NenuFAR correlator NICKEL built in collaboration with ASTRON

BY ALICE SPRUIT (ASTRON)

A highly successful collaboration between the Netherlands Institute for Radio Astronomy (ASTRON) and Nançay Radio Observatory (NRO) has resulted in a state-of-the-art GPU correlator for the SKA pathfinder NenuFAR telescope array at Nançay, France.

Saving years of work in a matter of weeks has resulted in making high-quality NenuFAR data available to astronomers worldwide. The GPU correlator, called NenuFAR Imaging Compute Kluster Elaborated from LOFAR’s (NICKEL), was inspired by the correlator of another SKA pathfinder, LOFAR’s mega mode correlator COBALT2.0.

The correlator is the brain for radio telescopes like LOFAR and NenuFAR (and eventually the SKA) as it processes and combines in real time the digital streams arriving from the antennas (stations). The output astronomical data is then ready for astronomical analysis.

The COBALT2.0 correlator design was adapted, scaled and optimised, in collaboration between NRO and ASTRON, for the requirements of NenuFAR’s antenna field.NICKEL also makes use of LOFAR’s GPU-based correlation software suite (originally developed at ASTRON) after incorporating significant necessary adaptations and optimisations for NenuFAR operations.

The sharing, reuse and re-optimisation of the correlator hardware design and the software suite is a shining example of open science and technical collaboration between SKA pathfinders and across the SKA community. Such collaboration is key to the success of large international endeavours like the SKA Project.

NenuFAR is now carrying out astronomical observations utilising its new imaging mode. The images at the top of the picture below are the first-light images resulting from a 10-hour observation of the giant elliptical galaxy Virgo A (also known as M87) in the lower part (29.5-32 MHz) and the upper part (63.5-66.5 MHz) of the NenuFAR band. The orange ellipses show the primary beam of the instrument. At low frequencies, we clearly see the diffuse emission from the tip of the Loop I region. At high frequencies, several 3C radio sources are also seen, including M84, another giant elliptical galaxy very close to Virgo A, which appears as a powerful unresolved source.

Credit: NenuFAR/ASTRON

The observations were carried out using only central 56 mini arrays having a maximum baseline of about 400m. The angular resolution and sensitivity will significantly improve when observations with all the 96 mini arrays having up to 3km baselines and full bandwidth (10-85MHz) are carried out in the near future.

More information can be found at the NenuFAR webpage: https://nenufar.obs-nancay.fr/en/2021/01/06/first-light-of-the-nenufar-radio-imager/