Astronomy Picture of the Day . A Network of Microlensing Caustics . ... 15.12.2002 . ... Potsdam ) Explanation: A virtual sky map like this would be of interest to astronomers studying gravitational microlensing. In microlensing , the gravity of stars near the line of sight can act to magnify the light of background objects such as distant stars , or quasars . ... When a background quasar moves across a microlensing caustic, it can appear dramatically brighter . ... December 2002 . ...