As part of the performance verification of HIFI in space, spectroscopic observations of the 12C16O 576.3 GHz transition were obtained at the position of the chemically-active bow shock B1 driven by the LDN 1157 Class 0 protostar (Bachiller et al. 2001). Initial observations in HIFI's Band 1B were made on August 1, 2009; a second set was obtained 186 days later on February 4, 2010. The spectral shape of the source remained essentially unchanged but initially the signal from the high-frequency edge of the line appeared stronger in the V direction, when the H and V peaks were matched, whereas half a year later it appeared stronger by the same ratio in the H direction.
This ``false polarisation'' effect was due to the small misalignment of the H and V beams described earlier, which imaged slightly different portions of a scene onto the two receivers (Attard et al. 2008). Unless otherwise specified, observations of a targeted source are centred on a position halfway between the two beam centres. Because the viewing direction of Herschel at all times is constrained by the need to keep the plane of the sun-shield roughly perpendicular to the radius vector to the Sun, sources close to the ecliptic plane can only be viewed at half year intervals. Over this interval the telescope aperture rotates 180 about its viewing direction on the sky. Thus, portions of LDN 1157 B1 initially viewed by the V channel were viewed, half a year later, by the H channel and vice versa. It is clear that this effect was not due to linear polarisation because a 180 rotation of the telescope leaves the polarisation direction unchanged.
For VY CMa, where the entire source was relatively well centred on each beam, this effect was negligible. But for linear polarisation observations of an extended source the best strategy may be to make a small map with HIFI over the area of interest, where the spectra obtained in the H and V channels at each position of the source, can be individually and directly compared.
Overlapping with the second epoch of 620.701 GHz observations, we observed the VY CMa 22.235 GHz maser with the Effelsberg 100-m telescope of the Max-Planck-Institut für Radioastronomie. Those observations were begun on April 11, 2010 at 16:40 UT and lasted until 17:20 UT and are shown in Fig. 3. Data in two linear polarisations were obtained with the two channels of the K-band or 1.3 cm receiver at the primary focus. The frequency resolution was 6.104 kHz, corresponding to a velocity resolution of 0.082 km s-1. Although these observations were realised at only one epoch and are, therefore, insufficient for a full characterisation of the linear polarisation state, a search by Vlemmings et al. (2002) found that the linear polarisation of the VY CMa 22.235 GHz masers is well below 1%.