Generalised model-independent characterisation of strong gravitational lenses IV: formalism-intrinsic degeneracies

Understand

Based on the standard gravitational lensing formalism with its effective, projected lensing potential in a given background cosmology, we investigate under which transformations of the source position and of the deflection angle the observable properties of the multiple images, i.e.

• the time delay differences, the relative image positions, relative shapes, and magnification ratios, remain invariant.
• As these observables only constrain local lens properties, we derive general, local invariance transformations in the areas covered by the multiple images.
• We show that the known global invariance transformations, e.g.