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One of the key science goals for a diffraction limited imager on an ExtremelyLarge Telescope (ELT) is the resolution of individual stars down to faintlimits in distant galaxies. The aim of this study is to test the proposedcapabilities of a multi-conjugate adaptive optics (MCAO) assisted imagerworking at the diffraction limit, in IJHK$_s$ filters, on a 42m diameter ELT tocarry out accurate stellar photometry in crowded images in an Elliptical-likegalaxy at the distance of the Virgo cluster. As the basis for realisticsimulations we have used the phase A studies of the European-ELT project,including the MICADO imager (Davies & Genzel 2010) and the MAORY MCAO module(Diolaiti 2010). We convolved a complex resolved stellar population with thetelescope and instrument performance expectations to create realistic images.We then tested the ability of the currently available photometric packagesSTARFINDER and DAOPHOT to handle the simulated images. Our results show thatdeep Colour-Magnitude Diagrams (photometric error, $\pm$0.25 at I$\ge$27.2;H$\ge$25. and K$_s\ge$24.6) of old stellar populations in galaxies, at thedistance of Virgo, are feasible at a maximum surface brightness, $\mu_V \sim$17 mag/arcsec$^2$ (down to M$_I > -4$ and M$_H \sim$ M$_K > -6$), andsignificantly deeper (photometric error, $\pm$0.25 at I$\ge$29.3; H$\ge$26.6and K$_s\ge$26.2) for $\mu_V \sim$ 21 mag/arcsec$^2$ (down to M$_I \ge -2$ andM$_H \sim$ M$_K \ge -4.5$). The photometric errors, and thus also the depth ofthe photometry should be improved with photometry packages specificallydesigned to adapt to an ELT MCAO Point Spread Function. We also make a simplecomparison between these simulations and what can be expected from a SingleConjugate Adaptive Optics feed to MICADO and also the James Webb SpaceTelescope.

An E-ELT Case Study: Colour-Magnitude Diagrams of an Old Galaxy in the Virgo Cluster