Applying full polarization A-Projection to very wide field of view instruments: An imager for LOFAR

TL;DR

This paper demonstrates the application of full polarization A-Projection to LOFAR, enabling correction of complex direction-dependent effects for high-dynamic-range wide-field imaging, crucial for deep extragalactic surveys.

Contribution

It introduces advanced implementations of A-Projection for LOFAR that handle non-unitary beams and complex Mueller matrices, improving calibration accuracy and imaging speed.

Findings

A-Projection significantly improves dynamic range in LOFAR images.

Algorithmic optimizations enable fast, wide-field imaging.

Simulations show effective correction of ionospheric and beam effects.

Abstract

The aimed high sensitivities and large fields of view of the new generation of interferometers impose to reach high dynamic range of order $\sim$1:$10^6$ to 1:$10^8$ in the case of the Square Kilometer Array. The main problem is the calibration and correction of the Direction Dependent Effects (DDE) that can affect the electro-magnetic field (antenna beams, ionosphere, Faraday rotation, etc.). As shown earlier the A-Projection is a fast and accurate algorithm that can potentially correct for any given DDE in the imaging step. With its very wide field of view, low operating frequency ($\sim30-250$ MHz), long baselines, and complex station-dependent beam patterns, the Low Frequency Array (LOFAR) is certainly the most complex SKA precursor. In this paper we present a few implementations of A-Projection applied to LOFAR that can deal with non-unitary station beams and non-diagonal Mueller matrices. The algorithm is designed to correct for all the DDE, including individual antenna, projection of the dipoles on the sky, beam forming and ionospheric effects. We describe a few important algorithmic optimizations related to LOFAR's architecture allowing us to build a fast imager. Based on simulated datasets we show that A-Projection can give dramatic dynamic range improvement for both phased array beams and ionospheric effects. We will use this algorithm for the construction of the deepest extragalactic surveys, comprising hundreds of days of integration.