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Published online by Cambridge University Press: 07 August 2017
The Hubble Space Telescope key project to determine the extragalactic distance scale aims to measure H0 to 10%. To achieve this goal will require Cepheid distance measurements for some 20 galaxies within a redshift of approximately 103 km/s. These galaxies in turn will calibrate five secondary distance indicators which will extend the volume over which the expansion rate has been measured to some 106 Mpc3.