Daily Image

10-12-2018
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Polarized imaging with the Image Domain Gridder

Submitter: André Offringa
Description: Recently, a team of ASTRON scientists has been working hard to get the so called "Image Domain Gridder" (IDG) working. IDG is an imaging technique invented by ASTRON's Bas van der Tol and Bram Veenboer, that allows extremely fast imaging of very wide field of views, and it is therefore well suited for imaging LOFAR data. For the SKA, fast imaging will be even more crucial. One of the major features of this imager is that it can undo some of the effects of the instrument while imaging. In particular, it can correct the (polarized) response of the beam as well as correct for the ionosphere. It can do this with an accuracy that was so far not possible.

These images show the 3C 196 field in Stokes Q polarization as observed with LOFAR. From previous LOFAR observations it was discovered that the 3C 196 field, which is one of the fields targetted by the LOFAR Epoch of Reionization project, has intriguing linear polarized structures in the EoR field that as of yet are not well understood (see Jelić et al. 2015 and daily image 20151110). For testing IDG, a single subband of a 3C 196 observation was imaged and corrected for LOFAR's beam response.

Both the left and right image were cleaned as deep as possible using WSClean's auto-masking cleaning strategy, with one important difference: IDG was used to produce the right image to correct for the beam, leading to a dramatic improvement of image quality. The left image shows strong artefacts from sources that are not polarized, but end up in the image because the instrumental leakage of LOFAR is not corrected for. In the IDG image, sources that are not polarized are 'cleaned' from the image, and the real polarized structure is much more clearly visible. This process is fully automated.

IDG was developed by Bas van der Tol, Bram Veenboer, Tammo Jan Dijkema and André Offringa, with the help of several scientists that have tested the imager, and was recently published in A&A (Van der Tol, Veenboer & Offringa 2018) and IEEE (Veenboer, Petschow & Romein 2017).
Copyright: CC BY 4.0
 
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