PAFs beyond ASKAP
Through an agreement with the Max Planck Institute for Radioastronomy (MPIfR), CSIRO has built a PAF receiver for the 100-metre Effelsberg telescope in Germany. Modified for the RFI environment in Germany, the Effelsberg PAF will operate from 1150-1800 MHz and use a bespoke GPU cluster and 2.5 terabit per second Ethernet switch. MPIfR, in collaboration with the University of Manchester, is developing specialised data processing software, as the high data rates produced by a PAF require real-time data management and archiving.
Over the past eight months the Effelsberg PAF has been tested and commissioned on CSIRO’s 64-metre Parkes Radio Telescope. This is the first time a PAF has been installed on a large single-antenna radio telescope and made available to astronomers. Pleasingly, it demonstrated the expected high performance while also achieving several observing ‘firsts’.
Trial observations demonstrated the viability of transient and spectral-line science observations with the sensitivity of a large dish and the wide field of view of the PAF. Sensitivity and field of view were characterised (Chippendale et al. ICEAA arXiv:1606.03533 < https://arxiv.org/abs/1606.03533>) and found to be consistent with expectations, as were observations of familiar pulsars and extragalactic sources of atomic hydrogen emission. Indeed, the latter showed very flat bandpass responses, replicating almost instantly spectra that were obtained previously only after significant processing effort.
For the first time three pulsars in three different beams in the same field of view were simultaneously timed (an article is being prepared for publication) and another first was achieved with sub-1 GHz very long baseline interferometry (VLBI) between two PAF systems: the Bonn PAF on Parkes and one on the ASKAP radio telescope (see story below).
Significant data were collected for a project led by researchers from the Australian Research Council Centre of Excellence for All-Sky Astrophysics (CAASTRO) and the International Centre for Radio Astronomy Research (ICRAR) to demonstrate wide-field intensity mapping of distant atomic hydrogen using a ‘drift-scan’ observing mode. This is where the dish is stationary while the target drifts through the telescope’s line of sight, in this case using 16 beams and 18 kHz spectral resolution.
Towards MPIfR’s goal of competitive wide-field searching for Fast Radio Bursts on Effelsberg, the Parkes deployment demonstrated ingest of the full duty-cycle voltage time-series for 18 beams in two polarisations with a bandwidth of 336 MHz into a Graphical Processor Unit (GPU) cluster, where search algorithms for transient sources could be run in real-time. Work is ongoing to tune the parameters of the single-pulse transient detection algorithms for the MPIfR PAF data recorded at Parkes, and also to scale the time-series ingest from 18 beams to 36 beams, for when the PAF is installed on Effelsberg.
There were successful tests of new radio frequency interference mitigation techniques via live adaptation of the beamformer weights: providing up to 30 dB of interference suppression. Also, the polarisation response of a PAF beam was measured by observing a pulsar with a well-known polarisation profile. This showed a high degree of polarisation purity and proved to be an efficient method of determining the polarisation characteristics of the PAF (through Jones matrix determination).
Now the Effelsberg PAF is on its way to Germany, CSIRO is planning to install a third generation (Mk III) PAF on Parkes in place of Parkes’ highly successful multibeam receiver. Nicknamed the Rocket PAF for obvious reasons, it melds the technological concepts behind CSIRO’s multi-award winning chequerboard planar array and the more common PAF technology, a Vivaldi array (Dunning et al. Proceedings of the 13th European Radar Conference: EuRAD 2016). Also, the Rocket PAF is cryogenically cooled and expected to achieve an on-sky system temperature below 20 Kelvin. It is hoped that the low system temperature and increased field of view of the Rocket PAF over the original mutlibeam receiver will reap significant scientific rewards in the fields of atomic hydrogen intensity mapping, fast radio burst detections, and pulsar and spectral line surveys.
In recognition of its role in testing innovative new receiver systems, the SKA Organisation granted CSIRO’s iconic Parkes Radio Telescope the status of SKA Pathfinder.