IM3SHAPE: A maximum-likelihood galaxy shear measurement code for cosmic
  gravitational lensing
  We present and describe im3shape, a new publicly available galaxy shape
measurement code for weak gravitational lensing shear. im3shape performs a
maximum likelihood fit of a bulge-plus-disc galaxy model to noisy images,
incorporating an applied point spread function. We detail challenges faced and
choices made in its design and implementation, and then discuss various
limitations that affect this and other maximum likelihood methods. We assess
the bias arising from fitting an incorrect galaxy model using simple noise-free
images and find that it should not be a concern for current cosmic shear
surveys.
  We test im3shape on the GREAT08 Challenge image simulations, and meet the
requirements for upcoming cosmic shear surveys in the case that the simulations
are encompassed by the fitted model, using a simple correction for image noise
bias. For the fiducial branch of GREAT08 we obtain a negligible additive shear
bias and sub-two percent level multiplicative bias, which is suitable for
analysis of current surveys. We fall short of the sub-percent level requirement
for upcoming surveys, which we attribute to a combination of noise bias and the
mis-match between our galaxy model and the model used in the GREAT08
simulations. We meet the requirements for current surveys across all branches
of GREAT08, except those with small or high noise galaxies, which we would cut
from our analysis. Using the GREAT08 metric we we obtain a score of Q=717 for
the usable branches, relative to the goal of Q=1000 for future experiments. The
code is freely available from https://bitbucket.org/joezuntz/im3shape
