  The Transient Double-beam Spectrograph (TDS) is designed for optical
low-resolution observations of non-stationary and extragalactic sources with
the 2.5-m telescope of Caucasus Mountain Observatory of the Sternberg
Astronomical Institute. It operates simultaneously in a short-wavelength
(360--577 nm, reciprocal dispersion 1.21 A/pixel, resolving power R=1300 with a
1 arcsec slit) and long-wavelength (567--746 nm, 0.87 A/pixel, R=2500)
channels. The light is split by a dichroic mirror with a 50% transmission at
574 nm. In the "blue" channel, the automatic replacement of the grating by a
grism with a double resolving power is possible. Two CCD-cameras use E2V 42-10
detectors cooled down to $-70^\circ$C with a readout noise of 3 $e-$ at a
readout rate of 50 kHz. The spectrograph is equipped with a back slit viewer
camera and a calibration unit allowing to record a comparison spectrum from a
hollow cathode lamp for wavelength calibration or from an LED source with a
continuous spectrum (the "flat field") to take into account the vignetting and
uneven slit illumination. The throughput of the entire optical path without
slit loss is 20% at the zenith in the "blue" and 35% in the "red" channel.
Excluding the atmosphere and the telescope, the efficiency of the TDS itself
reaches a maximum of 47% and 65% respectively. The spectrograph is permanently
mounted in the Cassegrain focus of the 2.5-m telescope of CMO SAI MSU sharing
the port with a wide-field photometric CCD-camera. The spectrograph is fed by
the light from a folding mirror introduced into the optical path. Since
November 2019, TDS has been used for regular observations of non-stationary
stars and extragalactic sources up to 20-th mag in a 2-h exposure with a
signal-to-noise ratio >5 per pixel.
