Adaptive Optics pushes the resolution of ground-based optical telescopes towards their diffraction limit by correcting the aberrations caused by atmospheric turbulence. A specific flavour of AO is Multi-Conjugate Adaptive Optics (MCAO), which expands the corrected field of view by using multiple guide stars and multiple deformable mirrors.

GeMS is the first facility-class MCAO and the first to use laser guide stars. During the Science Verification phase, with its GSAOI camera we have taken images of the Galactic globular cluster NGC 1851. This cluster is known through HST photometry to house a double stellar population identified by the double subgiant branch in its colour-magnitude diagram. The possibility provided by GeMS to resolve the stars of a crowded field allowed to recover this double sequence for the first time from the ground thanks to the precise photometry. By increasing the Strehl ratio, GeMS also pushes the depth of the photometry by reaching the faint main sequence, a region that can be used for a novel approach to the determination of a globular cluster's age.

I will also discuss some of the tricks used to obtain precise and deep photometry with GSAOI and how they are adapted to the peculiarity of MCAO images. An understanding of these techniques not only can produce better scientific results with present instrumentation but can also help to understand and be prepared for the MCAO systems of the next generation of Extremely Large telescopes.