  The deepest optical image ever in a globular star cluster, a Hubble Space
Telescope (HST) 123 orbit exposure in a single field of Messier 4, was obtained
in 2 filters (F606W, F814W) over a 10 week period in early 2001. A somewhat
shallower image obtained in 1995 allowed us to select out cluster and field
objects via their proper motion displacement resulting in remarkably clean
color-magnitude diagrams that reach to V = 30, I = 28. The cluster main
sequence luminosity function contains very few stars fainter than MV = 15.0, MI
= 11.8 which, in both filters, is more than 2 magnitudes brighter than our
limit. This is about the faintest luminosity seen among field Population II
subdwarfs of the same metallicity. However, there remains a sprinkling of
potential cluster stars to lower luminosity all the way down to our limiting
magnitudes. These latter objects are significantly redder than any known
metal-poor field subdwarf. Comparison with the current generation of
theoretical stellar models implies that the masses of the lowest luminosity
cluster stars observed are near 0.09 Msun. We derive the mass function of the
cluster in our field and find that it is very slowly rising towards the lowest
masses with no convincing evidence of a turnover even below 0.1 Msun. The
formal slope between 0.65 and 0.09 Msun is alpha = 0.75 (Salpeter of 2.35) with
a 99% confidence interval 0.55 - 1.05. A consistency check between these slopes
and the number of observed cluster white dwarfs yields a range of possible
conclusions, one of which is that we have indeed seen the termination of the
white dwarf cooling sequence in M4.
