Study of the photon flux from the night sky at La Palma and Namibia, in the wavelength region relevant for imaging atmospheric Cherenkov telescopes

The level of the night-sky background light at La Palma and Namibia was determined, with emphasis on the wavelength region and solid angle coverage relevant for the operation of imaging atmospheric Cherenkov telescopes. The dependence of the night-sky background light both on celestial coordinates (alt,az) and on galactic coordinates (b,l) was measured, with an angular resolution of about 1°. Average light levels near the zenith are similar in both locations −2.2×1012–2.6×1012 photons sr−1 s−1 m−2 for 300 nm<λ<650 nm. With increasing zenith angle the level of background light increases at La Palma, whereas a constant level is measured in Namibia. Near the center of the Milky Way, background light levels are increased by a factor up to 4 and more. Also the level of light backscattered from the ground has been studied.