  Weak gravitational lensing observations are a key science driver for the NASA
Wide Field Infrared Survey Telescope (WFIRST). To validate the performance of
the WFIRST infrared detectors, we have performed a laboratory emulation of weak
gravitational lensing measurements. Our experiments used a custom precision
projector system to image a target mask composed of a grid of pinholes,
emulating stellar point sources, onto a 1.7 micron cut-off Teledyne HgCdTe/H2RG
detector. We used a 880nm LED illumination source and f/22 pupil stop to
produce undersampled point spread functions similar to those expected from
WFIRST. We also emulated the WFIRST image reconstruction strategy, using the
IMage COMbination (IMCOM) algorithm to derive oversampled images from dithered,
undersampled input images. We created shear maps for this data and computed
shear correlation functions to mimic a real weak lensing analysis. After
removing only 2nd order polynomial fits to the shear maps, we found that the
correlation functions could be reduced to O(10^-6). This places a conservative
upper limit on the detector-induced bias to the correlation function (under our
test conditions). This bias is two orders of magnitude lower than the expected
weak lensing signal. Restricted to scales relevant to dark energy analyses (sky
separations > 0.5 arcmin), the bias is O(10^-7): comparable to the requirement
for future weak lensing missions to avoid biasing cosmological parameter
estimates. Our experiment will need to be upgraded and repeated under different
configurations to fully characterize the shape measurement performance of
WFIRST IR detectors.
