[asac] [almanews] ALMA Memo 505 Released

[Stacy Oliver]

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. 

http://www.alma.nrao.edu/memos/html-memos/alma505/memo505.pdf

 

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listmgr.nrao.edu/pipermail/asac/attachments/20041005/a27ce428/attachment.html>
-------------- next part --------------
_______________________________________________
Almanews mailing list
Almanews at listmgr.cv.nrao.edu
http://listmgr.cv.nrao.edu/mailman/listinfo/almanews