We present RingFinder , a tool for finding galaxy-scale strong gravitational lenses in multi-band imaging data .
By construction , the method is sensitive to configurations involving a massive foreground early-type galaxy and a faint , background , blue source . RingFinder detects the presence of blue residuals embedded in an otherwise smooth red light distribution by difference imaging in two bands .
The method is automated for efficient application to current and future surveys , having originally been designed for the 150-deg2 Canada France Hawaii Telescope Legacy Survey ( CFHTLS ) .
We describe each of the steps of RingFinder .
We then carry out extensive simulations to assess completeness and purity .
For sources with magnification \mu > 4 , RingFinder reaches 42 % ( resp .
25 % ) completeness and 29 % ( resp .
86 % ) purity before ( resp .
after ) visual inspection .
The completeness of RingFinder is substantially improved in the particular range of Einstein radii 0 \farcs 8 \leq R _ { Ein } \leq 2 \farcs 0 and lensed images brighter than g = 22.5 , where it can be as high as \sim 70 % . RingFinder does not introduce any significant bias in the source or deflector population .
We conclude by presenting the final catalog of RingFinder CFHTLS galaxy-scale strong lens candidates .
Additional information obtained with Hubble Space Telescope and Keck Adaptive Optics high resolution imaging , and with Keck and Very Large Telescope spectroscopy , is used to assess the validity of our classification , and measure the redshift of the foreground and the background objects .
From an initial sample of 640,000 early type galaxies , RingFinder returns 2500 candidates , which we further reduce by visual inspection to 330 candidates .
We confirm 33 new gravitational lenses from the main sample of candidates , plus an additional 16 systems taken from earlier versions of RingFinder .
First applications are presented in the SL2S galaxy-scale Lens Sample paper series .