Expected uncertainty of the Lyman-α flux of M dwarfs as a function of their distance in a single exposure of HST/STIS using the G140M grating and for two different densities of H I in the ISM. The flux was computed in the Doppler velocity range −50 km s⁻¹ to 10 km s⁻¹ and we assumed the radial velocity of the ISM to be null in the rest frame of the telescope. The dashed line represents the excess transit absorption level in a 0.1 R_⊙ M dwarf.