Accuracy of the recovered peak column density (left frames) and FWHM (right frames) values of the compact isothermal sources. The plots show the relative errors for method B (crosses) and method E (plus signs) relative to idealised observations at 18.3″ resolution. The colours are used to separate 10 K and 20 K sources, blue and red for method E and cyan and magenta for method B, respectively. The upper frames correspond to a noise level of k_n = 0.1, the lower frames to k_n = 0.3. Values outside the range of [–0.6, 0.6] are plotted at those boundaries. In the FWHM plots, to reduce the overlap of the symbols, we have added jitter in the x-axis values.