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ALMA MEMO #478
Distance to Possible Calibration Sources as a Function of Frequency for ALMA
Bryan Butler
National Radio Astronomy Observatory
2003-10-07
The assumption that so-called "fast switching" calibration will always be done by observing a calibrator at 90 GHz
is examined. It is found that it may be better to observe the calibrator at the same frequency as the target, since
suitable calibration sources may be just as close at higher frequencies as they are at 90 GHz (all roughly 1° for continuum bandwidths). In the end it will depend on the characteristics of the sources (size, mostly). Observing the calibrator at 90 GHz will require an additional calibration to be performed on fairly short timescales (of order minutes) to calibrate the relative complex gain between the target and calibrator frequency, since the electronics will change between scans. This extra calibration is not required if calibrator and target are observed at the same frequency. In addition, observing the calibrator at the same frequency as the target removes the requirement of having very accurate atmospheric modelling in order to transfer the phase and amplitude from calibrator frequency to target frequency (especially problematic in regions of dispersion near strong telluric lines). Because of the uncertainty in the sizes of the sources, it is not recommended to change the current paradigm of calibrating at 90 GHz, but the option of calibrating at the target frequency should be kept in mind, and exercised if it turns out that the sources considered
herein are appropriate to use as calibrators (small enough).
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Last modified: 2003-10-17
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