If wide-field or spectral line imaging is required, then multi-channel data is used. It is necessary to correct for variations in phase and amplitude as a function of frequency. This is usually done using the point-cal as a bandpass calibration source (bp-cal), after it has been self-calibrated, and the resulting solutions, known as a bandpass table, are applied to all data. A separate stronger source (Appendix C) can be used for observations made in very narrow spectral channels. For spectral line observations, particularly at 1.6 GHz and lower frequencies, the data should be carefully inspected for interference at an early stage.