Keywords: mira

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## 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.

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