Cross section, y = 0, of the geometrical model adopted in our light-echo modelling. The source of the light is at x = 0, y = 0, z = 0 (black point in the figure). The observer is at x = 0, y = 0, z = ∞. x and z are in units of ct. Dust is uniformely distributed in the space satisfying z ≤ z₀ and . Dashed curves: light-echo paraboloids, Eq. (1), for two epochs differing by a factor of 2. If the source produced a flash of light, which is observed between these two epochs, dust scattering the light at present (ct = 0) towards the observer is situated between these two paraboloids.