Earth as a Turntable
Aperture synthesis exploits Earth's rotation to transform a small array of antennas into a virtual telescope spanning the maximum baseline distance. As the Earth turns, each antenna pair traces an elliptical track in the UV (spatial frequency) plane. After hours of observation, the accumulated UV coverage approximates a filled aperture, enabling imaging with resolution determined by the longest baseline rather than any individual dish diameter.
The UV Plane
The UV plane is the Fourier dual of the sky brightness distribution. Each point (u,v) sampled by a baseline measures one spatial frequency component of the sky. The set of all sampled (u,v) points determines which angular scales are captured in the image. Complete UV coverage produces a clean point spread function; gaps create sidelobes that contaminate the image.
From Dirty to Clean
The raw image produced by Fourier-transforming the sampled visibilities is called the 'dirty image' — the true sky convolved with the dirty beam (PSF). The CLEAN algorithm, developed by Jan Hogbom in 1974, iteratively identifies point sources, subtracts their dirty beam response, and replaces them with clean Gaussian components. Modern variants (multi-scale CLEAN, MEM) handle extended emission more effectively.
Array Design Philosophy
Array configurations balance conflicting requirements: closely spaced antennas provide sensitivity to large-scale structure but poor resolution, while widely spaced antennas give high resolution but miss extended emission. The VLA uses a Y-shaped configuration with logarithmic spacing to achieve uniform UV coverage across a wide range of angular scales. The SKA-Mid will use a spiral configuration optimized for survey speed and imaging fidelity.