One doublet, two planes: top = FODO (ray goes up: focus → defocus), bottom = DOFO (ray goes down: defocus → focus). Trajectories are reflected at the axis so FODO stays above and DOFO below (no crossing). θmax is chosen automatically so the outermost ray fills the QP bore without clipping. Drag Q1/Q2; Focus E₀ images both planes to \(z_{\mathrm{img}}\).