Bandpass Calibration for ALMA Abstract
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ALMA Memo # 505
Bandpass Calibration for ALMA
A. Bacmann (ESO), S. Guilloteau (IRAM/ESO)
2004-09-30
Keywords: Calibration, Bandpass
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.
