VLBI Connection between APEX and Effelsberg

First Interferometric Signal Detection Between MPIfR’s two Radio Telescopes

June 03, 2025

A symbolic moment in millimetre-wave VLBI at the Max Planck Institute for Radio Astronomy (MPIfR): For the first time, the 12-metre APEX telescope in Chile and the 100-metre Effelsberg radio telescope in Germany have successfully detected VLBI fringes between them.

First connection between both MPIfR telescope, Effelsberg (left) and APEX (right) at a baseline of 9637 km length. — **Fig. 1**: *First interferometric fringes between both MPIfR telescope, Effelsberg (left) and APEX (right) at a baseline of 9637 km length, as detected by the MPIfR correlator in the GMVA Session from April 2025.*

/MPIfR, C. Durán/ESO/APEX (MPIfR/ESO/OSO)

**Fig. 1**: *First interferometric fringes between both MPIfR telescope, Effelsberg (left) and APEX (right) at a baseline of 9637 km length, as detected by the MPIfR correlator in the GMVA Session from April 2025.*

/MPIfR, C. Durán/ESO/APEX (MPIfR/ESO/OSO)

Both telescopes are operated by the MPIfR. APEX, located at 5100 m altitude on the Chajnantor Plateau in the Chilean Andes, and Effelsberg, nestled in the Eifel mountains, have vastly different designs and operating environments. Effelsberg has been a station of the global Very Long Baseline Interferometry (VLBI) network since the mid-1970s; APEX since 2015. Thanks to the completion of N3AR at APEX, both now support a common frequency band.

In April 2025, as part of the Global mm-VLBI Array at 3.5-mm wave, the two telescopes observed the bright quasar 3C 273, among other sources. This observation marks a symbolic handshake between both MPIfR telescopes—separated by 9637 km—as they detected the same wave from a distant quasar.

The figure shows the fringe plot confirming this detection. Adding APEX to the Global mm-VLBI Array (GMVA) will greatly improve the array's north-south resolution, resulting in much more enhanced images of quasars and black hole environments.

This milestone highlights the expansion of mm-wave VLBI capabilities and paves the way for better imaging of radio source located in the southern hemisphere and for for future high resolution studies.

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Additional Information

Effelsberg: With a diameter of 100 metres, the Radio Telescope Effelsberg is one of the largest fully steerable radio telescopes on earth. Its inauguration took place on May 12, 1971. Since the full start of operations in 1972, the technology has been continually improved (i.e. new surface for the antenna-dish, better reception of high-quality data, extremely low noise electronics) making it one of the most advanced modern telescopes worldwide.

The telescope is employed to observe pulsars, cold gas- and dust clusters, the sites of star formation, jets of matter emitted by black holes and the nuclei (centres) of distant far-off galaxies and also radio emission and magnetic fields in our Milky Way and nearby galaxies.

Effelsberg is an important part of the worldwide network of radio telescopes (Very Long Baseline Interferometry, VLBI). The combination of different telescopes in interferometric mode makes possible to obtain the sharpest images of the Universe.

APEX: The Atacama Pathfinder Experiment is a world-class millimetre & sub-millimetre observatory located at the so-called Chajnantor plateau, on the Chilean Andes. This is an ideal place with outstanding weather conditions for this type of observations.

The APEX project is run by MPIfR and hosted in Chile. The telescope has been in operation for more than 20 years and has evolved since its construction and the start of its operations.

Given the experimental nature of the project, a large variety of instruments are available, some of them very versatile and for general use, and some other more experimental. These, in combination with the excellent weather conditions at the site, enable many astronomers in the world to get the data they need to conduct their research on a variety of astronomical topics.