The PLANCK mission will offer a powerful possibility to observe the cosmic microwave background (CMB) fluctuations with great sensitivity and high resolution. The generation of high resolution full sky maps, consistent with the COBE- DMR normalisation of the CMB anisotropies and based on reasonable angular and frequency extrapolations of available maps with angular resolution of deg, is a basic step for simulating the mission performance. The goal of high resolution observations of CMB anisotropy requires the use of large aperture telescopes. A large frequency range coverage is necessary to efficiently separate the CMB fluctuations from different astrophysical components, galactic emission, extragalactic foregrounds, SZ effects, which are themselves very informative. Not all the feedhorns can be located very close to the centre of the focal plane, where optical distortions are minimum. Optical distortions make the main beam response somewhat different from a pure Gaussian, centrally symmetric shape. In addition sidelobe effects may became more prominent and affect the measurements, an effect that will be studied in a forthcoming work. Here we describe a first estimate of the effect of main beam distortions on anisotropy measurements.
In section 2 we outlined the basic framework for generating full sky maps including CMB anisotropy and Galaxy emission. In section 3 we present the method adopted to convolve the simulated map with a beam of general shape, taking into account the mission observational strategy (we refer here to the standard PLANCK scanning strategy, but the method can be applied to other observational schemes). The main results of our beam tests are presented in section 4. Finally in section 5 we discuss the main implications of this analysis.