The next generation of large aperture ground based telescopes will offer the opportunity to perform accurate stellar photometry in very crowded fields. This future capability will allow one to study in detail the stellar population in distant galaxie
s. In this paper we explore the effect of photometric errors on the stellar metallicity distribution derived from the color distribution of the Red Giant Branch stars in the central regions of galaxies at the distance of the Virgo cluster. We focus on the analysis of the Color-Magnitude Diagrams at different radii in a typical giant Elliptical galaxy obtained from synthetic data constructed to exemplify observations of the European Extremely Large Telescope. The simulations adopt the specifications of the first light high resolution imager MICADO and the expected performance of the Multi-Conjugate Adaptive Optics Module MAORY. We find that the foreseen photometric accuracy allows us to recover the shape of the metallicity distribution with a resolution $lesssim 0.4$ dex in the inner regions ($mu_{rm B}$ = 20.5 mag arcsec$^{-2}$) and $simeq 0.2$ dex in regions with $mu_{rm B}$ = 21.6 mag arcsec$^{-2}$, that corresponds to approximately half of the effective radius for a typical giant elliptical in Virgo. At the effective radius ($mu_{rm B} simeq 23$ mag arcsec$^{-2}$), the metallicity distribution is recovered with a resolution of $simeq 0.1$ dex. It will thus be possible to study in detail the metallicity gradient of the stellar population over (almost) the whole extension of galaxies in Virgo. We also evaluate the impact of moderate degradations of the Point Spread Function from the assumed optimal conditions and find similar results, showing that this science case is robust.
Over the last few years increasing consideration has been given to the study of Laser Guide Stars (LGS) for the measurement of the disturbance introduced by the atmosphere in optical and near-infrared astronomical observations from the ground. A poss
ible method for the generation of a LGS is the excitation of the Sodium layer in the upper atmosphere at approximately 90 km of altitude. Since the Sodium layer is approximately 10 km thick, the artificial reference source looks elongated, especially when observed from the edge of a large aperture. The spot elongation strongly limits the performance of the most common wavefront sensors. The centroiding accuracy in a Shack-Hartmann wavefront sensor, for instance, decreases proportionally to the elongation (in a photon noise dominated regime). To compensate for this effect a straightforward solution is to increase the laser power, i.e. to increase the number of detected photons per subaperture. The scope of the work presented in this paper is twofold: an analysis of the performance of the Weighted Center of Gravity algorithm for centroiding with elongated spots and the determination of the required number of photons to achieve a certain average wavefront error over the telescope aperture.
