  The 10-100 keV region of the electromagnetic spectrum contains the potential
for a dramatic improvement in our understanding of a number of key problems in
high energy astrophysics. A deep inspection of the universe in this band is on
the other hand still lacking because of the demanding sensitivity (fraction of
microCrab in the 20-40 keV for 1 Ms integration time) and imaging (~15" angular
resolution) requirements. The mission ideas currently being proposed are based
on long focal length, grazing incidence, multi-layer optics, coupled with focal
plane detectors with few hundreds microns spatial resolution capability. The
required large focal lengths, ranging between 8 and 50 m, can be realized by
means of extendable optical benches (as foreseen e.g. for the HEXIT-SAT, NEXT
and NuSTAR missions) or formation flight scenarios (e.g. Simbol-X and XEUS).
While the final telescope design will require a detailed trade-off analysis
between all the relevant parameters (focal length, plate scale value, angular
resolution, field of view, detector size, and sensitivity degradation due to
detector dead area and telescope vignetting), extreme attention must be
dedicated to the background minimization. In this respect, key issues are
represented by the passive baffling system, which in case of large focal
lengths requires particular design assessments, and by the active/passive
shielding geometries and materials. In this work, the result of a study of the
expected background for a hard X-ray telescope is presented, and its
implication on the required sensitivity, together with the possible
implementation design concepts for active and passive shielding in the
framework of future satellite missions, are discussed.
