Contaminants in ATCA baselines with shadowing: a case study of cross talk in short-spacing interferometers

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.