Separation of the astronomical linear polarization from instrumental contributions for the example of R Leo. The solid red and blue lines show least-squares fits to the fractional Stokes parameters Q_N/I and U_N/I (red and blue dots, respectively) of R Leo vs. χ₀ = ε − η, averaged across a velocity interval framing the Stokes I emission peak (0.68 to 2.04 km s⁻¹) and expressed in the Nasmyth reference frame. The dashed lines indicate the corresponding fractional instrumental contributions. For details, see text.