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The Extremely Large Telescopes currently under construction have a collectingarea that is an order of magnitude larger than the present largest opticaltelescopes. For seeing-limited observations the performance will scale as thecollecting area but, with the successful use of adaptive optics, for manyapplications it will scale as $D^4$ (where $D$ is the diameter of the primarymirror). Central to the success of the ELTs, therefore, is the successful useof multi-conjugate adaptive optics (MCAO) that applies a high degree correctionover a field of view larger than the few arcseconds that limits classicaladaptive optics systems. In this letter, we report on the analysis of crowdedfield images taken on the central region of the Galactic globular cluster NGC1851 in $K_s$ band using GeMS at the Gemini South telescope, the onlyscience-grade MCAO system in operation. We use this cluster as a benchmark toverify the ability to achieve precise near-infrared photometry by presentingthe deepest $K_s$ photometry in crowded fields ever obtained from the ground.We construct a colour-magnitude diagram in combination with the F606W band fromHST/ACS. As well as detecting the "knee" in the lower main sequence at$K_s\simeq20.5$, we also detect the double subgiant branch of NGC 1851, thatdemonstrates the high photometric accuracy of GeMS in crowded fields.

Towards Precision Photometry with Extremely Large Telescopes: the Double Subgiant Branch of NGC 1851