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 M _ { V } = 15.0 , M _ { I } = 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 M _ { \odot } .
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 M _ { \odot } .
The formal slope between 0.65 and 0.09 M _ { \odot } is \alpha = 0.75 ( Salpeter = 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 ( WDs ) yields a range of possible conclusions , one of which is that we have indeed seen the termination of the WD cooling sequence in M4 .