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G. Skinner, P. von Ballmoos (CESR, Toulouse), N. Gehrels, J. Krzmanic (NASA GSFC)
Refractive indices at gamma-ray wavelengths are such that material thicknesses of the order of millimeters allow the phase of a wavefront to be changed by up to 2\pi. Thus a phase Fresnel lens can be made from a simple profiled thin disk of, for example, aluminium or plastic. Such a lens can easily have a collecting area of several square meters and an efficiency >90%. Ordinary engineering tolerances allow the manufacture of a lens which can be diffraction limited in the pico-meter wavelength band (up to ~MeV) and thus provides a simple optical system with angular resolution better than a micro arc second i.e. the resolution necessary to resolve structures on the scale of the event horizon of super-massive black holes in AGN. However the focal length of such a lens is very long - up to a million km. Nevertheless studies have shown that a mission `Fresnel' using a detector and a phase Fresnel lens on two station-keeping spacecraft separated by such a distance is feasible. Results from these studies and work on other proof of concept studies are presented.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.