Effective transmission curves for the ESO/[O III]/0 (blue), ESO[O III]/6000 (red) and HST/F502N (magenta) filters. The velocity is centered on the rest velocity of [O III] λ5007 in the local gas in the LMC, which we assume has a systemic shift of + 273 km s⁻¹ compared to Earth (cf. Morse et al. 2006). The red wings of the curves ESO/[O III]/0 and HST/F502N are due to emission from [O III] λ4959 that enters the filter with a shift of ∼ 2870 km s⁻¹. We have assumed that this line is a factor of three weaker than [O III] λ5007. The black curve shows twice the effective transmission for HST/F673N centered on [S II] λ6731, assuming the intensity of [S II] λ6716 and [S II] λ6731 to be equal.