M82 FTS spectrum corrected with the method developed in this work. a) Spectrum of M82 observed by the FTS on 2010−11−08 (OD 543). Red and blue indicate the spectra observed by the SLWC3 and SSWD4, respectively. The dashed line shows the spectrum calibrated by the point-source calibration mode. The solid line shows the spectrum corrected by the method presented in this work with a best-fit source r_e = 11′′ (see Eq. (18).) b) Comparison of the CO SLEDs from the starburst core of M82. Red indicates the fluxes derived from the central detector spectrum (SLWC3 and SSWD4), corrected for the source distribution and convolved to a Gaussian beam of FWHM = 42′′ (red). Cyan is the same spectrum corrected by a source-beam coupling factor derived from the SPIRE photometry 250   μm map with appropriate profiles to produce the continuum light distribution seen with the FTS (cyan, Kamenetzky et al. 2012).