Synthesis telescopes suffer from a number of aberrations which increase with distance from the field centre. The radii at which the amplitude of a point source is reduced by 1 and 10% at each frequency are listed in Table 1.1. We distinguish between the normal observing mode, corresponding to 16 s integrations and a single averaged frequency channel with width equal to the maximum bandwidth, and a special wide-field mode with the shortest possible integration time and the bandwidth divided into the maximum possible number of channels. This gives a large increase in the unaberrated field of view, at the expense of a increase in data volume and data reduction time. It is possible to reduce the aberrations still further, but only at the cost of a loss in sensitivity. For further details see §4.5.4.