Similar to Fig. 2 but assuming that the effective beam corresponds to the real SPIRE 250 μm beam as opposed to a strictly Gaussian beam. (a) Convolution of a model filament with a p = 2 Plummer-like radial profile by a non-Gaussian beam corresponding to the SPIRE beam at 250 μm. The dash-dotted purple curve shows this non-Gaussian beam on top of the same uniform background as in Fig. 2a. The dashed purple curve is the filament profile after convolution with this beam (assuming the same ratio as in Fig. 2a). The solid and dash-dotted black curves are the same as in Fig. 2a. (b) Same as in Fig. 2b but when the convolved synthetic data are constructed with a non-Gaussian beam, with a shape as shown in panel (a), and the estimated diameters are derived as in Fig. 2b, assuming a strictly Gaussian beam.