Spatial Structure of the Source

The second striking feature of the emitted radiation is that its measurement yields the spatial multipole structure of the source. Measure the angular distribution for a given radial coordinate $r$ in the plane transverse to the direction of acceleration. Do a least squares fit to the measured data points in order to determine each of the Fourier coefficients in Eq.(52). In order to obtain the radial distribution of each of these coefficients, repeat this determination for various $r$ values. A second least squares analysis yields the radial derivatives and hence the amplitude of each multipole moment $S^m_{I,II},~m=0,\pm 1,\pm 2, \cdots$ in Eq.(54).