Left: the observed profiles of the Hβ flux (relative units, in black), extinction c (magenta) and dust-to-gas ratio (A_V/N_H, in red) along a strip 2′′ wide along the minor axis of NGC 7009. The extinction refers to the nebular component and an assumed interstellar component (of c = 0.04) has been subtracted. Right: plausible model profiles of emission (assumed Hβ), extinction c and dust-to-gas ratio (from sums of model gas and dust columns) plotted with scaling and plot symbols similar to the observed profiles. The model consists of two prolate ellipsoidal dust shells; see text for details.