  We used HST to obtain surface brightness fluctuation (SBF) observations of
four nearby brightest cluster galaxies (BCG) to calibrate the BCG Hubble
diagram of Lauer & Postman (1992). This BCG Hubble diagram contains 114
galaxies covering the full celestial sphere and is volume limited to 15,000
km/s, providing excellent sampling of the far field Hubble flow. The SBF zero
point is based on the Cepheid calibration of the ground I_KC method (Tonry et
al. 1997) as extended to the WFPC2 F814W filter by Ajhar et al. (1997). The BCG
globular cluster luminosity functions give distances essentially identical to
the SBF results. Using the velocities and SBF distances of the four BCG alone
gives H_0 = 82 +/- 8 km/s/Mpc in the CMB frame, valid on ~4,500 km/s scales.
Use of BCG as photometric redshift estimators allows the BCG Hubble diagram to
be calibrated independently of recession velocities, yielding a far field H_0 =
89 +/- 10 km/s/Mpc with an effective depth of ~11,000 km/s. The error in this
case is dominated by the photometric cosmic scatter in using BCG as distance
estimators. The concordance of the present results with other recent H_0
determinations, and a review of theoretical treatments on perturbations in the
near field Hubble flow, argue that going to the far field removes an important
source of uncertainty, but that there is not a large systematic error to be
corrected for to begin with. Further improvements in H_0 depend more on
understanding nearby calibrators than on improved sampling of the distant flow.

Astrophysics