Galaxy shear estimation from stacked images
  Statistics of the weak lensing of galaxies can be used to constrain cosmology
if the galaxy shear can be estimated accurately. In general this requires
accurate modelling of unlensed galaxy shapes and the point spread function
(PSF). I discuss suboptimal but potentially robust methods for estimating
galaxy shear by stacking images such that the stacked image distribution is
closely Gaussian by the central limit theorem. The shear can then be determined
by radial fitting, requiring only an accurate model of the PSF rather than also
needing to model each galaxy accurately. When noise is significant asymmetric
errors in the centroid must be corrected, but the method may ultimately be able
to give accurate un-biased results when there is a high galaxy density with
constant shear. It provides a useful baseline for more optimal methods, and a
test-case for estimating biases, though the method is not directly applicable
to realistic data. I test stacking methods on the simple toy simulations with
constant PSF and shear provided by the GREAT08 project, on which most other
existing methods perform significantly more poorly, and briefly discuss
generalizations to more realistic cases. In the appendix I discuss a simple
analytic galaxy population model where stacking gives optimal errors in a
perfect ideal case.
