A comparison of the absolute [O iii] magnitudes of M31 PNe measured by Merrett et al. (2006) to the number of λ5007 counts recorded in our spectra. The scatter is due to imperfect astrometry and fiber positioning errors, and the effect of clouds on one of the Hydra setups. The dotted line shows the one-to-one relation. The figure illustrates that we should be able to detect PNe that are more than ~6 mag down the luminosity function. For reference, M* = −4.5, which in M31 is equivalent to a monochromatic flux of 2.9 × 10 ⁻¹⁴ erg cm ⁻² s ⁻¹.