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 #darcangelo2010.
coordinate system, placed in the center of the FPU and with the ZRDP axis aligned along the chief ray of the telescope.
[LFI meaning]: absolute calibration refers to the 0th order calibration for each channel, 1 single number, while the relative calibration refers to the component of the calibration that varies pointing period by pointing period.
(Planck) Low Frequency Instrument
(Planck) High Frequency Instrument
LFI Ortho Module Transducer
European Space Agency
Focal Plane Unit