Transient Double-Beam Spectrograph for the 2.5-m Telescope of the Caucasus Mountain Observatory of SAI MSU

The Transient Double-beam Spectrograph (TDS) is designed for low-spectral-resolution optical observations of nonstationary and extragalactic sources at the 2.5-m telescope of the Caucasus Mountain Observatory (CMO) of SAI MSU. The spectra are recorded simultaneously in two channels, short-wavelength (360-577 nm, reciprocal dispersion 1.21 Å/pixel, resolution $R=1300$ with a $1^''$-wide working slit) and long-wavelength (567-746 nm, 0.87 Å/pixel, $R=2500$) ones, with the light between them being split by a dichroic mirror with a 50$%$ transmittance at 574 nm. In the ``blue'' channel, it is possible to automatically replace the main grating by an additional one with a double resolving power. Two CCD cameras based on E2V 42-10 detectors cooled down to ${-}70^{\circ}$C with a readout noise of 3 electrons at a readout rate of 50 kHz serve as detectors. The height of the entrance slit is 3 arcmin. The spectrograph incorporates a back-slit viewing camera and a calibration unit to record the line spectrum of a gas-discharge lamp and a continuum LED source (``flat field'') to take into account the vignetting and slit width nonuniformity. The transmittance of the entire optical path without losses on the slit at the zenith is 20 and 35$%$ in the ``blue'' and ``red'' channels, respectively. Excluding the atmosphere and the telescope, the efficiency of the TDS itself reaches 47 and 65$%$ at maximum, respectively. The spectrograph is permanently mounted at the Cassegrain focus of the 2.5-m CMO SAI MSU telescope together with a wide-field photometric CCD-camera; the light is fed into the spectrograph by a flat diagonal mirror inserted into the optical path. Regular observations of nonstationary stars and extragalactic sources to ${∼}20^{\textrm{m}}$ with a signal-to-noise ratio above 5 in 2 h of observations have been carried out with the TDS since November 2019.