  We present new measurements of the galaxy luminosity function (LF) and its
dependence on local galaxy density, color, morphology, and clustocentric radius
for the massive z=0.83 cluster MS1054-0321. Our analyses are based on imaging
performed with the ACS onboard the HST in the F606W, F775W and F850LP passbands
and extensive spectroscopic data obtained with the Keck LRIS. Our main results
are based on a spectroscopically selected sample of 143 cluster members with
morphological classifications derived from the ACS observations. Our three
primary findings are (1) the faint-end slope of the LF is steepest in the
bluest filter, (2) the LF in the inner part of the cluster (or highest density
regions) has a flatter faint-end slope, and (3) the fraction of early-type
galaxies is higher at the bright end of the LF, and gradually decreases toward
fainter magnitudes. These characteristics are consistent with those in local
galaxy clusters, indicating that, at least in massive clusters, the common
characteristics of cluster LFs are established at z=0.83. We also find a 2sigma
deficit of intrinsically faint, red galaxies (i-z>0.5, Mi>-19) in this cluster.
This trend may suggest that faint, red galaxies (which are common in z<0.1 rich
clusters) have not yet been created in this cluster at z=0.83. The
giant-to-dwarf ratio in MS1054-0321 starts to increase inwards of the virial
radius or when Sigma>30 Mpc^-2, coinciding with the environment where the
galaxy star formation rate and the morphology-density relation start to appear.
(abridged)
