  We present the analysis of the luminosity function of a large sample of
galaxy clusters from the Northern Sky Optical Cluster Survey, using latest data
from the Sloan Digital Sky Survey. Our global luminosity function (down to
M_r<= -16) does not show the presence of an upturn at faint magnitudes, while
we do observe a strong dependence of its shape on both richness and
cluster-centric radius, with a brightening of M^* and an increase of the dwarf
to giant ratio with richness, indicating that more massive systems are more
efficient in creating/retaining a population of dwarf satellites. This is
observed both within physical (0.5 R_200) and fixed (0.5 Mpc) apertures,
suggesting that the trend is either due to a global effect, operating at all
scales, or to a local one but operating on even smaller scales. We further
observe a decrease of the relative number of dwarf galaxies towards the cluster
center; this is most probably due to tidal collisions or collisional disruption
of the dwarfs since merging processes are inhibited by the high velocity
dispersions in cluster cores and, furthermore, we do not observe a strong
dependence of the bright end on the environment. We find indication that the
dwarf to giant ratio decreases with increasing redshift, within 0.07<z<0.2. We
also measure a trend for stronger suppression of faint galaxies (below M^*+2)
with increasing redshift in poor systems, with respect to more massive ones,
indicating that the evolutionary stage of less massive galaxies depends more
critically on the environment. Finally we point out that the luminosity
function is far from universal; hence the uncertainties introduced by the
different methods used to build a composite function may partially explain the
variety of faint-end slopes reported in the literature as well as, in some
cases, the presence of a faint-end upturn.
