  The main factors that influence the success of observations in the infrared
range (central wavelengths of the photometric bands at 3.75 and 4.8~$\mu$m) on
the multipurpose optical telescope are considered. Estimates of the sky
background brightness are obtained for the Caucasus Mountain Observatory (CMO)
of Moscow State University: $1.3\cdot10^6$~photons/(s pixel) in the 3.75~$\mu$m
band and $3.4\cdot10^6$~photons/(s pixel) in the 4.8~$\mu$m; and the
instrumental background for the 2.5-m CMO telescope at $0^\circ$C:
$3.2\cdot10^6$~photons/(s pixel) in the 3.75~$\mu$m band and
$4.3\cdot10^6$~photons/(s pixel) in the 4.8~$\mu$m band. It is shown that at
this background signal level with the currently available commercial cameras in
the $3-5$~$\mu$m spectral range, the telescope-camera coupling capabilities for
observing faint objects will still be limited by the thermal background. For
different observational conditions, estimates of the limiting magnitudes of
objects available for observations in the 3.75 and 4.8~$\mu$m ranges are
obtained. For average observation conditions (instrument temperature of
$0^\circ$C and stellar image size of 1 arcsec), the limit is $\sim10.6^m$ and
$\sim8.4^m$, respectively.
