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Observations with C-BASS

The University of Manchester, Manchester, UK
Publication Date
  • C-Bass
  • Polarization Calibration


The C-Band All Sky Survey (C-BASS)\nomenclature{C-BASS}{C-Band All-Sky Survey} is a project to image the whole sky at a frequency of 5~GHz, measuring both the brightness and the polarization of the sky. The principal aim of C-BASS is to allow the subtraction of polarized Galactic synchrotron emission from the data produced by Cosmic Microwave Background (CMB)\nomenclature{CMB}{Cosmic Microwave Background} polarization experiments such as \emph{Planck}, Wilkinson Microwave Anisotropy Probe (\emph{WMAP})\nomenclature{WMAP}{Wilkinson Microwave Anisotropy Probe} and future $B$-mode polarization experiments. Hence, it is important that the polarization be measured with utmost accuracy.It is difficult to measure the Stokes parameters correctly because the degree of polarization in astronomical sources is typically small and the radioastronomical receiving system modifies the incoming polarization with instrumental effects. Thus, careful calibration of the polarimeter was a critical step in the development of C-BASS pipeline. In this thesis, calibration of the C-BASS polarimeter was achieved by both developing the polarization calibration module in the pipeline and applying it to the observations of the Orion (an unpolarized source) and Tau A (a polarized source) over a range of parallactic angles over a period of some hours with C-BASS telescope.The data were reduced in the C-BASS data reduction pipeline. A baseline fit was performed on the reduced data and binned maps of 3 arcmin pixels for Stokes $I$, $Q$, $U$ and $V$ were made. Gaussian fits were made on Orion and Tau A maps to determine the flux of the Stokes parameters. Stokes $Q$ and $U$ for Tau A were also corrected for parallactic angle rotation. They showed the presence of leakage.The binned maps of complete raster schedules were made in beam coordinates in order to determine the leakage parameters. The polarization beams in the maps were found to be not circular, and an offset in Stokes $Q$ and $U$ was also observed. This showed that at least some of the leakage is due to the optics and not the receiver. The observed polarization of Orion was found to be $\thickapprox$ 2$\%$. Since Orion is expected to be intrinsically unpolarized, this was attributed to leakage. A leakage correction was applied to Tau A maps using the leakage parameters derived from Orion and the maps were also corrected for parallactic angle rotation. The effectiveness of the correction process was then verified on Tau A by using Gaussian fitting and calculating polarized intensity, degree of polarization, and the position angle.The raster schedule after calibration, for which this thesis presents a detailed analysis, shows a degree of polarization of $\Pi=3.51\pm0.13\%$ and a position angle of $\chi=138.44^{\circ}\pm0.39^{\circ}$. At 5 GHz, a position angle of $141^{\circ}\pm3^{\circ}$ and a degree of polarization between 3.5\% and 4.5\% is expected for Tau A. Our results are consistent with this.

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