  The tilt, rotation, or offset of each CCD with respect to the focal plane, as
well as the distortion of the focal plane itself, cause shape distortions to
the observed objects, an effect typically known as field distortion (FD). We
point out that FD provides a unique way of quantifying the accuracy of cosmic
shear measurement. The idea is to stack the shear estimators from galaxies that
share similar FD-induced shape distortions. Given that the latter can be
calculated with parameters from astrometric calibrations, the accuracy of the
shear estimator can be directly tested on real images. It provides a way to
calibrate the multiplicative and additive shear recovery biases within the
scientific data itself, without requiring simulations or any external data
sets. We use the CFHTLenS images to demonstrate the accuracy of the
Fourier_Quad shear recovery method. We highlight some details in our image
processing pipeline, including background removal, source identification and
deblending, astrometric calibration, star selection for PSF reconstruction,
noise reduction, etc.. We show that in the shear ranges of -0.005 < g_1 < 0.005
and -0.008 < g_2 < 0.008, the multiplicative biases are at the level of < 0.04.
Slight additive biases on the order of 5E-4 (6 sigma) are identified for
sources provided by the official CFHTLenS catalog (not using its shear
catalog), but are minor (4 sigma) for source catalog generated by our
Fourier_Quad pipeline.
