The summer Milky Way at Sierrita Observatory is a spectacular sight. These wide angle views show the southern Milky Way setting along the SSW horizon. The left image runs from Cepheus at the top, through the Galactic Bulge in Sagittarius, just above the horizon. About 1/4 the way down from the top is the constellation Cygnus, with NGC7000 (“North America Nebula”) glowing red to the left of the bright star, Alpha Cygni (Deneb). The other two images show the sky below Sagittarius. To the right of the Galactic Bulge, notice the dark region called the “Pipe Nebula” which actually consists of several contiguous galactic dust clouds known as Barnard objects.

In this compass direction, there is no human development all the way to the Gulf of California, but the sky brightness increases toward the horizon as we view a longer optical path length through the upper atmosphere. Emission from the upper atmosphere, or “airglow”, occurs from forbidden transitions of OⅠ λ6300 (red) and NⅡ λ5577 (green), and other species. [ref]. Very close to the horizon, however, the extinction rapidly increases from looking through the low elevation dust layer.

Left: 2016-09-03, Nikon D800e + 12mm fisheye, f/2.8, 156 seconds @ ISO 1600″
Middle: 2016-09-04, Nikon D800e + 14-24mm @ 20mm, f/2.8, 30 seconds @ ISO 2000
Right: 2016-08-27, Nikon D800e + 14-24mm @ 24mm, f/2.8, 95 seconds @ ISO 1600
Not shown above: Comparison with labeled objects in SkyGazer