The Interferometric Principle
A single radio dish is limited in resolution by its physical diameter. Interferometry overcomes this by correlating signals from separated antennas: the fringe pattern encodes angular information at a resolution set by the baseline length, not dish size. Martin Ryle pioneered this technique at Cambridge in the 1950s, earning a Nobel Prize for aperture synthesis.
Fringe Patterns and Visibility
When two antennas observe the same source, the geometric path-length difference creates a sinusoidal fringe pattern as Earth rotates. The fringe spacing equals λ/B radians, and the fringe amplitude (visibility) encodes the source structure at that spatial frequency. A point source produces full-contrast fringes; extended sources reduce the visibility amplitude according to the van Cittert-Zernike theorem.
Building the UV Plane
Each antenna pair samples one point in the spatial frequency (UV) plane at any instant. Earth rotation sweeps each baseline through an elliptical track, gradually filling the UV plane. More antennas yield more baselines — the VLA's 27 elements provide 351 simultaneous baselines, enabling rapid UV coverage. Gaps in UV coverage produce imaging artifacts (sidelobes) that must be deconvolved.
From the VLA to the SKA
Modern interferometers push baselines from kilometers (VLA, ALMA) to intercontinental scales (VLBI, Event Horizon Telescope). The EHT's Earth-diameter baseline achieved the resolution to image the M87 black hole shadow. The Square Kilometre Array, under construction in Australia and South Africa, will combine thousands of elements with unprecedented sensitivity and survey speed.