Stokes parameters of the polarised standards as measured with the MOPTOP and the MOP-L filter. The solid circles correspond to the catalogue polarisation degree (Turnshek et al. 1990; Schmidt et al. 1992; Weitenbeck 1999), and the dotted lines mark the (q, u) = (0, 0). The three figures show (a) data corrected only for the instrumental offset (q_inst, u_inst), (b) data corrected for ellipticity (a, b, ψ), and (c) data additionally corrected via depolarisation (D_ϵ), as well as the angle rotation induced by the Liverpool Telescope (θ_tel) and MOPTOP (θ_inst).