Dips in bandpass every 64 channels

Extensive development work to eliminate bandpass artefacts has been done since the first call for Open Time proposals in late 2018. One final artefact remains, and cannot be eliminated due to fundamental constraints in the design of the correlator. It is present in all channelisation modes of the correlator. All 4k and 32k data show a periodic dip every 64 channels of roughly 0.005 - 0.010% in the visibility amplitudes, and much stronger in the autocorrelations. However, this effect has been shown to be stable and can be successfully calibrated and eliminated using standard bandpass calibration procedures. We include this effect here for completeness.

Further details are given in the Calibration and stability of the 64-channel dips commissioning report. Note that due to a different design, the narrowband modes do not share this effect. No other persistent periodic or spectral leakage effects have been found in the current production release of the correlator.