Fractional photoionisation curves of Na I gas at annotated radii in cm. The vertical dashed lines indicate the phases of the obtained spectra. The red curve at 1 × 10¹⁸ cm, defines the inner exclusion radius at which a gas cloud would have been ionised before the first spectrum was obtained. The blue curve defines the outer exclusion radius at 2 × 10¹⁹ cm, where photoionisation leads to negligible change.