  We report the characterization of an achromatic focusing metasurface at
oblique incident angles. We show that in addition to the inherent off-axis
aberrations that occurs due to the hyperbolic phase profile of the metasurface,
the focusing performance is significantly degraded due to the meta-atoms'
angular dispersion. To obtain insights into how the angular and spectral
bandwidth of meta-atoms relate to the metasurface focusing performance,
point-dipole models are used which incorporate different aspect's of the
meta-atoms' angular response. It is emphasized that despite the meta-atoms
being designed under the assumption that they support a single dipolar
resonance, other resonances exist within the meta-atom geometry and become
stronger at oblique incidence. These resonances disturb the designed phase and
amplitude responses, resulting in lower focusing efficiency at higher incident
angles. The modelling of higher order modes leads to good agreement with the
experimental measurements, confirming that angular dispersion of the meta-atoms
is the dominant mechanism in determining off-axis aberrations.
