Induced CMB quadrupole from pointing offsets
  Recent claims in the literature have suggested that the {\it WMAP} quadrupole
is not primordial in origin, and arises from an aliasing of the much larger
dipole field because of incorrect satellite pointing. We attempt to reproduce
this result and delineate the key physics leading to the effect. We find that,
even if real, the induced quadrupole would be smaller than claimed. We discuss
reasons why the {\it WMAP} data are unlikely to suffer from this particular
systematic effect, including the implications for observations of point
sources. Given this evidence against the reality of the effect, the similarity
between the pointing-offset-induced signal and the actual quadrupole then
appears to be quite puzzling. However, we find that the effect arises from a
convolution between the gradient of the dipole field and anisotropic coverage
of the scan direction at each pixel. There is something of a directional
conspiracy here -- the dipole signal lies close to the Ecliptic Plane, and its
direction, together with the {\it WMAP} scan strategy, results in a strong
coupling to the $Y_{2,\,-1}$ component in Ecliptic co-ordinates. The dominant
strength of this component in the measured quadrupole suggests that one should
exercise increased caution in interpreting its estimated amplitude. The {\it
Planck} satellite has a different scan strategy which does not so directly
couple the dipole and quadrupole in this way and will soon provide an
independent measurement.
