A demonstration of position angle-only weak lensing shear estimators on
  the GREAT3 simulations
  We develop and apply the position angle-only shear estimator of Whittaker et
al. (2014) to realistic galaxy images. This is done by demonstrating the method
on the simulations of the GREAT3 challenge (Mandelbaum et al. 2014b), which
include contributions from anisotropic PSFs. We measure the position angles of
the galaxies using three distinct methods - the integrated light method,
quadrupole moments of surface brightness, and using model-based ellipticity
measurements provided by IM3SHAPE. A weighting scheme is adopted to address
biases in the position angle measurements which arise in the presence of an
anisotropic PSF. Biases on the shear estimates, due to measurement errors on
the position angles and correlations between the measurement errors and the
true position angles, are corrected for using simulated galaxy images and an
iterative procedure. The properties of the simulations are estimated using the
deep field images provided as part of the challenge. A method is developed to
match the distributions of galaxy fluxes and half-light radii from the deep
fields to the corresponding distributions in the field of interest. We recover
angle-only shear estimates with a performance close to current well-established
model and moments-based methods for all three angle measurement techniques. The
Q-values for all three methods are found to be Q~400. The code is freely
available online at http://www.jb.man.ac.uk/~mbrown/angle_only_shear/.
