THE HUBBLE-SPACE-TELESCOPE EXTRAGALACTIC DISTANCE SCALE KEY PROJECT .2. PHOTOMETRY OF WFC IMAGES OF M81

The Extragalactic Distance Scale (H0) Key Project for Hubble Space Tel escope (HST) aims to employ the Cepheid period-luminosity (P - L) rela tion to measure galaxy distances out as far as the Virgo Cluster. The vital steps in this program are (1) to obtain precise photometry of st ellar images from the Wide Field Camera (WFC) exposures of selected ga laxies, and (2) to calibrate this photometry to obtain reliable distan ces to these galaxies from the Cepheid P - L relation. We have used th e DAOPHOT II and ALLFRAME programs to determine 28 instrumental magnit udes-22 of F555W (approximately V) and six of F785LP (approximately I) -of all stars brighter than V approximately 25 in each of two 2.56' x 2.56' WFC fields of M81. The reductions use a varying point-spread fun ction to account for the field effects in the WFC optics and yield ins trumental magnitudes with single epoch precision ranging from 0.09 to 0.24 mag, at V approximately 21.8 to 23.8-the magnitude range of the 3 0 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 determi ned from independent ground-based CCD observing at the CFHT 3.6 m (con firmed by the KPNO 4.0 m) and from the Palomar 5.0 m (using the wide-f ield COSMIC camera) and 1.5 m telescopes. Secondary standards, taken f rom the COSMIC and CFHT frames, were established in each of the WFC fi elds in V and I, allowing a direct transformation from ALLFRAME magnit udes to calibrated V and I magnitudes, giving mean V approximately 23 magnitudes accurate to approximately +/- 0.1 mag. The stellar populati ons 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.