What is the primary beam response of an interferometer with unequal elements?

TL;DR

This paper reviews how the primary beam response of an interferometer with unequal elements differs from that of individual elements, emphasizing its importance for wide-field radio astronomy applications.

Contribution

It provides a detailed review and summary of the primary beam envelopes for interferometers with unequal elements, highlighting practical implications.

Findings

Different station pairs produce distinct primary beam envelopes.

The combined primary beam differs from individual element beams in unequal-element interferometers.

Proper determination of the beam envelope is crucial for wide-field observations.

Abstract

The EVN stations encompass elements with a range of diameters, even including an interferometer (the Westerbork Telescope, with up to 14 elements used together as a tied array). In combination, the various station pairs will each produce their own primary beam envelopes, with which the interferometer pattern is modulated. People sometimes forget that in the case of unequal elements, this combined primary beam envelope is different from the beam of each element separately. The reason for this is reviewed, the results for a number of station pairs are summarized, and some of the practical consequences are discussed. The increased interest in wide-field applications, as illustrated by several recent results, underlines the need for a proper determination of the interferometer beam envelope.