Scattered photons may still be observed on the detector. The grey-shaded region between the two black lines indicates the volume swept out by the observing beam of the telescope as it extends into space. Photons (light and dark blue lines) that arrive at the detector at the end of this region (marked “To observer”) will be observed as though they originated at the star. The grey circles represent a distribution of dust along the line of sight toward the star being observed. The dark full lines show the contributions we consider here: “undeflected” photons (a) which are forward scattered and do not leave the beam, and photons which are back scattered (b) into the beam. The pale dashed lines (c, d) show cases where the scattering event leads to the photon leaving the observing beam. Finally, the dotted lines (e, f) represent cases that may contribute to observations, but occur with significantly lower probabilities and are hence not considered in this paper.