Milky Way season is coming. Currently, now at the end of May, the Milky Way rises over the southern flank of Snowhoe Mountain around midnight. A month later, near the end of June, it will be rising two hours earlier.
Last night the sky brightness was measured (near the zenith) as sqml=21.60 mpsas, a very respectably dark value. What the unaided eye can not easily see is the strong airglow at low elevations. The camera, however, records it easily and it makes a colorful overlay on the distant star clouds of the Milky Way.
These images were obtained with a Sony A7iii + Sony 20mm f/1.8 G lens (ISO 1600, 30s on a tracking mount).
The Milky Way rising over McCall Gulch on the southern flank of Snowshoe Mountain:
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| Sparkle-6 filter. |
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| Softon-A filter. |
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| Softon-A filter. |
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| Sparkle-6 filter. |
Airglow is caused by the night-time recombination of molecules dissociated by UV light from the sun. It occurs at altitudes near 100 km and higher, which is considered to be the boundary of space. Green and purple colors arise from different molecules that stratify at different altitudes. It is possible to see moving bands of color in images taken only minutes apart. Airglow imposes a natural limit to how dark the sky can get when viewed from the Earth's surface.
Airglow is not a static feature. Here is what it looked like about 1/2 hour before the previous images:
As usual, click on an image to enter Gallery View and get access to the full-size versions.
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