Lensing degeneracies revisited

This paper shows that the mass-sheet degeneracy, as well as other degeneracies in lensing, have simple geometric interpretations. They are mostly rescalings of the arrival-time surface. Different degeneracies appear in Local Group lensing than in cosmological lensing, because in the former the absolute magnification is measured but the image structure is not resolved, whereas in the latter the reverse usually applies. The most dangerous of these degeneracies is a combination we call the mass-disk degeneracy, which appears in multiply imaging galaxy lenses and may lead to large systematic uncertainties in estimates of cosmological parameters from these systems.