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Atmospheric phase and phase calibration considerations

With the old ATCA correlator, phase calibration at 3-mm wavelength is significantly more difficult than at the longer wavelengths. This is because the atmospheric phase errors are comparatively larger, the secondary calibrator grid is sparser and the system sensitivity is poorer. To use a tolerably nearby calibrator, you will need to use a calibrator that is weaker than you might wish - particularly if you wish to observe in a narrow bandwidth. Alternatively you might use a nearby strong SiO maser rather than continuum point source phase calibrator.

These lead to a different approach to calibration compared to that at longer wavelengths. At 3-mm wavelengths, the dual frequency band capability of the old ATCA correlator could be used to good effect in the calibration process. When sensitivity to calibrators was an issue, one of the two bands could be specifically set aside for calibration purposes. The correlator configuration could be selected so that one of the bands was either 128 MHz wide (and so as sensitive as possible), or suitable for observing an SiO maser. This calibration band could also be used for reference pointing during the course of the observation.

When using one of the bands for calibration purposes, the two bands need to be jointly calibrated.

SiO masers should be used with caution: they are often strongly polarised, and they may be structurally complex. Note, also, that the old ATCA correlator limited the dual observing bands to be set within 2.7 GHz of each other. If you use an SiO source as a calibrator, the part of the 3-mm band that is accessible to the observation is limited.

For data taken with the CABB correlator, the extra bandwidth provides a large increase in sensitivity. Also, since CABB provides continuum data at all times, regardless of whether spectral line data has been gathered as well, it is possible to do reference pointing and phase calibration in the same way for all observations at all wavelengths.