Probabilistic direction-dependent ionospheric calibration for LOFAR-HBA

Abstract

Direction dependent calibration and imaging is a vital part of producing deep, high fidelity, high-dynamic range radio images with a wide-field low-frequency array like LOFAR. Currently, state-of-the-art facet-based direction dependent calibration algorithms rely on the assumption that the isoplanatic-patch size is much larger than the separation between bright in-field calibrators. This assumption is often violated due to the dynamic nature of the ionosphere, and as a result direction dependent errors affect image quality between calibrators. In this paper we propose a probabilistic physics-informed model for inferring ionospheric phase screens, providing a calibration for all sources in the field of view. We apply our method to a randomly selected observation from the LOFAR Two-Metre Sky Survey archive, and show that almost all direction dependent effects between bright calibrators are corrected and that the root-mean-squared residuals around bright sources is reduced by 32\% on average.

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• The LOFAR Two-metre Sky Survey -- V. Second data release
• A Unified Calibration Framework for 21 cm Cosmology
• Bayesian radio interferometric imaging with direction-dependent calibration
• Investigating ionospheric calibration for LOFAR 2.0 with simulated observations
• Cygnus A jointly calibrated and imaged via non-convex optimisation from VLA data
• Exploring the cosmic dawn and epoch of reionization with 21cm line
• Ionospheric contributions to the excess power in high-redshift 21-cm power-spectrum observations with LOFAR
• Simulations of ionospheric refraction on radio interferometric data
• Robust direction-dependent gain-calibration of beam-modelling errors far from the target field
• Metrewave Galactic Plane with the uGMRT (MeGaPluG) Survey: Lessons from the Pilot Study