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12. The future

Any future ground-based attempts to detect Cepheids in external galaxies must carefully consider the question of how the periods are going to be unambiguously determined. The rapid success enjoyed by the recalibration of the extragalactic distance scale using infrared detectors was in no small part due to the fact that enormous amounts of telescope time had already been invested in discovering those Cepheids and determining their periods. Unlike all previous investigations we are now confronted by a new problem in the application of Cepheids to the extragalactic distance scale. When the light curves are sparsely sampled over time intervals that are only a few cycles in duration there is a tight coupling between the accuracy of the period determinations and the photometric accuracy of the observations. Co-ordinated ground-based observations are difficult to schedule, and even more difficult (if not impossible) to guarantee. With space observations there is however the immense advantage that the exposures can be optimally scheduled to minimize aliasing effects and also provide for uniform coverage of the phased light curve. Details of the methodology employed by the Key Project will soon be given in Madore & Freedman (1998).