Airglow, Paranal Observatory

Yuri Beletsky took this image and those below at Paranal, Chili. Paranal, part of the European Southern Observatory, is the world’s most advanced optical observatory with four 8.2m and four auxiliary 1.8m telescopes that can be optically linked into a giant interferometer.

The site, high in the Atacama Desert was chosen for its ultra-dark and steady skies but Earth’s skies are rarely completely dark.  Take away the last vestiges of twilight, take away moonlight, try to evade the pollution of thoughtlessly bright and unshielded artificial lights and we find that sometimes the upper atmosphere itself glows – airglow.

Fortunately for astronomy the brightness of the red airglow in these pictures was exceptional.     

Airglow is light emitted by excited atoms and molecules 85-300 km high in the atmosphere.   The excitation derives from extreme ultra violet radiation from the sun (aurorae are collisionally excited).    Green is the most common airglow colour, see Yuri's third image below.   The less common red airglow is probably from vibrationally and rotationally excited OH radicals in a band 87 km high. Oxygen atoms at 150 – 300 km might also have formed it.

The airglow at left is banded.    Waves propagating up from the lower atmosphere influence collisional de-excitation that competes with the airglow-forming radiative de-excitation.

A yellow laser beam shines up from the Yepun telescope to create an artificial star in the upper atmosphere that controls the telescope's adaptive optics.

All images ©Yuri Beletsky, shown with permission.
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Red (upper image) and green (lower picture) airglow over Paranal's 1.8m auxiliary telescopes.   The fainter green airglow near the horizon represents the 'normal' sky over the Atacama Desert. The green comes from forbidden transitions ( O1S to 1D ) of oxygen atoms 90-100 km high in a band that is easily visible from earth orbit. The yellow patch in the upper image is light pollution from a copper mine 200km away. In the lower image, satellite galaxies, the Large and Small Magellanic Clouds blaze in the sky and at left is an upside-down Orion

Paranal's four auxiliary telescopes can re-locate on railway tracks and are optically linked to each other and the four large telescopes via mirrors and delay lines in underground tunnels. They form a giant interferometer with high resolving power and light gathering ability.