Top panel: footprint of the SPIRE spectrometer detectors on the sky for OH 30.1 −0.7. The blue and red circles represent the beams of the SLW and SSW arrays, respectively (the SSW arrays are identified individually). Two SSW detectors are not considered for background subtraction, and are not represented, which explains the apparent asymmetry of the SSW detector array. Lower panel: smoothed off-axis detector intensities and mean off-axis intensities for the SSW array. The black and red curves represent the smoothed spectra measured in the off-axis detectors, i.e. off-source. The spectra marked in red at the edge of the distribution are not used for background subtraction. The others (in black) are averaged, and the average (dotted blue curve) is used for background subtraction.