  Diffractive/refractive optics, such as Phase Fresnel Lenses (PFL's), offer
the potential to achieve excellent imaging performance in the x-ray and
gamma-ray photon regimes. In principle, the angular resolution obtained with
these devices can be diffraction limited. Furthermore, improvements in signal
sensitivity can be achieved as virtually the entire flux incident on a lens can
be concentrated onto a small detector area. In order to verify experimentally
the imaging performance, we have fabricated PFL's in silicon using gray-scale
lithography to produce the required Fresnel profile. These devices are to be
evaluated in the recently constructed 600-meter x-ray interferometry testbed at
NASA/GSFC. Profile measurements of the Fresnel structures in fabricated PFL's
have been performed and have been used to obtain initial characterization of
the expected PFL imaging efficiencies.

Optics;Astrophysics