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The H$_0$ Key Project for Hubble Space Telescope aims to employ the Cepheid period-luminosity relation to measure galaxy distances out as far as the Virgo cluster, and to calibrate secondary distance indicators to reach to the Coma cluster and beyond. These secondary distance indicators include the Tully-Fisher relation, the surface brightness fluctuation color relation, the planetary nebula luminosity function, the SNIa standard candle, and the expanding photospheres method for SNIIe. Observations of M81 (Freedman $et~al.$, this meeting) and IC4182 by Sandage and coworkers have shown that HST is capable of locating Cepheids in significant numbers and measuring periods from light curves two magnitudes fainter than has been possible from the ground, yielding distances as well determined as those in the Local Group. After the HST servicing mission in December Cepheids as distant as the Virgo and Fornax clusters should be similarly accessible to WFPC2. In the Key Project Cepheid distances to twenty galaxies are designed to calibrate the five secondary distance indicators specified above so that systematic differences between their zeropoints can be measured to 0.1 mag and eliminated. The goal of the project is determination of the Hubble Constant to 10\% accuracy.
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