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Time smearing or time-average smearing is the degradation of the reconstructed image of a celestial body observed by a ground-based interferometer that occurs because of the duration of the observation. Unlike single telescopes or cameras that can compensate the Earth's rotation in real time with a dedicated mount, the different telescopes of the interferometer are at fixed positions on the Earth. As a result, maps obtained with interferometers feature the elongated orthoradial features similar to those of night sky photographs taken with a fixed tripod, unless they use short enough integration times.
The smearing is a problem for long integration times or very separated telescopes. Mostly an issue in radioastronomy, it severely limits the usable field of view of observations in very long baseline interferometry.