Aperture synthesis instruments providing a generally highly uniform sampling of the visibility function often leave an unsampled hole near the origin of the (u,v)-plane. One of the common configurations of the Westerbork Synthesis Radio Telescope (WSRT) provides sampling in concentric rings with radii of n × 18 m. However, the samples with n=0 and n=1 cannot be obtained directly with the instrument because of rather obvious physical constraints. This missing short spacing information causes a

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... ntral depression of the synthesized beam pattern, resulting in large negative, bowl shaped areas around (especially) extended sources. Very large sources will only partially be present in the maps. Annoying as this can be for continuum observations, in the case of spectral line data it causes serious distortions of the individual line profiles.