  We present deep near-infrared HST/WFC3 observations of the young supermassive
Galactic star cluster Westerlund 1 and an adjacent control field. The depth of
the data is sufficient to derive the mass function for the cluster as a
function of radius down to 0.15 M$_\odot$ in the outer parts of the cluster. We
identify for the first time a flattening in the mass function (in logarithmic
units) at a mass range that is consistent with that of the field and nearby
embedded clusters. Through log-normal functional fits to the mass functions we
find the nominal peak mass to be comparable to that of the field and nearby
embedded star clusters. The width of a log-normal fit appears slightly narrow
compared to the width of the field IMF, closer to the values found for globular
clusters. The subsolar content within the cluster does not appear to be mass
segregated in contrast to the findings for the supersolar content. The total
mass of Westerlund 1 is estimated to be 44-57 $\times 10^3$ M$_\odot$ where the
main uncertainty is the choice of the isochrone age and the higher mass slope.
Comparing the photometric mass with the dynamically determined mass, Westerlund
1 is sufficiently massive to remain bound and could potentially evolve into a
low-mass globular cluster.
