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Difference imaging provides a new way to discover gravitationally lensedquasars because few non-lensed sources will show spatially extended, timevariable flux. We test the method on lens candidates in the Sloan Digital SkySurvey (SDSS) Supernova Survey region from the SDSS Quasar Lens Search (SQLS)and their surrounding fields. Starting from 20768 sources, including 49 SDSSquasars and 36 candidate lenses/lensed images, we find that 21 sourcesincluding 15 SDSS QSOs and 7 candidate lenses/lensed images are non-periodicvariable sources. We can measure the spatial structure of the variable flux for18 of these sources and identify only one as a non-point source. This sourcedoes not display the compelling spatial structure of the variable flux of knownlensed quasars, so we reject it as a lens candidate. None of the lenscandidates from the SQLS survive our cuts. Given our effective survey area oforder 0.71 square degrees, this indicates a false positive rate of order oneper square degree for themethod. The fraction of quasars not found to bevariable and the false positive rate should both fall if we analyze the full,later data releases for the SDSS fields. While application of the method to theSDSS is limited by the resolution, depth, and sampling of the survey, severalfuture surveys such as Pan-STARRS, LSST, and SNAP will avoid these limitations.

Difference Imaging of Lensed Quasar Candidates in the SDSS Supernova Survey Region