Gravitational Telescopes

Abstract

Some ways in which gravitational lenses can act as crude telescopes are reviewed. Magnification limits associated with finite source size, corrugation of the gravitational potential and finite wavelength are specified. It should be possible to obtain rotation curves for a small sample of ultra-faint galaxies imaged as giant arcs. There is an appreciable chance of eventually observing a radio jet component cross a caustic. The successful observation of microlensing in 2237+031 suggests that the continuum emission from this source at blue wavelengths is at least partly non-thermal. The maximum possible magnification observable, granted existing constraints is argued to be from γ-ray pulsars crossing caustic sheets formed by galaxies or a hypothetical population of intergalactic massive objects.