Planck is a European Space Agency (ESA) satellite,launched in May 2009, which will map the cosmic microwave backgroundanisotropies inintensity and polarisation with unprecedented detail and sensitivity.Itwill also provide full-sky maps of astrophysical foregrounds. Anaccurate knowledge of the telescope beam patterns is an essentialelement for a correct analysis of the acquired astrophysical data. Wepresent a detaileddescription of the optical design of the High Frequency Instrument(HFI)together with some of the optical performances measured during thecalibrationcampaigns. We report on the evolution of the knowledge of thepre-launch HFI beam patterns when coupled to ideal telescope elements,and on their significance for the HFI data analysis procedure.Key words: space vehicles: instruments -submillimeter: general - telescopes - cosmic microwave background -instrumentation: polarimeters - instrumentation: detectors
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