Polarization measurements

The VESPA backend at the 30m telescope can simultaneously measure the four Stokes parameters, by computing the cross-correlation of an orthogonally polarized receiver pair. However, the phase difference between the signals from both receivers has to be calibrated, in order to attribute the real and imaginary part of the complex cross correlation to the Stokes U and V parameters. This phase difference is measured by a dedicated observation (a calibration with an additional subscan towards the cold load, equipped with a polarizer). If VESPA is in its polarimetry mode, this calibration is the default. MIRA then calculates the amplitude and phase (step 1) and applies the corrections to the astronomical observations (step 2).  

SCAN calScan	Reads a calibration including a polarizer measurement
	in scan calScan.
	The spectra are (for each backend part) in order H, V,
	R, I (for horizontal, vertical, real part, imaginary part).
CAL all [/GAINS /TCAL /PHASE]	Determines, for all backend parts, the relative phase of
	the orthogonally polarized receiver pair, and the
	amplitude loss of the cross correlation due to phase
	noise. The options /GAINS /TCAL /PHASE do not
	need to be specified, they are the default for
	polarisation calibrations.
VIEW iPart /XPOL	Plots the phase and amplitude of the complex cross
	correlation of the polarizer measurement (Fig. 7) for
	an assessment the validity of the calibration.
	iPart is the number of the VESPA part to be plotted.
SCAN obsScan	Reads the astronomical observation.
CAL all [/GAINS /TCAL /OFF /PHASE]	Calibrates this observation, and applies the phase
	calibration previously determined.

 


Then the calibrated Stokes spectra can be written out to a CLASS file. The data can then be further reduced using CLASS enhanced by the XPOL macros.

 

Figure 7: Example of the phase (top) and amplitude (bottom) of a VESPA phase calibration measurement towards a polarizer.