The Effelsberg 100-m Radio Telescope is a radio telescope in the Ahr Hills (part of the Eifel) in Bad Münstereifel, Germany. Inaugurated in 1972, for 29 years the Effelsberg Radio Telescope was the largest fully steerable radio telescope on Earth, surpassing the Lovell Telescope in the UK. In 2000, it was surpassed by the Green Bank Observatory's Robert C. Byrd Green Bank Telescope in Green Bank, US, which has a slightly larger elliptical 100 by 110-metre aperture.

The telescope is located about 1.3 km northeast of Effelsberg, a southeastern part of the town of Bad Münstereifel in North Rhine-Westphalia. It is less than 300 m west of the 398 m high Hünerberg, which is in neighbouring Rhineland-Palatinate. The boundary is a stream, the Effelsberger Bach, which runs only a few metres east of the telescope. The Effelsberger Bach is 6.5 km long, flowing from the Effelsberger Wald into the Sahrbach, which in turn flows south and into the Ahr river.

A hiking path leads past the telescope; in 2004 part of this was turned into a planet trail with information panels about the Solar System with its planets. The trail ends at the 39 cm model of the Sun next to the visitor centre.

The Effelsberg radio telescope is operated by the Max Planck Institute for Radio Astronomy in Bonn, the radio astronomy institute of the Max-Planck-Gesellschaft. It was constructed from 1968 to 1971 and inaugurated on 1 August 1972. A major technical difficulty in building a radio telescope of 100 m diameter was how to deal with the deformation of the mirror due to gravity when it is rotated to point in a different direction. The mirror must have a precise parabolic shape to focus the radio waves, but a conventionally-designed dish of this size would "sag" slightly when rotated so the mirror loses its parabolic shape. The Effelsberg telescope uses a novel computer-designed mirror support structure which deforms in such a way that the deformed mirror will always take a parabolic shape. The focus will move during such deformation, and the feed antenna suspended in front of the mirror is moved slightly by the computer control system as the telescope is rotated to keep it at the focus. Tests after completion of the telescope showed that the intended accuracy of the mirror surface of 1 mm had not only been met, but exceeded significantly.

About 45% of the observing time is available to external astronomers. The Effelsberg 100-m telescope was involved in several surveys, including the one at 408 MHz (73 cm) by Haslam et al.

Technical data of the antenna

Reflector Diameter	100 m
Aperture	7,854 m
Number of Surface Elements (Panels)	2,352
Shape Accuracy of Surface	< 0.5 mm
Focal Length in Prime Focus	30 m
Secondary Mirror Diameter (Gregory-Reflector)	6.5 m
Aperture Stop
– in Prime Focus	f/0.3
– in Secondary Focus	f/3.85
Angular Resolution (Beam Width)
– at 21 cm wavelength (1.4 GHz)	9.4' (arc minutes)
– at 3 cm wavelength (10 GHz)	1.15' (arc minutes)
– at 3.5 mm wavelength (86 GHz)	10" (arc seconds)
Azimuth Track Diameter	64 m
Setting Accuracy of Track	±0.25 mm
Azimuth Range	480°
Maximum Rotation Speed	30°/min.
Pointing Accuracy
– Blind Pointing	10"
– Repeatability	2"
Power Output of the 16 Azimuth Drives	10.2 kW each
Radius of Elevation Gear Track	28 m
Elevation Range	from 7° to 94°
– during observations	from 8.1° to 89°
Maximum Tilt Speed	16°/min.
Power Output of the 4 Elevation Drives	17.5 kW each
Total Weight	3,200 t
Construction Period	1968–1971
Height of Track above Sea Level	319 m
Commencement of Operation	1 August 1972
Constructed by	Arbeitsgemeinschaft Krupp/MAN

• Stockert Radio Telescope
• Lovell Telescope – at Jodrell Bank Observatory

• Official website of the Max Planck Institute for Radio Astronomy
• Dr Norbert Junkes video interview on Astrotalkuk.org
• Effelsberg Official Webpage
• Bill Keel's "Telescopes I've seen"