Adaptive Optics for Extremely Large Telescopes 4 – Conference Proceedings

Multi-conjugate adaptive optics is a central technology for the Extremely Large Telescopes (NFIRAOS on TMTand MAORY on E-ELT). GeMS on the 8-m Gemini South telescope is the rst facility-class MCAO and therst to use laser guide stars. We have observed the Galactic globular cluster NGC 1851 (and 5 other targets)and here we present the results of the prole-tting photometry in the near-infrared. This is the most precisephotometry to date of a cluster taken from the ground, conrmed by our ability to detect the double subgiantbranch, previously observed only from space. The high Strehl ratio of the images pushes the depth of the stellardetections well below the main sequence knee of the colour-magnitude diagram, making this also the deepestnear-infrared CMD yet obtained from ground. The large number of stars allows to evaluate the performanceof the instrument in terms of position-dependent PSF. We demonstrate how the analysis of the spatial andtemporal PSF variations allows us to develop eective photometric techniques for MCAO to be used for the nextgeneration of large telescopes.