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.
View a pdf version of ALMA Memo #505.
Last modified: 2004-10-04
alma-memos@nrao.edu