The Sloan Lens ACS (SLACS) Survey is a project that combines the massive data volume of the Sloan Digital Sky Survey (SDSS) with the unmatched high-resolution imaging capability of the Hubble Space Telescope to identify and study a large and uniform sample of strong gravitational lens galaxies.

SLACS "lens candidates" are selected from within the spectroscopic database of the SDSS for the presence of two galaxies along the same line of sight in the sky, one much more distant than the other.  Such spectra occur with a frequency of only 1 in 500 to 1 in 1000, so a survey such as the SDSS -- with nearly a million galaxy spectra -- is key to obtaining a statistically significant sample.

Following this selection, the SLACS targets are observed with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST).  The exquisite angular resolution of the HST often reveals the image of the more distant galaxy distorted into a ring by the gravity of the nearer galaxy.  The following images show the difference in image sharpness between the ground-based 2.5-meter SDSS telescope (left) and the space-based 2.5-meter HST (right):

The HST images allow us to measure the angular size of these "Einstein rings", which in combination with the distances measured from the SDSS spectra give us direct measurements of the enclosed masses of the nearer galaxies (or "lens galaxies").  These masses are then combined with measurements of the sizes, brightnesses, and stellar velocities of the lens galaxies to yield insights into their structure and evolution.

The SLACS sample is currently the largest single collection of these "strong gravitational lenses" with known distances (redshifts) to both components.  The results of the survey to date are summarized on the "Major Results" page, and described in detail in the papers listed on the "Publications" page.