Mueller matrix element that modulates the Stokes-Q component of the sky signal, shown as a function of the HWP rotation angle θ. We assume an ideal HWP. In blue, we show the ideal case of vanishing polarization angle and vanishing uncertainty on the HWP rotation angle, α, δθ = 0 [rad]. In solid orange, we show the case of ζ₁ = ζ₂ = 0.2 and α, δθ = 0 [rad]. In green, the ideal HWP with δθ = 0, α = −0.2 [rad]. In dashed orange, we have the ideal HWP with δθ = 0.1, α = 0 [rad]. We note that the orange dashed and the green lines overlap perfectly. They also partly overlap with the solid orange line, albeit the latter shows a slightly different amplitude. The purple line shows all the effects combined together. Finally, the red line corresponds to α = 3 × ( − 0.2) [rad]. We can see that the shift of the red curve is equivalent to three times the effects described with the yellow, green, and dashed curves.