A likelihood-based method for measuring weak gravitational lensing shear in deep galaxy surveys is described and applied to the Canada-France-Hawaii Telescope ( CFHT ) Lensing Survey ( CFHTLenS ) .
CFHTLenS comprises 154 deg ^ { 2 } of multicolour optical data from the CFHT Legacy Survey , with lensing measurements being made in the i ^ { \prime } band to a depth i ^ { \prime } _ { AB } < 24.7 , for galaxies with signal-to-noise ratio \nu _ { SN } \ga 10 .
The method is based on the lens fit algorithm described in earlier papers , but here we describe a full analysis pipeline that takes into account the properties of real surveys .
The method creates pixel-based models of the varying point spread function ( PSF ) in individual image exposures .
It fits PSF-convolved two-component ( disk plus bulge ) models , to measure the ellipticity of each galaxy , with bayesian marginalisation over model nuisance parameters of galaxy position , size , brightness and bulge fraction .
The method allows optimal joint measurement of multiple , dithered image exposures , taking into account imaging distortion and the alignment of the multiple measurements .
We discuss the effects of noise bias on the likelihood distribution of galaxy ellipticity .
Two sets of image simulations that mirror the observed properties of CFHTLenS have been created , to establish the method ’ s accuracy and to derive an empirical correction for the effects of noise bias .