From the van-Cittert Zernike theorem (see Chapter 2) it follows that if one knows the mutual coherence function of the electric field, then the source brightness distribution can be measured4.1. The electric field from the cosmic source is measured using an antenna, which is basically a device for converting the electric field into a voltage that can then be further processed electronically (see Chapter 3). In this chapter we will examine exactly how the mutual coherence function is measured.
We start by looking at the relationship between the output of a two element interferometer and the wanted mutual coherence function. Large interferometric arrays can be regarded as collections of two element interferometers, and for this reason it is instructive to understand in detail the working of a two element interferometer.