Beams LFI

Main Beams and Focal Plane calibration

In its flight configuration, LFI is coupled to the telescope by eleven dual-profiled, corrugated, conical horns (Villa et al. 2010): six feed horns at 70 GHz (FH18 – FH23), three feed horns at 44 GHz (FH24 – FH26), and two feed horns at 30 GHz (FH27 and FH28). Figure 2 shows the arrangement of the horns inside the LFI main frame. It should be noted that the feed position in the focal surface is axisymmetric (for instance, FH27 is sym- metric to FH28 at 30 GHz), a natural design choice based on the symmetry of the telescope and satellite. As a consequence, only six different feed elements have been considered in the optimi- sation analysis: one feed at 30 GHz, two at 44 GHz, and three at 70 GHz (Villa et al. 2010). The center of the focal surface is occupied by the HFI horns. This optical layout, with one in- strument (LFI) around the other (HFI), required that aberration effects in the LFI beams be accurately controlled in the tele- scope and instrument design optimization phases. Corrugated horns were selected as the most suitable solution in terms of cross polarization levels, sidelobes levels, return and insertion coordinate system, placed in the center of the FPU and with the ZRDP axis aligned along the chief ray of the telescope.

A CAD model of the Planck focal plane, which is located di- rectly below the telescope primary mirror. It comprises the HFI bolo- metric detector array (small feed horns on golden circular base) and the LFI radio receiver array (larger feed horns around the HFI). The box holding the feedhorns appears to be transparent in this view, to also show the elements inside and behind it. ESA/Thales.