2015ApJ...812...25M

We present deep near-infrared J, photometry of the old, metal-poor Galactic globular cluster M15 obtained with images collected with the LUCI1 and PISCES cameras available at the Large Binocular Telescope (LBT). We show how the use of First Light Adaptive Optics (FLAO) system coupled with the PISCES camera allows us to improve the limiting magnitude by ∼2 mag in. By analyzing archival Hubble Space Telescope data, we demonstrate that the quality of the LBT/PISCES color-magnitude diagram is fully comparable with analogous space-based data. The smaller field of view is balanced by the shorter exposure time required to reach a similar photometric limit. We investigated the absolute age of M15 by means of two methods: (i) by determining the age from the position of the main-sequence turnoff (MSTO), and (ii) by the magnitude difference between the MSTO and the well-defined knee detected along the faint portion of the MS. We derive consistent values of the absolute age of M15, that is, 12.9±2.6 Gyr and 13.3±1.1 Gyr, respectively.