ABSTRACT
In Chapter 5, we introduced the fundamental principles of aperture synthesis by considering a two-element interferometer and restricting our discussion to a simplied situation in which the radio sources, any source structure, and the antennas are all contained in the equatorial plane. is simplied the mathematics, but of course does not describe the most general situation. In this chapter, we expand our presentation of radio interferometry with the goal of providing a deeper-but still basic-understanding that will serve for embarking on aperture synthesis observations. More advanced and in-depth discussions are provided in Interferometry and Synthesis in Radio Astronomy by ompson et al. (2001)* and Synthesis Imaging in Radio Astronomy II Volume CS-180 edited by Taylor et al. (1999).†
Let us rst review some important denitions. e conguration we set up in Chapter 5 involves two antennas separated by a distance b, called the baseline, and they receive radiation from a source at an angle q relative to the zenith. Because of the extra path length distance to one of the antennas, the arrival of the wave fronts to one antenna involves a time delay, t, given by Equation 5.1 as τ θ=b csin , which causes a phase dierence between the two signals given by ∆Φ = 2piντ . e signals from the antennas are multiplied and averaged over an integration time, which is much longer than the period of the waves, but shorter than the time for any signicant change in the delay, and typically of order a few seconds. e phase dierence between the two signals changes as the source moves across the sky (due to the Earth’s rotation), and so the product of the two signals results in an oscillation in time called fringes. e amplitude of the fringes is called the visibility amplitude. For a point source, the visibility amplitude, aer calibration, equals the source ux density. A reference position, called the phase center, is chosen by the observer, and the fringe function for that position is removed from the interferometer output versus time (see Section 5.5).
