Interferometry Baseline Resolution

Interferometer baseline and angular resolution

In a radio interferometer it's the baseline B between two antennas that fixes the synthesised angular resolution, not how big each dish is on its own: θ (rad) ≈ λ / B. Stretch the baseline out and the beam gets sharper, which is exactly why these arrays end up spread across kilometres, sometimes whole continents.

A few benchmarks to anchor the scale: the Karl G. Jansky VLA tops out near 36 km of baseline in its A configuration; ALMA reaches 16 km up on the Chajnantor plateau; the VLBA stretches 8 611 km across North America; the Event Horizon Telescope builds baselines of roughly 12 000 km, almost the width of Earth’s diameter; and SKA‑Mid is slated to work with baselines as long as 150 km in South Africa.

Applications

Reach for this when you want a quick read on the angular resolution a given baseline buys you at some radio frequency, when you're weighing a compact (more sensitive) layout against an extended (higher-resolution) one, or when you need to know whether something like a jet, an accretion disk or a stellar surface is even within reach to resolve.

FAQ

Is resolution exactly λ/B? Not quite. The real synthesised beam carries a numerical prefactor that shifts with uv-coverage and weighting, so treat λ/B as the ballpark figure.

Does a longer baseline also increase sensitivity? No. Sensitivity tracks the total collecting area you have. The baselines decide the resolution and the largest scale that gets filtered out, nothing more.

Why not just one giant dish? A single dish runs into mechanical and thermal limits somewhere around 100 m and can't grow much past that. Interferometry gets around the ceiling by tying together a lot of smaller antennas.