The ASKAP Roll Axis Preliminary study of its benefits to calibration and imaging

A standard test field containing numerous strong sources has been observed twice with the ASKAP BETA system, with the third axis of motion – the roll axis – disabled for the second run. These two data sets are subjected to identical calibration and imaging pipelines in order to gauge the e↵ects of the roll axis when observing with a Phased Array Feed. Present beam forming techniques only allow meaningful comparisons to be performed with the on axis ‘boresight’ beam. Observing with the roll axis tracking reduces the standard deviation of the close-in imaging artefacts around the bright sources by factors of 2–3. Applying directional calibration techniques and examining the solutions reveals that the elevated artefacts are imparted by position and time dependent gain drifts with amplitude modulations of up to 50%, consistent with a rotating primary beam of non trivial shape.