Approaching the Cramer-Rao Bound in Weak Lensing with PDF Symmetrization
  Weak lensing statistics is typically measured as weighted sum of shear
estimators or their products (shear-shear correlation). The weighting schemes
are designed in the hope of minimizing the statistical error without
introducing systematic errors. It would be ideal to approach the Cramer-Rao
bound (the lower bound of the statistical uncertainty) in shear statistics,
though it is generally difficult to do so in practice. The reasons may include:
difficulties in galaxy shape measurement, inaccurate knowledge of the
probability-distribution-function (PDF) of the shear estimator,
misidentification of point sources as galaxies, etc.. Using the shear
estimators defined in Zhang et al. (2015), we show that one can overcome all
these problems, and allow shear measurement accuracy to approach the Cramer-Rao
bound. This can be achieved by symmetrizing the PDF of the shear estimator, or
the joint PDF of shear estimator pairs (for shear-shear correlation), without
any prior knowledge of the PDF. Using simulated galaxy images, we demonstrate
that under general observing conditions, this idea works as expected: it
minimizes the statistical uncertainty without introducing systematic error.
