  Weak lensing by large scale structure or 'cosmic shear' is a potentially
powerful cosmological probe to shed new light on Dark Matter, Dark Energy and
Modified Gravity. It is based on the weak distortions induced by large-scale
structures on the observed shapes of distant galaxies through gravitational
lensing. While the potentials of this purely gravitational effect are great,
results from this technique have been hampered because the measurement of this
weak effect is difficult and limited by systematics effects. In particular, a
demanding step is the measurement of the weak lensing shear from wide field CCD
images of galaxies. We describe the origin of the problem and propose a way
forward for cosmic shear. Our proposed approach is based on Monte-Carlo Control
Loops and draws upon methods widely used in particle physics and engineering.
We describe the control loop scheme and show how it provides a calibration
method based on fast image simulations tuned to reproduce the statistical
properties of a specific cosmic shear data set. Through a series of iterative
loops and diagnostic tests, the Monte Carlo image simulations are made robust
to perturbations on modeling input parameters and thus to systematic effects.
We discuss how this approach can make the problem tractable and unleash to full
potential of cosmic shear for cosmology.
