Advanced simulation techniques for straylight prediction of high performance mm-wave reflecting telescope

The prediction of the antenna radiation pattern can be extremely complex for antenna system composed of several reflector surfaces. Typically, at mm-wavelengths, telescopes are composed of mirrors and shields, and an accurate prediction of the whole system response is therefore complicated. This is the case for the optical system of the ESA Planck mission. Planck represents the third generation of mm-wave instruments designed for space observations of CMB anisotropies over the whole sky, and has unprecedented sensitivity and angular resolution. The Low Frequency Instrument (LFI) is one of the two instruments onboard the satellite. LFI consists of an array of 46 cooled HEMT-based receivers coupled to the 1.5 meter off-axis dual reflector telescope by an array of 23 dual profiled corrugated feed horns working at four frequencies between 30 and 100 GHz. Owing to the small level of CMB anisotropies, the control of the systematic effects is mandatory, and this is assured by means of a dedicated instrument design and by an accurate knowledge of instrument characteristics. The telescope is a source of systematic errors both in the main beam region (aberrations) and in the sidelobes region (straylight contamination) and efficient and accurate radiation pattern simulations of the telescope-feed system are needed. We report on the simulations of the full pattern applied to the Planck/LFI-Telescope system. These simulations, based on advanced GTD methods, are used to evaluate the straylight rejection of the entire optical system, by applying a new approach for evaluating the sidelobes in a rigorous way, even at very low levels.