Calibration of colour gradient bias in shear measurement using
  HST/CANDELS data
  Accurate shape measurements are essential to infer cosmological parameters
from large area weak gravitational lensing studies. The compact
diffraction-limited point-spread function (PSF) in space-based observations is
greatly beneficial, but its chromaticity for a broad band observation can lead
to new subtle effects that could hitherto be ignored: the PSF of a galaxy is no
longer uniquely defined and spatial variations in the colours of galaxies
result in biases in the inferred lensing signal. Taking Euclid as a reference,
we show that this colourgradient bias (CG bias) can be quantified with high
accuracy using available multi-colour Hubble Space Telescope (HST) data. In
particular we study how noise in the HST observations might impact such
measurements and find this to be negligible. We determine the CG bias using HST
observations in the F606W and F814W filters and observe a correlation with the
colour, in line with expectations, whereas the dependence with redshift is
weak. The biases for individual galaxies are generally well below 1%, which may
be reduced further using morphological information from the Euclid data. Our
results demonstrate that CG bias should not be ignored, but it is possible to
determine its amplitude with sufficient precision, so that it will not
significantly bias the weak lensing measurements using Euclid data.
