At V band, the emission from airglow is V=22 per square arc-second at a high-altitude observatory on a moonless night; in excellent seeing conditions, the image of a star will be about 0.7 arc-second across with an area of 0.4 square arc-second, and so the emission from airglow over the area of the image corresponds to about V=23.
The best conditions give a seeing disk diameter of ~0.4 arcseconds and are found at high-altitude observatories on small islands such as Mauna Kea or La Palma.
The seeing statistics at ORM make it the second-best location for optical and infrared astronomy in the Northern Hemisphere, after Mauna Kea Observatory, Hawaii.
Astronomical seeing, the blurring effects of air turbulence in the atmosphere
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A key measure of the quality of atmospheric turbulence is the seeing diameter, also known as Fried's seeing diameter.