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No distance indicator is perfect. Confidence in the results, though, is built by extensive testing against other distance indicators, and through the development of the astrophysical theory supporting the indicator's use.

The PNLF technique has been well tested against Cepheids, SBF, and against itself in clusters. Constancy of the bright end of the PNLF has been easy to reproduce theoretically if the ages of the stars producing PN are within a plausibly wide range. Thus, PNLF distances appear reliable at this time. The main points of this paper are:

  1. PNLF distances to spirals are consistent with Cepheid distances.
  2. There are now 7 PNLF calibrators; their dispersion is 8%.
  3. Good agreement with Cepheid and SBF distances must mean that systematic errors (due to age, metallicity, extinction, PN sample sizes, PNLF methodology) are under control to the limits of the deviations of these methods (typically, 8%).
  4. The M87 deep survey demonstrates that the bright end of the PNLF is not a power law as some have maintained and that PNLF distances are not sensitive to the number of PN in the sample.
  5. Both the Virgo and Fornax distances agree with the Cepheid-based distances, but the Fornax SN Ia distance is ~ 25% larger. The Virgo SN Ia distance is not discrepant, within the errors.

In terms of the future application of the PNLF, it is somewhat costly in telescope time (exposure time is proportional to distance4). So, while the method yields results as reliable as the Cepheids or SBF, its application may be reserved for galaxies that present problems for other methods. For example, it can be used to measure distances to SN Ia hosts such as Sa/S0 galaxies where Cepheids won't be found easily and where galaxy structure compromises the SBF.

This paper derives from other papers in press. The discussions on spirals was based on information provided by John Feldmeier; preliminary results on the deep M87 survey were provided by Robin Ciardullo. I am grateful to John Graham who gave me his HST Cepheid distance to NGC 3351 prior to publication.

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