Revisiting the spread spectrum effect in radio interferometric imaging: a sparse variant of the w-projection algorithm

ABSTRACT Next-generation radio interferometric telescopes will exhibit non-coplanar baseline configu-rations and wide field-of-views, inducing a w-modulation of the sky image, which induces thespread spectrum e ect. We revisit the impact of this e ect on imaging quality and study a newalgorithmic strategy to deal with the associated operator. In previous studies it has been shownthat image recovery in the framework of compressed sensing is improved due to this e ect,where the w-modulation can increase the incoherence between measurement and sparsify-ing signal representations. For the purpose of computational eciency, idealised experimentswith a constant baseline component w were performed. We extend this analysis to the morerealistic setting where the w-component varies for each visibility measurement. Firstly, incor-porating varying w-components into imaging algorithms is a computational demanding task.We propose a variant of the w-projection algorithm, which is based on an adaptive sparsifi-cation procedure, and incorporate it in compressed sensing imaging methods. Secondly, weshow that for varying w-components, reconstruction quality is significantly improved com-pared to no w-modulation, reaching levels comparable to a constant, maximal w-component.This finding confirms that one may seek to optimise future telescope configurations to pro-mote large w-components, thus enhancing the fidelity of image reconstruction.Key words: techniques: interferometric – methods: numerical.