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Table 2 Sky quality indicators for 13 sites representing a range from virtually pristine to a location inside a large city

From: The growing threat of light pollution to ground-based observatories

  Location Zenith (% above natural) Average (% above natural) Median (% above natural) Brightest square degree Illuminance (% above natural) Sky Quality Index
Mag arcsec−2 Mlux Horizontal max. Vertical
a Mauna Kea Summit, Hawaii  < 10  < 5  < 5 20.9 0.5 1.3 5.5 99.3
b Grand Canyon NP Point Imperial  < 10 6.0  < 5 20.3 0.8 1.1 13.2 97.8
c McDonald Observatory Mt. Fowlkes  < 10 18.8 15.0 19.7 1.4 8.5 13.9 90.9
d Death Valley NP Dantes View  < 10 33.8 17.8 17.6 9.6 12.7 72.4 87.8
e U.S. Naval Observatory Flagstaff Station 14 43.7 35.1 19.8 1.3 27.8 72.1 80.6
f Mojave National Preserve Sunrise Rock 22 87.0 48.6 16.6 23.8 39.5 217 70.8
g Lake Mead NRA Temple Bar 22 139 53.8 16.5 27.7 60.2 157 65.9
h Carlsbad Caverns NP Helipad 44 129 99.0 17.5 11.0 64.3 250 57.8
i Joshua Tree NP Keyes View 45 181 96.4 17.4 11.9 91.6 397 54.1
j Shenandoah NP Hawksbill Summit 102 197 229 16.9 19.3 143 450 43.2
k Palomar Observatory, California 108 403 250 16.7 22.0 184 706 38.8
l Tule Springs NM, Hwy 95 & Mt. Charleston 1,030 2,780 1,350 16.3 32.1 1,486 7,116 18.2
m Rock Creek Park, Washington D.C 3,880 6,440 7,263 15.2 88.6 4,825 6,979 9.2
  1. The indicators were derived from all sky images of estimated artificial sky glow, using methods described in the text. The "brightest square degree" indicator is reported in luminance values which apply to the artificial component of the observed sky brightness only. For indicators expressed as a percentage above the natural background, the natural reference values are as follows: zenith and median—171 μcd m−2; average—250 μcd m−2; horizontal illuminance— .79 mlux; and max. vertical illuminance—0.4 mlux. The sky quality index ranges from 0 to 100, where 100 is pristine and zero represents a sky with no features such as stars and the Milky Way visible because they are totally overwhelmed by artificial sky glow