Apercal-The Apertif calibration pipeline

Full list of authors: Adebahr, B.; Schulz, R.; Dijkema, T. J.; Moss, V. A.; Offringa, A. R.; Kutkin, A.; van der Hulst, J. M.; Frank, B. S.; Vilchez, N. P. E.; Verstappen, J.; Adams, E. K.; de Blok, W. J. G.; Denes, H.; Hess, K. M.; Lucero, D.; Morganti, R.; Oosterloo, T.; Pisano, D. -J.; Ivashina, M. V.; van Cappellen, W. A.; Connor, L. D.; Coolen, A. H. W. M.; Damstra, S.; Loose, G. M.; Maan, Y.; Maccagni, F. M.; Mika, A.; Mulder, H.; Oostrum, L. C.; Orrú, E.; Smits, R.; van der Schuur, D.; van Leeuwen, J.; Vohl, D.; Wijnholds, S. J.; Ziemke, J.
Apertif (APERture Tile In Focus) is one of the Square Kilometre Array (SKA) pathfinder facilities. The Apertif project is an upgrade to the 50-year-old Westerbork Synthesis Radio Telescope (WSRT) using phased-array feed technology. The new receivers create 40 individual beams on the sky, achieving an instantaneous sky coverage of 6.5 square degrees. The primary goal of the Apertif Imaging Survey is to perform a wide survey of 3500 square degrees (AWES) and a medium deep survey of 350 square degrees (AMES) of neutral atomic hydrogen (up to a redshift of 0.26), radio continuum emission and polarisation. Each survey pointing yields 4.6 TB of correlated data. The goal of Apercal is to process this data and fully automatically generate science ready data products for the astronomical community while keeping up with the survey observations. We make use of common astronomical software packages in combination with Python based routines and parallelisation. We use an object oriented module-based approach to ensure easy adaptation of the pipeline. A Jupyter notebook based framework allows user interaction and execution of individual modules as well as a full automatic processing of a complete survey observation. If nothing interrupts processing, we are able to reduce a single pointing survey observation on our five node cluster with 24 physical cores and 256 GB of memory each within 24 h keeping up with the speed of the surveys. The quality of the generated images is sufficient for scientific usage for 44% of the recorded data products with single images reaching dynamic ranges of several thousands. Future improvements will increase this percentage to over 80%. Our design allowed development of the pipeline in parallel to the commissioning of the Apertif system. © 2021 Elsevier B.V. All rights reserved.
JMvdH and JV acknowledge support from the European Research Council under the European Union‘s Seventh Framework Programme (FP/2007–2013), ERC Grant Agreement nr. 291531 (HIStoryNU). EAKA is supported by the WISE research programme, which is financed by the Netherlands Organization for Scientific Research (NWO). The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007–2013)/ERC Advanced Grant RADIOLIFE-320745. JvL, LDC, LCO, YM and RS acknowledge funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007–2013)/ERC Grant Agreement n. 617199 (‘ALERT’). JVL and LCO were further supported through Vici research programme ‘ARGO’ with project number 639.043.815, financed by NWO. DV acknowledges support from the Netherlands eScience Center (NLeSC) under grant ASDI.15.406. The Westerbork Synthesis Radio Telescope is operated by ASTRON (Netherlands Foundation for Research in Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO). This research made use of Astropy,34 a community-developed core Python package for Astronomy (Astropy Collaboration et al., 2013, Astropy Collaboration et al., 2018). This research made use of the Python Kapteyn Package (Terlouw and Vogelaar, 2015).
With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.