SIMBAD references

We demonstrate that gravitationally lensed quasars are easily recognized using image subtraction methods as time variable sources that are spatially extended. For Galactic latitudes|b|≳20°, lensed quasars dominate the population of spatially extended variable sources, although there is some contamination from variable star pairs, variable star-quasar pairs, and binary quasars that can be easily controlled using other information in the survey such as the object light curves and colors. This method will allow planned large-scale synoptic surveys to find lensed quasars almost down to their detection limits without the need for extensive follow-up observations.