Weighted average of the point spread functions (PSFs) for the horizontal polarization mixer (blue circle) and vertical polarization mixer (red circle) obtained at various times during the HIFI observation towards comet C/2006 W3 (Christensen). The orientation of the orthogonal polarizations is shown by the thick blue and red lines (H and V). The trajectory of the comet nucleus projected on the sky during the HIFI integration of 48 min is shown by the solid line according to the HORIZONS ephemeris. The projected direction toward the Sun and projected velocity vector of the comet with respect to the origin are indicated by the dotted and dashed arrows. The figure is centered on the average synthetic beam aperture located in the center of the two polarization beams and shown by the black point.