Photometric Recovery of Crowded Stellar Fields Observed withHST/WFPC2 and the Effects of Confusion Noise on the Extragalactic Distance Scale

We explore the limits of photometric reductions of crowded stellar fieldsobserved with the Wide Field and Planetary Camera 2 on board the Hubble SpaceTelescope. Two photometric procedures, based on the DoPHOT and DAOPHOT/ALLFRAMEprograms are tested, and the effects of crowding, complex sky background andcosmic-ray contamination are discussed using an extensive set of artificialstar simulations. As a specific application of the results presented in thispaper, we assess the magnitude of photometric biases on programs aimed atfinding Cepheids and determining distances. We find that while the photometryin individual images can be biased too bright by up to 0.2 mag in the mostcrowded fields due to confusion noise, the effects on distance measurementsbased on Cepheid variables are insignificant, less than 0.02 mag (1% indistance) even in the most problematic cases. This result, which is at oddswith claims recently surfaced in the literature, is due to the strict criteriaapplied in the selection of the variable stars, and the photometric crosschecks made possible by the availability of multiple exposures in differentfilters which characterizes Cepheid observations.