u-v plane

A u-v plane is a geometric plane defined for the analysis/processing of interferometer observations, for which the axes are conventionally termed u and v. The plane's attitude within geometric space is tangent to the celestial sphere at the position of the astronomical object under observation, with v virtually always chosen to be northward, the axes scaled using the observation's wavelength as their unit of distance. Its purpose is to represent/analyze/display a Fourier transform of the signal from that portion of the sky, which corresponds to the interferometer correlator output (visibilities), per the van Cittert-Zernike theorem. In such discussions, the corresponding plane of the actual signal (i.e., the plane of the sky) is termed the l-m plane, I presume to stand for latitude and meridian. Aperture synthesis can be described as determining the signal from the l-m plane based upon the input data expressed in the u-v plane.