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 #sandri2010,villa2010: 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 1 below 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 symmetric 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 optimisation analysis: one feed at 30 GHz, two at 44 GHz, and three at 70 GHz #villa2010. The center of the focal surface is occupied by the HFI horns. This optical layout, with one instrument (LFI) around the other (HFI), required that aberration effects in the LFI beams be accurately controlled in the telescope 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 loss. Dual-profiled corrugation shaping was chosen for the control of the main lobe shape, the phase centre location, and compactness. The corrugation profile of each horn was designed to achieve a trade-off between angular resolution and straylight rejection. Each feed horn is connected to an orthomode transducer (OMT) to divide the field propagating into the horn into two orthogonal linear polarization components, X and Y #darcangelo2010a.

Figure 1. A CAD model of the Planck focal plane, which is located di- rectly below the telescope primary mirror. It comprises the HFI bolometric 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.

The feeds and corresponding OMTs are adjusted in the focal surface so that the main beam polarization directions of the two symmetrically located feed horns in the focal plane unit (FPU) are at an angle of 45 degrees when observed in the same direc- tion in the sky. This configuration permits measurement of the Q and U Stokes parameters and thus the linear polarization of the CMB. The location and orientation of each horn is reported in Table 1, with respect to the reference detector plane (RDP) coordinate system, placed in the center of the FPU and with the ZRDP axis aligned along the chief ray of the telescope.

Effective beams

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1. References

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