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The imaging survey observations occur in the frequency range 1130-1430 MHz

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^[1], corresponding to a redshift of 0.26 for HI. While the lower half of this band is significantly affected by RFI, it offers a large cosmic volume for observing HI. The 24576 channels provide a frequency resolution of 12.2 kHz, or 2.6–3.2 km/s across the band for HI. The angular resolution of the imaging survey observations is ∼ 15′′×15′′/sin(δ). Imaging survey observations occur roughly four weeks out of every six weeks, with the other two weeks dedicated to time-domain surveys. The exact cadence of observations is adapted to suit maintenance and operational needs. A full description of the imaging survey observing strategy will be given in Hess et al. (in prep).

Briefly, survey fields are observed for 11.5 hours, instead of a full 12-hour sythesis measurement, as a compromise between flexibility in scheduling and depth/maximum uv coverage. Calibrator observations alternate between a flux/bandpass calibrator (3C147 or 3C196, occasionally 3C295) and polarization calibrator (3C286 or 3C138); calibrators are observed in every Apertif beam as the different beams have different calibration solutions. Up to two survey fields can be observed in a row. Thus a survey field may be separated in time from its calibrator by ∼ 15 hours. Occasionally an observing session will start or end with a survey field in order to increase observing efficiency; then the separation in time may be longer. The calibration solutions were found to vary by less than 10% on timescales of ten days within the same beam weights model. This demonstrates that the system is stable enough to allow long periods between calibration scans with minimal impact on the absolute calibration.

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