Very Long Baseline Resolution

Very Long Baseline Interferometry (VLBI) combines radio telescopes separated by thousands of kilometres to synthesize a telescope with an effective aperture equal to the longest baseline. This technique allows astronomers to achieve angular resolutions far beyond what a single dish can provide, revealing fine details in distant cosmic radio sources.

The angular resolution θ of an interferometer is fundamentally limited by the observing wavelength λ and the projected baseline length B, following the diffraction relation θ approx frac{lambda}{B}. Shorter wavelengths or longer baselines yield finer resolution, enabling the imaging of structures such as black‑hole shadows and maser spots.

In practice, astronomers often specify the observing frequency ν instead of wavelength, using λ = c / ν, where c is the speed of light. By inserting realistic values—e.g., a 5 000 km baseline observing at 22 GHz—the VLBI array can reach sub‑milliarcsecond resolution, opening a window onto the most compact astrophysical phenomena.