Optical depth to the F = 2−1 hyperfine transition of N₂H⁺ as a function of the ratio of the line intensities of the F = 2−1 to the F = 0−1 shown in black. The line width used in the SLIM simulations is Δv = 2 km s⁻¹ to avoid blending between hyperfine transitions. In red and blue we represent the propagated uncertainties considering that the F = 2−1 hyperfine transitions of N₂H⁺ are detected with S/Ns of 50 and 20, respectively. See Sect. 4.3 for details.