  The shapes of galaxies can be quantified by ratios of their quadrupole
moments. For faint galaxies, observational noise can make the denominator close
to zero, so the ratios become ill-defined. Knowledge of these ratios (i.e.
their measured standard deviation) is commonly used to assess the efficiency of
weak gravitational lensing surveys. Since the requirements cannot be formally
tested for faint galaxies, we explore two complementary mitigation strategies.
In many weak lensing contexts, the most problematic sources can be removed by a
cut in measured size. We investigate how a size cuts affects the required
precision of the charge transfer inefficiency model and find slightly wider
tolerance margins compared to the full size distribution. However, subtle
biases in the data analysis chain may be introduced. Instead, as our second
strategy, we propose requirements directly on the quadrupole moments
themselves. To optimally exploit a Stage-IV dark energy survey, we find that
the mean and standard deviation of a population of galaxies' quadrupole moments
must to be known to better than $1.4\times10^{-3}$ arcsec$^{2}$, or the Stokes
parameters to $1.9\times10^{-3}$ arcsec$^2$. This testable requirement can now
form the basis for future performance validation, or for proportioning the
requirements between subsystems to ensure unbiased cosmological parameter
inference.
