Combined χ² values in PEP analysis of ¹³CO spectra. The cloud model an isothermal BE sphere with M = 10 M_⊙ and T_kin = 20 K. The frames a-d correspond, respectively, to the combinations of the 1–4 first rotational transitions. The circles indicate the reference values towards its centre (smallest circle), averaged over a FWHM = R₀/3 beam (middle circle that corresponds to the synthetic observations), and averaged over a FWHM = R₀ beam (largest circle).