  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. In this paper, originally published in 1979, we first describe the
common solution of retrieving the information from scans made with a large
single-dish telescope. However, this is not the only means by which short
spacing visibility data can be obtained. We propose an alternative technique
that employs a short-baseline interferometer to scan the entire primary beam
area. The obvious advantage is that a short-baseline pair from the synthesis
instrument can be used, ensuring uniformity in instrumental characteristics.
This technique is the basis for the mosaicing algorithms now commonly used in
aperture synthesis radio astronomy imaging.
