© CEP Project team, team Ruby, HPC group, L2COM team

Correlation of radio telescope signals is a highly computationally intensive job. The old LOFAR correlator Cobalt2 did not meet the needs to correlate the complete LOFAR2.0 system. Developments in both hardware and software allow for a smaller, more energy efficient system. We therefore proudly present: Cobalt3!

Cobalt3 includes new hardware and new software. To ensure minimal impact on LOFAR2.0 commissioning, the first step in building Cobalt3 was rolling out the new hardware, which was installed in CIT late 2025. It consists of five nodes each with three NVIDIA L40S GPUs, four operational nodes and one hot spare.

On January 27th and 28th the current Cobalt2.2 software was rolled out on the new hardware. Thanks to the Multicast network the new system could be verified and tested without impacting the ongoing commissioning observations, which still used Cobalt2. The improved capabilities of Cobalt3, especially the higher network bandwidth, have increased the performance to allow LOFAR2.0 commissioning observations to increase the number of stations.

The next step is to upgrade the software to Cobalt3.0, which will use TensorCore algorithms for correlation and beamforming. The HPC group is developing this and optimizing the software for efficient use of the GPUs. The software roll-out is planned for March and will further increase performance by tailoring the algorithms specifically for the Cobalt3 hardware.

The above diagnostic plot produced with Cobalt3 shows visibility phases (top right triangle) and amplitudes (bottom left triangle) of an interferometric observation on Cas A using the 23 available HBA antenna fields from the LOFAR core stations. Besides known issues, all baselines show generally flat visibility phases and comparable visibility amplitudes. For the Superterp stations (CS002-CS007) fixed delays between stations have been removed.