Gravitational Lensing by Galaxies
		• Galaxy lensing of background quasars leads to image separations on scales of arcseconds. Multiple images. • Must confirm not binary quasars – sometimes easy, sometimes difficult.  How do you do this? • Lens/binary quasar database: CASTLES • http://cfa-www.harvard.edu/castles/ • Note that lens modeling software by Keeton is publicly available on this page.