  The Point Spread Function (PSF) is a key figure of merit for specifying the
angular resolution of optical systems and, as the demand for higher and higher
angular resolution increases, the problem of surface finishing must be taken
seriously even in optical telescopes. From the optical design of the
instrument, reliable ray-tracing routines allow computing and display of the
PSF based on geometrical optics. However, such an approach does not directly
account for the scattering caused by surface microroughness, which is
interferential in nature. Although the scattering effect can be separately
modeled, its inclusion in the ray-tracing routine requires assumptions that are
difficult to verify. In that context, a purely physical optics approach is more
appropriate as it remains valid regardless of the shape and size of the defects
appearing on the optical surface. Such a computation, when performed in
two-dimensional consideration, is memory and time consuming because it requires
one to process a surface map with a few micron resolution, and the situation
becomes even more complicated in case of optical systems characterized by more
than one reflection. Fortunately, the computation is significantly simplified
in far-field configuration, since the computation involves only a sequence of
Fourier Transforms. In this paper, we provide validation of the PSF simulation
with Physical Optics approach through comparison with real PSF measurement data
in the case of ASTRI-SST M1 hexagonal segments. These results represent a first
foundation stone for future development in a more advanced computation taking
into account microroughness and multiple reflection in optical systems.
