the "Roque de los Muchachos" observatory on La Palma in the Canary Islands. Twelve medium-sized telescopes, specialized in Cherenkov radiation (Medium-Sized Telescopes, MST) are also planned.

The second CTA Observatory site is the Paranal facility in Chile. Together, the CTA telescopes therefore provide coverage of both the northern and southern celestial hemispheres. There will be over 100 telescopes across both sites; in Chile, very small telescopes, the Small-Sized Telescopes (SST), are also planned, in addition to LST and MST.

The three telescope models cover different energy ranges, allowing astrophysicists to investigate the origins of cosmic gamma radiation in detail.


Dr. Masahiro Teshima
Director of Experimental Astroparticle Physics at the Max Planck Institute for Physics
LST project spokesperson
+49 89 32354-301