A view on the Planck LFI optics

This document presents the optical performances of the Low Frequency Instrument (LFI) on-board the Planck satellite, which is making observations of the cosmic microwave background (CMB) with unprecedented accuracy. It reviews the study that has led to the flight design of the LFI feed horns, their characteristics, arrangement, and orientation in the focal plane in order to perform polarisation measurements of the CMB. The relevant beams properties are shown, and the results obtained assuming the design telescope are compared with those obtained taking into account the realistic model of the telescope.