First 28-antenna ASKAP image approaches confusion limit

The ASKAP team recently made 28 ASKAP antennas available as an observing array. In the course of testing the array, ASKAP commissioning team member Emil Lenc (CSIRO) has made the first image from observations with 28 ASKAP antennas. The image shows that ASKAP is on track to achieve the sensitivity and resolution needed for the EMU (Evolutionary Map of the Universe) project.

The radio continuum image is centred on the well-known calibrator B1934–638. Every dot in the image is a radio source.

ASKAP single-beam 14.5h observation, 28 antennas, 288-MHz bandwidth, centred on 888 MHz, 19 x 14 arcsec resolution. The central source (B1934-638) peaks at 14.1 Jy and the image noise is 17  uJy/beam. This gives a dynamic range of approximately 830,000:1. (Observed 9 August 2018, imaged 15 August 2018). Image: Emil Lenc (CSIRO)

This is the first ASKAP image to approach the confusion limit – the fundamental ‘noise floor’ created by faint radio sources. In other words, the image quality is not limited by telescope performance.

In earlier years it had been thought that ASKAP would achieve a dynamic range of 100,000:1. This is vastly surpassed by the figure for the new image, which is 830,000:1

Emil processed the data ‘by hand’ using casa (the Common Astronomy Software Applications package). This allowed him to inspect the quality of the data – which he describes as ‘awesome’. All antennas produced usable data.

The processing was relatively quick, leaving some artefacts in the image. The data will be reprocessed more carefully with askapsoft.

Below is the image compared with a one made with the ATCA, plus the parameters of the images. The ASKAP observation took just a tenth of the time of the ATCA’s, but ASKAP’s lower frequency and smaller dishes reveal many more sources, and of course its field of view is much larger.

This ASKAP image was made with only a single beam. A full, 36-beam image will cover an even greater area, with an even higher survey speed.

Parameters for the ATCA and ASKAP images above