First VLBI with two phased-array feeds

In a world first, CSIRO has shown that high-resolution observations, a technique called 'very long baseline interferometry' VLBI, can be made using two phased-array feeds (PAFs).

The CSIRO-designed PAFs are a key feature of the Australian SKA Pathfinder, ASKAP, which is sited at the Murchison Radio astronomy Observatory (MRO) in Western Australia. PAFs can be thought of as 'radio cameras', able to image much larger regions of sky than traditional single pixel feeds.

For their experiment on 27 October, the VLBI test team observed with one of the 12 ASKAP antennas now operating with PAFs and, simultaneously, with the 64-m Parkes radio telescope in eastern Australia, equipped with a slightly modified ASKAP PAF.

A previous link-up with an ASKAP antenna had shown it was feasible to use a single PAF for VLBI.

The dual-PAF observations were more complex. New firmware had to be installed on the ASKAP antenna to allow access to the raw data, and the data streaming from both PAFs had to be resampled and converted to VLBI format.

Three hours' worth of data was recorded and will be analysed over coming weeks.

The goal was to observe the galaxy PKS B1740-517. CSIRO astronomers had discovered neutral hydrogen gas (H I) in this galaxy when using just six ASKAP antennas, its so-called BETA array. They hoped to use VLBI to pinpoint where the gas lies in the galaxy. They used the dual-PAF set-up because they needed to observe at 986 MHz, a frequency covered by the PAFs but not by the conventional feeds available at Parkes or other Australian telescopes used for VLBI.

As well as confirming that the signals from two PAFs can be combined for VLBI, the tests showed how easily ASKAP can 'multitask'. While one ASKAP antenna was being used for VLBI, another ten were working together to study the same galaxy in a different way.

Chris Phillips, Aaron Chippendale and Aidan Hotan carried out the VLBI tests, helped by the MRO support staff and the Digital Backend group at CSIRO Astronomy and Space Science.

Added by Helen Sim on 2016-11-07