Seeing in the dark -- I. Multi-epoch alchemy
  Weak lensing by large-scale structure is an invaluable cosmological tool
given that most of the energy density of the concordance cosmology is
invisible. Several large ground-based imaging surveys will attempt to measure
this effect over the coming decade, but reliable control of the spurious
lensing signal introduced by atmospheric turbulence and telescope optics
remains a challenging problem. We address this challenge with a demonstration
that point-spread function (PSF) effects on measured galaxy shapes in current
ground-based surveys can be corrected with existing analysis techniques. In
this work, we co-add existing Sloan Digital Sky Survey imaging on the
equatorial stripe in order to build a data set with the statistical power to
measure cosmic shear, while using a rounding kernel method to null out the
effects of the anisotropic PSF. We build a galaxy catalogue from the combined
imaging, characterise its photometric properties, and show that the spurious
shear remaining in this catalogue after the PSF correction is negligible
compared to the expected cosmic shear signal. We identify a new source of
systematic error in the shear-shear auto-correlations arising from selection
biases related to masking. Finally, we discuss the circumstances in which this
method is expected to be useful for upcoming ground-based surveys that have
lensing as one of the science goals, and identify the systematic errors that
can reduce its efficacy.
