  We present an optical design of astronomic spectrograph based on a cascade of
volume-phase holographic gratings. The cascade consists of three gratings. Each
of them provides moderately high spectral resolution in a narrow range of 83
nm. Thus the spectrum image represents three lines covering region 430-680 nm.
Two versions of the scheme are described: a full-scale one with estimated
resolving power of 5300-7900 and a small-sized one intended for creation of a
lab prototype, which provides the resolving power of 1500-3000. Diffraction
efficiency modeling confirms that the system throughput can reach 75 %, while
stray light caused by the gratings crosstalk is negligible. We also propose a
design of image slicer and focal reducer allowing to couple the instrument with
the 6-m telescope. Finally, we present concept of the opto-mechanical design.
