The Hubble Space Telescope Extragalactic Distance Scale Key Project. II. Photometry of WFC Images of M81

The Extragalactic Distance Scale (H_0_) Key Project for Hubble Space Telescope (HST) aims to employ the Cepheid period-luminosity (P-L) relation to measure galaxy distances out as far as the Virgo Cluster. The vital steps in this program are (1) to obtain precise photometry of stellar images from the Wide Field Camera (WFC) exposures of selected galaxies, and (2) to calibrate this photometry to obtain reliable distances to these galaxies from the Cepheid P-L relation. We have used the DAOPHOT II and ALLFRAME programs to determine 28 instrumental magnitudes-22 of F555W (~V) and six of F785LP (~I)-of all stars brighter than V ~ 25 in each of two 2.56' x 2.56' WFC fields of M81. The reductions use a varying point-spread function to account for the field effects in the WFC optics and yield instrumental magnitudes with single epoch precision ranging from 0.09 to 0.24 mag, at V~21.8 to 23.8-the magnitude range of the 30 Cepheids that we have now identified in M81. For brighter stars (V < 22), single epoch magnitudes are precise to 0.09 mag. The photometric calibration onto the Johnson V and Kron-Cousins I systems was determined from independent ground-based CCD observing at the CFHT 3.6 m (confirmed by the KPNO 4.0 m) and from the Palomar 5.0 m (using the wide- field COSMIC camera) and 1.5 m telescopes. Secondary standards, taken from the COSMIC and CFHT frames, were established in each of the WFC fields in V and I, allowing a direct transformation from ALLFRAME magnitudes to calibrated V and I magnitudes, giving mean V ~ 23 magnitudes accurate to ~+/- 0.1 mag. The stellar populations in M81 have been analyzed in terms of the luminosity functions and color magnitude diagrams (CMD) derived from these data, from which we identify numerous supergiants, and a CMD morphology similar to M33.