  The photometric sky quality of Mt. Shatdzhatmaz, the site of Sternberg
Astronomical Institute Caucasian Observatory 2.5 m telescope, is characterized
here by the statistics of the night-time sky brightness and extinction. The
data were obtained as a by-product of atmospheric optical turbulence
measurements with the MASS (Multi-Aperture Scintillation Sensor) device
conducted in 2007--2013. The factors biasing night-sky brightness measurements
are considered and a technique to reduce their impact on the statistics is
proposed.
  The single-band photometric estimations provided by MASS are easy to
transform to the standard photometric bands. The median moonless night-sky
brightness is 22.1, 21.1, 20.3, and 19.0 mag per square arcsec for the $B$,
$V$, $R$, and $I$ spectral bands, respectively. The median extinction
coefficients for the same photometric bands are 0.28, 0.17, 0.13, and 0.09 mag.
The best atmospheric transparency is observed in winter.
