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28th of November 2023
ASKAP Update for November
The ASKAP update for November reports on survey progress, the latest ASKAPsoft pipeline software release and plans to improve ASKAP’s astrometry. It has long been known that individual ASKAP observations can exhibit larger than usual astrometry errors of up 6 arcsec in extreme cases, but usually better than 2 arcsec. ASKAP is susceptible to astrometry offsets because we do not conduct any phase referencing during an observation due to the large field of view. Extensive internal discussion has converged on a theory that the dominant cause of astrometry offsets in ASKAP is the fact that our bandpass calibration observations (which provide the fundamental phase reference) are spread out sequentially for each beam over a time span of more than 2 hours. The ionosphere and other environmental factors do not remain fixed during this time, so the astrometry of each beam can be inconsistent. Planning is underway to test this theory using a local sky model of a specifically chosen field to phase calibrate all beams simultaneously. This would use much less observing time than bandpass calibration and could therefore be done more often.

The figure above shows astrometry offsets (in arcseconds) measured in RACS-low2 fields by comparing the positions of sources visible in two beams simultaneously where they overlap. Different colours represent different survey fields, and the vector origins are placed between the beams from which they were derived, on a plane representing the beam footprint. This shows that neighbouring beams can have different astrometry, and that there are trends following the sequential pattern in which the bandpass calibration is performed. The fact that the pattern is similar for different fields suggests that the initial phase calibration derived from the bandpass observation is more significant than the contents of the science field itself. (Image credit: Emil Lenc)

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