Infra-Red Spectrum of the Night Sky

Abstract

WE photographed the infra-red spectrum of the night sky by means of a spectrograph with two prisms having an aperture of f/1·5, giving a dispersion of 2000 A./mm. near 8000 A. and of 3000 A./mm. near 10000 A. We used the only infra-red plates we had at our disposal, namely, "800" AGFA plates; we sensitized them before exposure. A ten-hour exposure made at the end of February 1942 at the Lyons Observatory gave us a spectrum¹ showing: (1) A broad band without any visible structure, stretching from λ 7400 A. to about λ 8500 A., with a notable fall of intensity beyond λ 8200 A.; according to the earlier observations made by Cabannes², it appears to be the first positive system of the nitrogen bands, the A oxygen band and the water-vapour bands. (2) A very broad band at about 0·97 μ and a line or band about 1·03 μ. Since the limit of sensitivity of the plates we used is normally about 0·85 μ, an image much beyond 0·9 μ is possible only if there is in the night sky an unusually intense emission in this spectral region.