Daily Image

30-08-2019
PreviousNext
Click here or on the picture for a full size image.

Discovery of a new systematic effect in LOFAR LBA data

Submitter: Francesco de Gasperin
Description: During the 28th imaging busy week a group of calibration and imaging enthusiasts focused on LBA calibration and improvement of Rapthor. A major systematic in LBA data is a time- and direction-dependent delay imprinted by the ionospheric free electrons on traveling radio waves. This delay is proportional to the observed wavelength and therefore large at the ultra-low frequencies explored by the LOFAR LBA system (30-70 MHz). One long standing problem was the extraction of clean total electron content (TEC) values in the direction of the scientific target. The problem is visible on the top-right panel of the image as series of "jumps" present in the TEC solutions.

We retrieved phase solution in the direction of the target (top-left panel), fitted TEC values and looked at the residual phases (left-bottom panel) once the fitted TEC was removed. Surprisingly, the residuals are not all zeros, but they show a frequency-dependent leftover. This effect can be associated with a delay (linearly proportional to the frequency) and we believe is associated with problematic station calibration. After removing this instrumental delay and re-fitting for the ionospheric delay we had a much more realistic set of TEC solutions (bottom right panel).

The characterization of instrument-related systematic effects is a crucial step in improving the performance of the telescope in sight of LOFAR 2.0.

Busy week participants: F. de Gasperin, T.J. Dijkema, M. Mevius, A.R. Offringa, D. Rafferty, S. van der Tol, R.J. van Weeren and W.L. Williams
Copyright: cc Francesco de Gasperin
 
  Follow us on Twitter
Please feel free to submit an image using the Submit page.