On deriving distances from Cepheids using the Hubble Space Telescope

In this paper we examine the methodology of obtaining Cepheid-based distances to galaxies from image data obtained with the Wide Field Planetary Camera (WFPC2) on the Hubble Space Telescope. We identify the sources of error and how these propagate to the derived distances for the particular method utilizing the modified DoPHOT program for photometry. Typical uncertainties from each of the identified sources are used to estimate the net uncertainties. In particular, the effect of skewed error distributions from object confusion and surface brightness fluctuation are studied, and tests to estimate the extent of such skewness in real data are shown. The effect of charge-transfer efficiency problems with the WFPC2 CCDs on photometric zero points is calibrated empirically. The corrections applicable to published data are given, which suggest that dereddened distance moduli using the DoPHOT methodology published to date have been underestimated by up to 0.1 mag.