This project aims to probe the main sequence mass function and substellar limit in old metal-poor Galactic globular clusters. The mass limit of stars at the bottom of the hydrogen-burning main sequence is predicted to be metallicity dependent: this prediction will be tested. Deep JHK images reaching the end of the population-II main sequence are required for several globular clusters. Measurements of proper motions, i.e., transverse motions across the sky, will be used to identify cluster members and exclude field stars.
Images of the old Galactic globular cluster NGC 6121 in the visible (left panels) and near-infrared (right panels) take from the ground (top panels) and with the Hubble Space Telescope (bottom panels). The fields of the GSAOI detectors are shown as four green squares in each frame.