  A new concept for an astronomical telescope in the MeV energy band is
presented. The concept builds on Bragg diffraction in crystals, which has been
discussed in the past, but so far a design with good sensitivity over a wide
energy range has seemed out of reach. In this paper we point out that if we
find ways to adjust, in orbit, the individual tilt of all the crystals in the
lens this would allow one single lens to cover with excellent efficiency the
full range of energies from 200 keV to 2.5 MeV in a few observation steps.
Secondly, we note that the use of lenses with double crystal layers will
increase the photon collection significantly. In an accompanying paper we
describe our overall lens design in more detail and present our first prototype
tilt adjustment pedestal for use with the individual lens facets.
  Keyords: Gamma-ray astronomy, Telescope technology, Laue lenses
