The Brightness of the Sky of the Caucasian Mountain Observatory of MSU in the Near Infrared

The results of measurements of background brightness in the near-infrared range (J, H, K bands), carried out in 2016–2023 at the Caucasus Mountain Observatory of Moscow State University was analyzed. It is shown that the instrumental background associated with the thermal radiation of the telescope is noticeable only in the \(K\) band, and at operating temperatures its contribution mainly determines the level of the overall background in this band. The coefficients of a polynomial taking into account the contribution of instrumental and extra-atmospheric backgrounds are presented. It is shown that the brightness of the sky background does not depend on air temperature, but a weak dependence on the water vapor content is observed, close to that expected from model calculations: in the \(J\) and \(H\) bands, the background brightness decreases at a rate of \( \approx {\kern 1pt} 1\% {\text{/}}1\) mm, and in the \(K\) band it grows at a rate of \( \approx {\kern 1pt} 2.5\% {\text{/}}1\) mm. The maximum amplitude of background brightness variability on short time scales (\( \sim {\kern 1pt} 30\) min) has been estimated: \( \approx {\kern 1pt} 10\)% in the \(J\) and \(K\) bands and \( \approx {\kern 1pt} 30\)% in the \(H\) band. The maximum contribution of Moon’s radiation scattered in the atmosphere to the overall background level has been determined. It is shown that this contribution can be ignored at an angular distance from the Moon greater than \( \sim {\kern 1pt} 10^\circ \) even during a full moon. The average background surface brightness mag/arcsec² in the J, H, and \(K\) bands was calculated: \({{m}_{J}} = 15.7\), \({{m}_{H}} = 13.9\), and \({{m}_{K}} = 13.1\).