  Interferometric telescopes made of close-packed antenna elements are an
important tool for imaging extended radio sources, specifically structures that
have angular sizes comparable to or even greater than the FWHM of the beams of
the antennas. They have proved useful in observations of cosmic microwave
background anisotropies that require high brightness-sensitivity. However, the
visibilities measured in baselines formed between close antenna elements -- in
particular, between shadowed elements -- of Fourier-synthesis arrays are often
observed to be corrupted. We discuss the multiplicative and additive errors
affecting such short-baseline interferometers.
  As a case study, we have examined the nature of the spurious correlations
between the Cassegrain-type paraboloidal reflectors that are elements of the
Australia Telescope Compact Array. In configurations with geometric shadowing,
the cross talk here appears as an additive component. Analysis of the
characteristics of this cross talk leads us to believe that when these
reflector antennas are in a shadowed configuration, the receivers in the
antenna pair pick up correlated emission from opposite sides of the main
reflector surface of the front antenna. The slots between the panels that make
up the main reflector surface provide the pathway for the coupling across the
reflector surface. This mode of cross talk may be avoided by constructing the
main reflectors of short spacing interferometers as continuous conducting
surfaces.
