Using CCD Archives of Caucasus Mountain Observatory of Sai MSU To Monitor Changes in The Night Sky Background

The paper describes the methodology and analyzes the results of measurements of the sky background brightness of the CMO SAI MSU in the visible and shortwave infrared ranges made from the archival frames of the observatory (2019-2024 years). The importance of controlling the bias level constancy in such measurements is shown. In the 2023-2024 year, the median brightness of 1 arcsec2 of the moonless night sky at zenith is: B = 21.3m, V = 20.6m, Rc = 20.1m, and Ic = 19.1m. Over the 2019 year period, background brightness increased by ΔB ≈ ΔV ≈ 0.8m, ΔRc ≈ 0.5m, ΔIc ~ 0.1m. The contribution of anthropogenic illumination to the overall increase in background brightness is extracted.