ALMA Memo XXX Bandpass Calibration for ALMA

This memo contains a detailed evaluation of the expected performance of the bandpass calibration for ALMA. We evaluate the limitations on the bandpass accuracy due to the imperfect knowledge of the atmospheric transmission. We show that the best bandpass accuracy is obtained when the amplitude calibration is applied in a single-load scheme. The proposed scheme is a variant of the so-called bandpass normalization technique. It uses normalization by the difference between the sky emission and the load emission, rather than normalization by the auto-correlation spectrum in the usual case. Bandpass calibration should be performed at the observing frequency. It will be limited by the knowledge of the sideband opacity difference. This knowledge can be based on a model, but a direct measurement is also possible, although time consuming. The bandpass calibration must be performed on strong point-like sources, of known spectral index. The knowledge of the spectral indexes will be a limiting factor in the wide band modes. Building up a database of suitable sources will be necessary, since the knowledge of these spectral indexes could be improved by bootstrapping techniques among several sources. Since astronomical sources have to be used, the required integration time varies substantially as a function of frequency. High accuracies can be reached in a minute of time at mm wavelengths, but integration time as large as an hour can be needed at sub-mm wavelengths. Occasional direct measurements of the sideband opacity difference could also be used to improve the accuracy of the atmospheric modelling.