Generalised model-independent characterisation of strong gravitational
  lenses I: Theoretical foundations
  We extend the model-independent approach to characterise strong gravitational
lenses of Wagner & Bartelmann (2016) to its most general form to leading order
by using the orientation angles of a set of multiple images with respect to
their connection line(s) in addition to the relative distances between the
images, their ellipticities and time-delays. For two symmetric images that
straddle the critical curve, the orientation angle additionally allows to
determine the slope of the critical curve and a second (reduced) flexion
coefficient at the critical point on the connection line between the images. It
also allows to drop the symmetry assumption that the axis of largest image
extension is orthogonal to the critical curve. For three images almost forming
a giant arc, the degree of assumed image symmetry is also reduced to the most
general case, allowing to describe image configurations for which the source
need not be placed on the symmetry axis of the two folds that unite at the
cusp. For a given set of multiple images, we set limits on the applicability of
our approach, show which information can be obtained in cases of merging
images, and analyse the accuracy achievable due to the Taylor expansion of the
lensing potential for the fold case on a galaxy cluster scale NFW-profile, a
fold and cusp case on a galaxy cluster scale SIE-profile, and compare the
generalised approach with the one of Wagner & Bartelmann (2016). The position
of the critical points is reconstructed with less than 5" deviation for
multiple images closer to the critical points than 30\% of the (effective)
Einstein radius and the slope of the critical curve at a fold and its shape in
the vicinity of a cusp deviate less than 20\% from the true values for
distances of the images to the critical points less than 15\% of the
(effective) Einstein radius.
