| Shaun M. Hughes, Barry F. Madore (2007) | |||||||||||||
Abstract | |||||||||||||
| The Distance Scale (HO) Key Project for Space Telescope ( aims employ the period-luminosity 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 the Wide Field Camera exposures of selected galaxies (and from 2 after the HST servicing mission), and (2) to calibrate this photometry to obtain reliable distances to these galaxies from the P-L relation. We have used the ALLFRAME program (based on to determine 28 instrumental magnitudes --- 22 of F555W V) and 6 of F785LP -- of all stars brighter than V 25.5 in each of two 2.56x2.56 arcmin WFC fields of M81. The reductions use a varying point-spread function to account for the 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 that we have now identified in M81. The photometric calibration onto the Johnson V and F785... | |||||||||||||
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