Difference between the demodulated and calibrated Stokes parameters and the predicted Rayleigh sky polarization spectra for April 21st 2012. Each panel shows observations calibrated at a different telescope azimuth. There are 4 to 5 independent observations at each telescope pointing. Different colors represent residual Stokes parameters for individual observations computed as data minus model. The residuals are always below 0.1 across the wavelength range. Note – only a single calibration point per spectral order has been applied and the data is shown binned to 50 spectral samples per order. Note that these residual Stokes parameters give rise to the few degree rotational error between a measured Stokes vector and a model Stokes vector projected on the Poincar’e sphere as shown above in Fig. 12.