Redshift z over transition strength fλ₀(a) and sensitivity coefficient Q(d) for an asymmetric line with underlying velocity field with peak velocity v_p = 10   km   s^-1, a symmetric line with artificial α variation of Δα/α = 0.5 × 10^-5(b, e), and an asymmetric line with v_p = 10   km   s^-1 and Δα/α = 0.5 × 10^-5(c, f).