Model-independent characterisation of strong gravitational lenses

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Table 1

Model-independent information that can be determined for different combinations of observables at folds and cusps.

Observables	Fold	Cusp

δ	–	Eq. (24)
δ, t_d	Eq. (9)	Eqs. (23), (24)
δ,r	Eq. (10)	Eqs. (24), (25)
δ, t_d ,r	Eqs. (9), (10)	Eqs. (23)−(25)
δ,r,μ	Eq. (10)	Eqs. (24), (25), (31), (32)
δ, t_d ,r,μ	Eqs. (9), (10)	Eqs. (21), (22), (24), (25), (31), (32)

Notes. δ denotes the relative distances between the images (directly measurable in the fold case and determined by Eqs. (26) to (28) at a cusp), r the axis ratios of the extended images, t_d time-delay information, and μ magnification ratios.

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