  In the context of atmospheric shower arrays designed for $\gamma$-ray
astronomy and in the context of the ALTO project, we present: a study of the
impact of heavier nuclei in the cosmic-ray background on the estimated
$\gamma$-ray detection performance on the basis of dedicated Monte Carlo
simulations, a method to calculate the sensitivity to a point-like source, and
finally the required observation times to reach a firm detection on a list of
known point-like sources.
