Dark Skies in the Australian Outback
August 29, 2019
Image provider: Andrew Cool
Summary Authors: Andrew Cool; Jim Foster
The luminance image above shows the exceptional sky darkness at Beltana Station in the northern reaches of South Australia (the outback), about 310 miles (500 km) north of Adelaide. This image was taken on April 6, 2019. The vertical scale is sky brightness (darkness) of the night sky in magnitude. So, a value of 19 is 2.51 times brighter than a scale of 20. The greenish-yellow strip across the upper center of the image is light emanating from the Milky Way, which raises the sky brightness by about 0.8 magnitude when it passes over the zenith.
A value of 19 is typical for the darkness of the night sky in the suburbs of Adelaide, a city of approximately 1.2 million people, and many large cities have brightness values closer to 17. Truly dark skies (blue/black on this image) have values of 21.5, and a value of 22 is incredibly dark – about as dark as it gets anywhere on Earth.
The 30 people or so who make their home in Beltana are looking to make their community a dark sky preserve. The bottom image shows the Milky Way and the zodiacal light (bottom right center) acquired from Beltana Station. You can see the lights from a few homestead buildings along the rim of the disk (the horizon) at upper right, but no other sources of light pollution can be detected.
Image Details: Top image - Canon 6D camera; Sigma 8mm Fisheye lens; processed with Prof. Zoltan Kollath's Dicalum package to report sky brightness in mag/arcsec^2. Bottom image - ISO 3200; 12-second exposure.