Space Science

Radio Interferometry: How ALMA Links Its Antennas

See how antenna spacing sets image detail and why a wider array can miss broad emission.

By AstroboatPublished 2026-11-092 min read

Prepared with AI assistance using the primary sources linked below. How Astroboat works

Compare the waves, not just their average brightness

A wave from a distant source reaches separated antennas with a small difference in arrival time. An interferometer accounts for those delays and correlates signals from antenna pairs. Each pair samples part of the sky's brightness structure. The array builds an image from many such measurements rather than stacking separate ordinary photographs of the same object.

Reference: ALMA Observatory: Interferometry; NRAO: ALMA Basics

A longer baseline can distinguish finer detail

The separation between a pair of antennas is called a baseline. At a given wavelength, larger projected separations can resolve finer features. A useful scale estimate is wavelength divided by baseline, in radians. For 1-millimetre waves and a 1-kilometre baseline, that is about one millionth of a radian, or 0.2 arcseconds. Actual resolution depends on the observing geometry and how the data are combined.

Reference: ALMA Observatory: Interferometry; NRAO: ALMA Basics

Fine resolution does not guarantee a complete image

An extended cloud can emit strongly across a broad area but have little contrast on the finest scales. An array without enough short baselines can recover compact knots while missing much of the smooth emission. ALMA uses compact antenna configurations and single-dish measurements to help recover larger structures. A spread-out array does not gather light like one solid dish the size of the array.

Reference: NRAO: ALMA Basics

Look at the beam and the scale together

Find the wavelength, image scale, and beam size in a published map. A beam indicates the resolution of the reconstructed view; features smaller than it are not resolved just because they cover several pixels. If two maps disagree about how extensive a cloud is, compare their baselines and sensitivity to broad emission before assuming the cloud changed.