  Dynamical diffraction effects in single crystals produce highly monochromatic
parallel X-ray beams with a mutual separation of a few micrometer and a
time-delay of a few fs -the so-called echoes. This ultrafast diffraction effect
is used at X-ray Free Electron Lasers in self-seeding schemes to improve beam
monochromaticity. Here, we present a coherent X-ray imaging measurement of
echoes from Si crystals and demonstrate that a small surface strain can be used
to tune their temporal delay. These results represent a first step towards the
ambitious goal of strain-tailoring new X-ray optics.
