  Spectrographs take snapshots of photon spectra with array detectors by
dispersing photons of different energies into distinct directions and spacial
locations. Spectrographs require optics with a large angular dispersion rate as
the key component. In visible light optics diffraction gratings are used for
this purpose. In the hard x-ray regime, achieving large dispersion rates is a
challenge. Here we show that multi-crystal, multi-Bragg-reflection arrangements
feature cumulative angular dispersion rates almost two orders of magnitude
larger than those attainable with a single Bragg reflection. As a result, the
multi-crystal arrangements become potential dispersing elements of hard x-ray
spectrographs. The hard x-ray spectrograph principles are demonstrated by
imaging a spectrum of photons with a record high resolution of $\Delta E \simeq
90 \mu$eV in hard x-ray regime, using multi-crystal optics as dispersing
element. The spectrographs can boost research using inelastic
ultra-high-resolution x-ray spectroscopies with synchrotrons and seeded XFELs.
