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Observed visibility amplitudes (blue crosses) for a flare interval as a function of subcollimator (SC=1-9) and position angle (PA=0-180°) of the grids. Each of the 9 vertical panels shows the amplitude as a function of PA for one subcollimator (labeled by red digits below the X axis). The red curve represents a model using two Gaussian sources, and the green squares show the residuals relative to the model. For a given subcollimator (6 and 7 are good examples), the amplitude rises and falls sinusoidally while the grids rotate from PA=0 to PA=180°. In general, such sinusoidal variation indicates an extended or double source.