  The study of Cosmic Microwave Background (CMB) anisotropies represents one of
the most powerful Cosmological tools. After the great success of the two NASA's
satellite missions COBE and WMAP, Planck represents the third generation of
mm-wave instruments designed for space observations of CMB anisotropies within
the new Cosmic Vision 2020 ESA Science Programme. The Planck survey will cover
the whole sky with unprecedented sensitivity, angular resolution and frequency
coverage, using two instruments that share the focal region of a 1.5 m off-axis
dual reflector telescope: the Low Frequency Instrument (LFI) and the High
Frequency Instrument (HFI). Within the LFI optical interfaces optimisation
activity, two concurrent demands have to be satisfied: the best angular
resolution (which impacts the ability to reconstruct the anisotropy power
spectrum of the CMB anisotropies at high multipoles) and the lowest level of
straylight contamination (that may be one of the most critical sources of
systematic effects). We present the results of the optical simulations aimed to
establish the trade-off between angular resolution and straylight rejection,
carried out for the 100 GHz channel of Planck Low Frequency Instrument. Antenna
pattern of different models of dual profiled corrugated conical feed horns have
been simulated using advanced simulation techniques, considering the whole
spacecraft geometry in order to obtain truthful sidelobe predictions. Optical
computation accuracy necessary to provide strong straylight evaluation in
reasonable computational time is shown and the inadequacy of a Gaussian feed
model in realistic far pattern predictions is demonstrated. This paper is based
on LFI activities.
