Some images on Darkenedsky.com are used with permission under the Creative Commons, By Attribution License.

http://www.flickr.com/photos/stevan/46655936/
http://www.flickr.com/photos/gaetanlee/267611943/
http://www.flickr.com/photos/chefranden/196695718/
http://www.flickr.com/photos/jimieye/495406380/