First VLBI observations with ASKAP PAF

A comparison of VLBI images, showing the results with and without the ASKAP baseline included. Credit: CSIRO

A comparison between the VLBI observations
show a clear improvement in image resolution
when ASKAP is included in LBA observations.
Click on the image for more detail.

22 July 2014

Promising results have been achieved from the first VLBI observations between an ASKAP antenna equipped with a first generation, or Mk I, phased array feed (PAF) receiver and CSIRO’s Parkes 64-m radio telescope, separated by approximately 3000km.

This result demonstrates the proof-of-concept of achieving VLBI observations in the later life of ASKAP with the Australian LBA (Long Baseline Array).

First fringes with the system were initially achieved in June, using raw data-capture from a Mk I PAF installed on an ASKAP antenna at the MRO and the 64-m dish in Parkes using a more ‘traditional’ VLBI system.

More recently, data processing was completed for a series of observations between a Mk I PAF installed on ASKAP Antenna 6 (at the MRO) and the LBA antennas at Parkes, Narrabri (Australia Telescope Compact Array), Mopra and Ceduna. The source chosen for these observations was a main VLBI calibrator, PKS 0537-441.

The results show promising ASKAP visibility data with stable phase and amplitude visibilities; the benefit of including ASKAP as part of the LBA is reflected in an improved resolution of the image.

According to Chris Phillips, LBA Lead Scientist, software development was required to correlate the data between the PAF and non-PAF systems, and the tests provided valuable insight into the ASKAP system.

“The most interesting technical challenge was to resample the data from the ASKAP beamformer to render it compatible with existing VLBI systems,” said Chris, “The advantage of running these observations is that not only do we learn about doing VLBI in the later life of ASKAP, but we also gain a better understanding of commissioning with the current digital systems hardware being used for commissioning by the ACES team.”

Though this is not the first time an ASKAP Antenna has been used in VLBI observations, it is the first time these observations have been performed using a phased array feed receiver. In July 2011, a live demonstration of eVLBI was performed using an ASKAP antenna (installed then with an X-band receiver) with other telescopes of the Australian LBA, controlled remotely from the International SKA Forum in Banff, Canada.

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