Na I D lines observed towards PN Tc 1 central star (black) and average of two positions in the nebula 2.7 arcsec away from the central star on either side (red) (from Williams et al. 2008). The dashed blue lines indicate the interstellar Na I D components at −6.8 and +25 km s^-1. The sharp emission close to 0 km s^-1 radial velocity is earth’s airglow. The absorption spectrum is in the stellar continuum units, while the emission spectrum is in the nebular continuum units.