  The Fresnel Diffractive Array Imager (FDAI) is based on a new optical concept
for space telescopes, developed at Institut de Recherche en Astrophysique et
Plan\'etologie (IRAP), Toulouse, France. For the visible and near-infrared it
has already proven its performances in resolution and dynamic range. We propose
it now for astrophysical applications in the ultraviolet with apertures from 6
to 30 meters, aimed at imaging in UV faint astrophysical sources close to
bright ones, as well as other applications requiring high dynamic range. Of
course the project needs first a probatory mission at small aperture to
validate the concept in space. In collaboration with institutes in Spain and
Russia, we will propose to board a small prototype of Fresnel imager on the
International Space Station (ISS), with a program combining technical tests and
astrophysical targets. The spectral domain should contain the Lyman-$\alpha$
line ($\lambda= 121$ nm). As part of its preparation, we improve the Fresnel
array design for a better Point Spread Function in UV, presently on a small
laboratory prototype working at 260 nm. Moreover, we plan to validate a new
optical design and chromatic correction adapted to UV. In this article we
present the results of numerical propagations showing the improvement in
dynamic range obtained by combining and adapting three methods : central
obturation, optimization of the bars mesh holding the Fresnel rings, and
orthogonal apodization. We briefly present the proposed astrophysical program
of a probatory mission with such UV optics.
  Keywords: Fresnel arrays -- Diffractive optics -- UV imaging -- Apodization
-- High dynamic range -- High angular resolution -- Exoplanets
