A symbolic moment in millimetre-wave VLBI at the Max-Planck-Institute für Radioastronomie (MPIfR): For the first time, the 12-meter APEX telescope in Chile and the 100-meter Effelsberg radio telescope in Germany have successfully detected VLBI fringes between them.
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 VLBI station since the mid-1970s; APEX since 2015. Thanks to the completion of N3AR at APEX, both now support a common frequency band.
In April 2026, as part of the Global mm-VLBI Array at 3.5-mm wave, the two telescopes observed the bright quasar 3C273, 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 GMVA greatly improves 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 even higher-resolution astronomy in the future.
Image: 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.