Previous
abstract 
Next abstract
The H$_0$ Key Project for Hubble Space Telescope 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 reliable photometry of stellar images from Wide Field Camera (WFC) exposures of selected galaxies, from which the Cepheids can be selected, and (2) to calibrate this photometry to obtain reliable distances to these galaxies from the Cepheid $P-L$\ relation. We have used Stetson's ALLFRAME program (based on DAOPHOT) to determine 28 instrumental magnitudes --- 22 of F555W ($\sim V$) and 6 of F785LP ($\sim I$) --- of all stars brighter than $V \sim 25.5$\ in each of two 25.6$\times$25.6 arcmin 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 errors ranging from 0.09 to 0.24 mag, at $V \sim 21.8$ to 23.8 --- the magnitude range of the $\sim$30 Cepheids that we have now identified in M81. The photometric calibration onto the Johnson $V$ and Cousins $I$ systems was determined from independent ground-based CCD observing at the CFHT 3.6m (confirmed by the KPNO 4.0m) and the Palomar 5.0m and 1.5m telescopes. Secondary standards, taken from the wide-field 5.0m COSMIC frames, were established in each of the WFC fields, allowing a direct transformation from ALLFRAME magnitudes to calibrated $V$ and $I$ magnitudes, giving mean $V \sim 23$ magnitudes accurate to $\sim \pm$0.09 mag.
