  One of the key science goals for a diffraction limited imager on an Extremely
Large Telescope (ELT) is the resolution of individual stars down to faint
limits in distant galaxies. The aim of this study is to test the proposed
capabilities of a multi-conjugate adaptive optics (MCAO) assisted imager
working at the diffraction limit, in IJHK$_s$ filters, on a 42m diameter ELT to
carry out accurate stellar photometry in crowded images in an Elliptical-like
galaxy at the distance of the Virgo cluster. As the basis for realistic
simulations 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 the
telescope and instrument performance expectations to create realistic images.
We then tested the ability of the currently available photometric packages
STARFINDER and DAOPHOT to handle the simulated images. Our results show that
deep 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 the
distance 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$_K > -6$), and
significantly deeper (photometric error, $\pm$0.25 at I$\ge$29.3; H$\ge$26.6
and K$_s\ge$26.2) for $\mu_V \sim$ 21 mag/arcsec$^2$ (down to M$_I \ge -2$ and
M$_H \sim$ M$_K \ge -4.5$). The photometric errors, and thus also the depth of
the photometry should be improved with photometry packages specifically
designed to adapt to an ELT MCAO Point Spread Function. We also make a simple
comparison between these simulations and what can be expected from a Single
Conjugate Adaptive Optics feed to MICADO and also the James Webb Space
Telescope.
