Spectroscopists must also calibrate the instrumental bandpass, often for both amplitude and phase. The amplitude calibration may sometimes be estimated by dividing the cross-correlation spectra by the geometric mean of the autocorrelation spectra of the relevant antennas. This approach is unsuitable however if there are lines (or interfering signals) in the passband that are strong enough to show up in the single-antenna spectra. It is inadequate if the phase variation across the band is significant relative to other uncertainties, for example those imposed by the signal-to-noise ratio.
The most complete approach to bandpass calibration is to observe a strong line-free continuum source to find the bandpass characteristics (amplitude and phase) of each antenna-receiver combination. In a well-engineered system, these characteristics will be only slow functions of time, so the calibration need not be repeated often. Notice however that the total time needed for the bandpass calibration is set by the sensitivity required to measure the gain in each channel as accurately as needed for the required spectral dynamic range. Projects that require high spectral dynamic range may therefore have to invest a large fraction of the total observing time in bandpass calibration. See Lecture 12 for details of bandpass calibration strategy.