Quasar lenses and galactic streams: outlier selection and Gaia multiplet detection

I describe two novel techniques originally devised to select strongly lensed quasar candidates in wide-field surveys. The first relies on outlier selection in optical and mid-infrared magnitude space; the second combines mid-infrared colour selection with Gaia spatial resolution, to identify multiplets of objects with quasar-like colours. Both methods have already been applied successfully to the Sloan Digital Sky Survey, ATLAS and Dark Energy Survey footprints: besides recovering known lenses from previous searches, they have led to new discoveries, including quadruply lensed quasars, which are rare within the rare-object class of quasar lenses. As a serendipitous by-product, at least four candidate Galactic streams in the South have been identified among foreground contaminants. There is considerable scope for tailoring the WISE−Gaia multiplet search to stellar-like objects, instead of quasar-like, and to automatically detect Galactic streams.