Diffraction-limited gamma-ray imaging with Fresnel lenses

Fresnel lenses can focus gamma-rays by using a combination of diffraction and refraction. Such lenses (and variations on them in which the chromatic aberration is much reduced) have the potential for revolutionizing gamma-ray astronomy. Diffraction-limited lenses of several meters in size are feasible and do not require high technology for their manufacture. Focal lengths are long - up to a million kilometers - but developments in formation flying of spacecraft make possible a mission in which the lens and detector are on two separate spacecraft separated by this distance. A telescope based on these principles can have angular resolution better than a micro second of arc - sufficient to resolve the event horizon of black holes in the nucleii of AGNs. At the same time, the sensitivity can be three orders of magnitude better than that of current instrumentation.