O. Wucknitz [1]
A promising approach is to select objects with time-variable extended emission. Since intrinsic variability (but also scintillation) is generally only possible for compact sources, the signature of extended variability provides evidence for several compact components with small separations. In a good fraction of these objects, the multiple images will just be an illusion caused by a gravitational lens.
I discuss the development of a difference imaging algorithm in radio interferometry. Independent deconvolution with subsequent subtraction is not an option, because difference images produced in this way would be dominated by deconvolution errors. A multi-channel adaption of CLEAN is a viable alternative. The availability of powerful deconvolution techniques for variable sources may also be of relevance for other fields of research.
Similar approaches are needed for wide-band imaging, where it is not
variability with time but with frequency that has to be taken into account.
A shorter version of this talk was presented at the Hamburg LOFAR meeting. An updated
version of selected parts can be found here.