'On-the-fly' mapping

The MeerKAT correlator does not innately support on-the-fly mapping, i.e. continous adjustments of the delay tracking centre while the antennas slew. However, it has been shown to be possible to correlate scanning observations after the fact to enable interferometric imaging. We briefly summarise the efforts in this direction below.

The HI intensity mapping project conducts scans at a fixed elevation angle while scanning at 5 arcmin/s, with a 2s integration time.

Previous behaviour of the Scan

The correlator delay tracking centre is set to the central azimuth, elevation point of the the scan, which effectively tracks the changing right ascension at that point.

Primary beam smearing is shown to be small (amplitude error <5%) at slow scanning speeds over a 2s integration, which may or may not be negligible depending on the scientific applications.

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The typical scanning pattern (one hour segment) for asynchronous OTF mapping.

Bandpass and gain calibrators are tracked for the standard duration and cadence.

New phase center tracking

The phase center is tracking the (ra, dec) position that drifts through the middle of the current linear scan when the antenna pointing centre passes that point. This results in a separate target for each scan which is tracked for the duration of the scan. As shown in the figure below labled as Option 3 . The correlator targets follow the same straight line along RA as in the original scan , but now hops along this line in discrete steps instead of smearing out along it.

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Delay centres for new scanning “Option 3”, is indicated by red squares. The red arrow indicates the largest distance between a field and its delay centre.

This way, the sky smearing is greatly reduced while the maximum distance between field and delay centre is still similar to that of the original behaviour.

This corresponds to option c (“Track a fixed point on the sky for the full duration of an OTF scan”) in ALMA Memo 331 .

This report describes the prototype data pipeline used to apply a phase rotation to each integration. The resulting test image shows excellent image quality with no visible trace of imaging artefacts and rms noise close to the expected level of ~0.33 mJy/bm (see Figure 4 of attached report). For further information contact Mario Santos.