Planck/LFI: Field Distribution on Primary Mirror for the Baseline Telescope

Abstract

In order to optimise the angular resolution (10 arcmin at 100GHz as a goal, 12 arcmin as a requirement) and at the same time to minimise all the systematics coming from the side lobes of the radiation pattern, it is necessary to carry out a detailed study of the optical interfaces. The side lobes level is driven by the so called Edge Taper, which is defined as the ratio between the power incident at the centre of the reflector and the power incident at the reflector edges. A strong taper (or an high value of the Edge Taper) means a strong under–illumination of the reflector which implies a degradation of the angular resolution. On the contrary, increasing the illumination of the telescope (low values of the Edge Taper) improves the angular resolution and degrades the stray light rejection of the telescope. The Edge Taper can be modified by changing the feed horn design, and thus how the horn illuminates the telescope can be controlled. A preliminary study of the primary mirror Edge Taper of the Planck telescope baseline configuration (CASE1 design) is reported here. Because of the symmetry of the Planck Focal Plane Unit, this study has been limited to 15 LFI feed horns. The simulations have been carried out in the transmitting mode using GRASP8 software package.