Flexion measurement in simulations of Hubble Space Telescope data
  We present a simulation analysis of weak gravitational lensing flexion and
shear measurement using shapelet decomposition, and identify differences
between flexion and shear measurement noise in deep survey data. Taking models
of galaxies from the Hubble Space Telescope Ultra Deep Field (HUDF) and
applying a correction for the HUDF point spread function we generate lensed
simulations of deep, optical imaging data from Hubble's Advanced Camera for
Surveys (ACS), with realistic galaxy morphologies. We find that flexion and
shear estimates differ in our measurement pipeline: whereas intrinsic galaxy
shape is typically the dominant contribution to noise in shear estimates, pixel
noise due to finite photon counts and detector read noise is a major
contributor to uncertainty in flexion estimates, across a broad range of galaxy
signal-to-noise. This pixel noise also increases more rapidly as galaxy
signal-to-noise decreases than is found for shear estimates. We provide simple
power law fitting functions for this behaviour, for both flexion and shear,
allowing the effect to be properly accounted for in future forecasts for
flexion measurement. Using the simulations we also quantify the systematic
biases of our shapelet flexion and shear measurement pipeline for deep Hubble
data sets such as Galaxy Evolution from Morphology and SEDs, Space Telescope
A901/902 Galaxy Evolution Survey or the Cosmic Evolution Survey. Flexion
measurement biases are found to be significant but consistent with previous
studies.
