Lessons from a blind study of simulated lenses: image reconstructions do
  not always reproduce true convergence
  In the coming years, strong gravitational lens discoveries are expected to
increase in frequency by two orders of magnitude. Lens-modelling techniques are
being developed to prepare for the coming massive influx of new lens data, and
blind tests of lens reconstruction with simulated data are needed for
validation. In this paper we present a systematic blind study of a sample of 15
simulated strong gravitational lenses from the EAGLE suite of hydrodynamic
simulations. We model these lenses with a free-form technique and evaluate
reconstructed mass distributions using criteria based on shape, orientation,
and lensed image reconstruction. Especially useful is a lensing analogue of the
Roche potential in binary star systems, which we call the $\textit{lensing
Roche potential}$. This we introduce in order to factor out the well-known
problem of steepness or mass-sheet degeneracy. Einstein radii are on average
well recovered with a relative error of ${\sim}5\%$ for quads and ${\sim}25\%$
for doubles; the position angle of ellipticity is on average also reproduced
well up to $\pm10^{\circ}$, but the reconstructed mass maps tend to be too
round and too shallow. It is also easy to reproduce the lensed images, but
optimising on this criterion does not guarantee better reconstruction of the
mass distribution.
