The QA0 criteria adopted by ALMA require that amplitude variations across a spectral window are < 3 dB in the autocorrelations (not due to atmospheric features). QA0 is given SEMIPASS if the bandpass calibrator signal is too weak, with amplitude variation > 3 dB or strong continuous wave (CW) signals, and the phase/gain calibrator cannot be used instead.
It is important to measure the spectral response during an observation by performing at least one bandpass observation per spectral setup (i.e., SB execution, because each re-execution entails a tuning optimization) for interferometric observations that require high spectral accuracy.
Some more information can be found in the chapter "Calibration and Calibration Strategies" of the ALMA Technical Handbook.
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