  Numerical hydro-N-body simulations are very important tools for making
theoretical predictions for the formation of galaxy clusters. They show that
the atmospheres of clusters of galaxies have quite complex angular and thermal
structures. The full understanding of the physical processes behind these
features can be only achieved by direct comparison of observations to
hydro-N-body simulations. Although simple in principle, these comparisons are
not always trivial. In fact, real data are convolved with the instrument
response which may substantially influence the apparent properties of the
studied features. To overcome this problem we build the software package X-MAS
devoted to simulate X-ray observations of galaxy clusters obtained from
hydro-N-body simulations. In this paper we present how this software package
works and discuss its application to the simulation of Chandra ACIS-S3
observations. We compare some of the main physical properties of the input data
to the ones derived from simulated observations after performing a standard
imaging and spectral analysis. We show that, if the thermal structure of the
cluster along a particular line of sight is quite complex, the projected
spectroscopic temperature obtained from the observation is significantly lower
than the emission-weighed value inferred directly from hydrodynamical
simulation. This implies that much attention must be paid in the theoretical
interpretation of observational temperatures.
