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Published: 18th September, 2026, Academia Sinica, Institute of Astronomy & Astrophysics (ASIAA), Taiwan
Very long baseline interferometry (VLBI) combines radio telescopes separated by large distances to achieve the resolving power of a telescope comparable in size to the separation between them. The Event Horizon Telescope used this technique at 230 GHz to image the black holes in M87 and the center of our Galaxy. Moving VLBI to still higher frequencies can provide even finer angular resolution, but it also becomes increasingly difficult because atmospheric fluctuations disturb the signals much more rapidly.
The First Attempt at 690 GHz
On November 21, 2024, an international team led by Dr. Ming-Tang Chen of Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), carried out the first VLBI experiment in the 690 GHz atmospheric window. The experiment involved the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) in Chile, the James Clerk Maxwell Telescope (JCMT) in Hawaii, and the Greenland Telescope (GLT). Poor weather conditions in Greenland prevented the GLT from participating in the observations. Using a newly developed Band 9 phasing capability at ALMA, we detected a VLBI fringe between ALMA and APEX from the quasar J0423−0120, with a signal-to-noise ratio of about 12. This represents the highest-frequency ground-based VLBI fringe detection reported to date. No fringe was detected on the much longer ALMA–JCMT baseline.
Why Is 690 GHz So Challenging?
The experiment shows both the promise and the difficulty of near-terahertz VLBI. Even under excellent weather conditions, atmospheric phase fluctuations at 690 GHz limit the time over which signals can be combined effectively. The experiment also tested an iodine-stabilized clock as a frequency reference, an important step toward developing compact and reliable timing systems for future high-frequency VLBI stations. Nevertheless, the successful ALMA–APEX detection demonstrates that VLBI operation at this frequency is technically feasible. The experiment also identifies the improvements in sensitivity, phasing, and atmospheric correction that will be needed before observations at these frequencies can become routine. Ultimately, extending VLBI toward 690 GHz could provide sharper views of the immediate environments of supermassive black holes and open a new regime for ground-based astronomical interferometry.
Telescopes involved in the 690 GHz VLBI experiment. The observing network included ALMA and APEX in Chile, JCMT in Hawaii, and the Greenland Telescope (GLT). Due to poor weather conditions in Greenland, GLT was unable to participate in the observations. Illustration generated with AI.
ALMA–APEX observations of J0423−012 at 690 GHz. The distinct peaks in delay and delay-rate space show that the signals recorded at the two telescopes were successfully matched in time, giving a statistically significant VLBI detection. The lower panels show large phase scatter across the observing band, illustrating the rapid atmospheric fluctuations at 690 GHz. Image Credit: Chen et al.
More Information:
This research was published in Publications of the Astronomical Society of the Pacific, volume 138, number 8, under the title “First Very Long Baseline Interferometry Fringe Detection at 690 GHz" by Chen st al.
Media Contact:
Dr. Ming-Tang Chen Email: mtchen@asiaa.sinica.edu.tw Tel: +886-2-2366-5348 (Taiwan), +1 (808) 938-4708 (Hilo)
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