Interferometry

The technique, developed by Martin Ryle, of building up an image from an interferometer by combining measurements taken as the Earth's rotation carries the antennas to different orientations — synthesising the resolution of a single huge aperture from many small ones. The basis of modern radio imaging.

Combining the light of separate optical or infrared telescopes to reach angular resolutions no single telescope could — far harder than in the radio because the light waves must be kept in step to a fraction of their wavelength. The VLT's four unit telescopes can be combined this way.

Linking many radio dishes so that they act together as one telescope as large as their separation, giving radio astronomy its sharpest images. Arrays like the VLA and ALMA correlate the signals from every pair of antennas to reconstruct the sky.

Radio interferometry stretched across continents — antennas thousands of kilometres apart record the sky independently with precise atomic clocks, and the data are combined later. It achieves the finest angular resolution in all of astronomy; a global VLBI network imaged the shadow of a black hole.