The gravitationally lensed double quasar HE 1104−1805 was discovered by Wisotzki et al. in 1993. The two images with magnitudes of 16.70 and 18.64 are separated by 3.195". Their redshift z=2.303 indicates a distance of 11.8 Gly (given a Hubble constant H0 = 74.2 (km/s)/Mpc).
The lensing galaxy itself has been imaged by Hubble Space Telescope and lies closest to the A component, with an angular separation of 1.15".
A gravitational lens is formed when the light from a very distant bright source, such as a quasar, is bent around a massive object, such as galaxy or a cluster of galaxies, between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Einstein's general theory of relativity.

Image details:
Taken on 20/02/2010 with 10" Newton F/5, Losmandy G11
ToUCam Pro SC1
25 x 45.4s exposures
Prime focus, no filters.