  In the present work we discuss a possibility to build an instrument with two
operation modes - spectral and imaging ones. The key element of such instrument
is a dispersive and filtering unit consisting of two narrowband volume-phase
holographic gratings. Each of them provides high diffraction efficiency in a
relatively narrow spectral range of a few tens of nanometers. Besides, the
position of this working band is highly dependent on the angle of incidence. So
we propose to use a couple of such gratings to implement the two operational
modes. The gratings are mounted in a collimated beam one after another. In the
spectroscopic mode the gratings are turned on such angle that the diffraction
efficiency curves coincide, thus the beams diffracted on the first grating are
diffracted twice on the second one and a high-dispersion spectrum in a narrow
range is formed. If the collimating and camera lenses are corrected for a wide
field it is possible to use a long slit and register the spectra from its
different points separately. In the imaging mode the gratings are turned to
such angle that the efficiency curves intersect in a very narrow wavelength
range. So the beams diffracted on the first grating are filtered out by the
second one except of the spectral component, which forms the image. In this
case the instrument works without slit diaphragm on the entrance. We provide an
example design to illustrate the proposed concept. This optical scheme works in
the region around 656 nm with F/# of 6.3. In the spectroscopic mode it provides
a spectrum for the region from 641 to 671 nm with reciprocal linear dispersion
of 1.4 nm/mm and the spectral resolving power higher than 14000. In the imaging
mode it covers linear 12mm x 12mm field of view with spatial resolution of
15-30 lines/mm.
