  We simulate Planck observations by adopting a detailed model of the microwave
sky including monopole, dipole, anisotropies of the cosmic microwave background
(CMB) and galactic and extragalactic foregrounds. We estimate the impact of
main beam optical aberrations on CMB anisotropy measurements in presence of
extragalactic source fluctuations and we discuss the main implications for the
Planck telescope design. By analysing the dipole pattern, we quantify the
Planck performance in the determination of CMB spectral distortion parameters
in presence of foreground contaminations.
