This 100-meter dish was the world's largest fully steerable telescope for 29 years. Its surface must be accurate to within 0.5mm despite weighing 3,200 tons.
Björn Golda, CC BY-SA 2.0
Engineering a precise giant
The Effelsberg Radio Telescope, inaugurated on May 12, 1971, is a precise engineering. Its 100-meter primary reflector has a total surface area of 7,850 square meters, roughly the size of a football field. The entire steerable structure weighs 3,200 tons, yet it can rotate 360 degrees in just twelve minutes and tilt from the horizon to the zenith in about six. The telescope is located in a quiet valley, a location deliberately chosen to shield its sensitive receivers from human-made radio frequency interference.
The instrument's immense size presents a significant challenge: gravity. As the massive dish tilts, its own weight causes the steel structure to deform. To counteract this, engineers employed a principle called "homologous deformation." The support structure is designed to bend in a controlled way, ensuring that as it deforms, the reflector surface always maintains a perfect parabolic shape, albeit with a slightly shifted focal point. This design allows the telescope to maintain a surface accuracy of about 0.5 mm—less than the thickness of a credit card—across its entire 100-meter span. In 2006, this system was augmented with a new subreflector featuring an active surface with 96 actuators that make tiny adjustments to further improve its accuracy.
Observing the radio universe
The Effelsberg telescope observes the universe at wavelengths ranging from 90 cm (300 MHz) down to 3.5 mm (90 GHz). This allows it to study many cosmic phenomena. Astronomers use the telescope to map cold gas and dust clouds where stars are born, investigate the powerful jets of matter ejected by supermassive black holes, and search for the faint pulses from pulsars—the spinning remnants of massive stars.
The telescope often works in concert with other radio observatories around the globe in a technique called Very Long Baseline Interferometry (VLBI). By combining data from multiple telescopes, astronomers can create a "virtual" telescope the size of a continent, achieving extraordinarily high angular resolution. Effelsberg's participation in VLBI networks, including the European VLBI Network (EVN), helped produce some of the sharpest images of the distant universe. For instance, it was part of the network that contributed to observations of the black hole at the center of the M87 galaxy.
In 1977, the telescope was the first to detect water molecules outside of the Milky Way. Another significant discovery was the detection of water vapor maser emission in a quasar 11.1 billion light-years away, providing a new tool to study the environments around distant supermassive black holes. Continuous upgrades to its receivers and electronics ensure that, decades after its construction, Effelsberg is one of the world's most advanced radio astronomy facilities.
💡Fun Facts
The telescope's foundation required 5,200 cubic meters of concrete and sits on 134 piles.
It can detect the faint radio signals from objects up to 15 billion light-years away.
The site was chosen because the surrounding hills of the Eifel mountains provide a natural shield against radio interference.
To reach the visitor pavilion, you can walk along an 800-meter "Planet Path," a scale model of the solar system.
The visitor pavilion and viewing platform are freely accessible. Public talks are offered on specific days, typically Tuesday to Saturday, but advance registration is required. Check the official website for the current schedule.
Admission
Access to the viewing platform is free. A small fee is charged for the public talks (e.g., 2 Euros for adults).
Accessibility
The telescope is located in a valley. An easy 10-15 minute walk is required from the public car park to reach the visitor pavilion and viewing area.
