Using a simple algorithm, Belokurov et. al discovered this almost perfect Einstein-ring around a luminous red galaxy in the SDSS database:
They called it the Cosmic Horseshoe. The ring has a diameter of 10 arcseconds, which counts as large. The lensing galaxy has a mass of about 5 x 1012 solar mass - about ten times the mass of the Milky Way! For comparison, here's an other lens, also from the SDSS, discovered serendipitously, the "8 O'clock Arc":
This is less ring-shaped, it has 3 images of the same background galaxy on the top and a fourth on the bottom. It shows a similar size, but the lensing galaxy is estimated to have a mass of about 1/5 of that in the center of the Cosmic Horseshoe.
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