  Telescope Point Spread Function (PSF) quality is critical for realising the
potential of cosmic weak lensing observations to constrain dark energy and test
General Relativity. In this paper we use quantitative weak gravitational
lensing measures to inform the precision of lens optical alignment, with
specific reference to the Dark Energy Survey (DES). We compute optics spot
diagrams and calculate the shear and flexion of the PSF as a function of
position on the focal plane. For perfect optical alignment we verify the high
quality of the DES optical design, finding a maximum PSF contribution to the
weak lensing shear of 0.04 near the edge of the focal plane. However this can
be increased by a factor of approximately three if the lenses are only just
aligned within their maximum specified tolerances. We calculate the E and
B-mode shear and flexion variance as a function of de-centre or tilt of each
lens in turn. We find tilt accuracy to be a few times more important than
de-centre, depending on the lens considered. Finally we consider the compound
effect of de-centre and tilt of multiple lenses simultaneously, by sampling
from a plausible range of values of each parameter. We find that the compound
effect can be around twice as detrimental as when considering any one lens
alone. Furthermore, this combined effect changes the conclusions about which
lens is most important to align accurately. For DES, the tilt of the first two
lenses is the most important.
