Examples of real asteroid trails visible in images taken with the VST, at an altitude of 2635 m. Panel a: S/N of 10.1, length of 49 pixels, altitude above the horizon of 68°, and exposure time of 45 s. Panel b: S/N of 3.5, length of 28 pixels, altitude above the horizon of 60°, and exposure time of 360 s; the smeared-out appearance of the trail is due to its very low S/N, which makes it difficult to distinguish from the background noise. The S/N quoted is per pixel.