Phase closure achieved on multiple ASKAP PAFs
20 August 2012
System verification tests on three ASKAP antennas installed with CSIRO's innovative new phased array feed (PAF) technology have achieved first fringes and successfully demonstrated phase closure on the three baseline system.
Phase closure is an important step in calibrating the antennas in preparation for interferometry with ASKAP by demonstrating the proper functioning of the antennas and their electronic systems. Fringes had been achieved in previous on-sky commissioning tests at the MRO, but now performing the correlation using a three-PAF baseline as a triangle removes phase offsets in observations and ensures a more accurate imaging process.
The Systems Commissioning (SCOM) team at the MRO used the strong compact astronomical source Virgo A to perform the correlations, using data captured simultaneously from the three beamformers and processed in real time. The three baselines were then combined to form the closure phase, the results of which were encouragingly close to zero degrees for both polarisations.
The software correlator developed by the Computing team allows for captured signals from multiple ASKAP antennas to be processed in real time and is crucial for end-to-end system testing in this configuration. The system uses 'raw data capture' from memory on beamformer cards, capable of handling data from 16 bands, each 1 MHz, spanning 300 MHz bandwidth to produce measurement sets just like a full correlator.
"There is a significant level of excitement among ASKAP team as a whole," says ASKAP Project Director Ant Schinckel, "Obtaining phase closure on a three baseline system reinforces that our systems work as expected, and confirms that our plans for BETA commissioning are on track."
This first demonstration of phase closure on three ASKAP antennas will be followed by delivery and commissioning of the next three antennas making up the full Boolardy Engineering Test Array (BETA), and commissioning of the six-antenna BETA hardware correlator.
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