I Zw 1 Top left panel: (u,v) coverage. The first observation of a stack is shown in black, other good observations (that are stacked to the preceding “black” one) in gray, bad observations in red. In the upper right hand the co-ordinates of the target are given for reference. Top right: model residuals on the (u,v) plane. The diameter of the filled circle indicates, in units of the measurement error, the difference between the observed correlated flux at that (u,v) position and the model predicted flux (see bottom left panel). The difference is displayed in green if the observed flux is lower than the model predicted one and in blue otherwise. Measurements that lie very close to the model predictions have very small filled circles and are therefore additionally encircled. Bottom left: radial plot with position angle (PA) color-coded. Note that PA is only unique modulo 180 degrees. Bottom right: χ2 plane. In it, the best fit is marked with a black cross, the 1σ limits to the best fit are marked with dotted lines. Since the total flux, Fν,tot, is well constrained, we approximate its error with zero in order to reduce the model to two parameters: Gaussian FWHM of the resolved source, Θg, and point source fraction fp. In the χ2 plot, the abscissae are the fluxes of the two modelled components, Fg ≡ (1 − fp)·Fν,tot and Fp ≡ fp·Fν,tot.
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