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UDSAC Sky Quality

The key feature of the UDSAC is providing a pristine Dark Sky to the institutional members. It is therefore important to understand what is scientifically meant by the term pristine Dark Sky. There are many methods and measurements that may be used to describe Sky Quality with respect to astronomy. Some are descriptive like the Bortler Scale that is a 1 - 9 scale  where 1 is the darkest locations on earth to 9, equivalent to inner-city skies ( The UDSAC location has been reported as 1-2 on the Bortler Scale. A more complex measurement of darkness where "radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area" ( The value of 0 indicates that there is no radiant energy, whereas, inner-cities may have a radiance values of >17 ( Radiance maps of the UDSAC location indicate a radiance of 0.27 (


The most accepted measure of darkness by astronomers is magnitudes per square arc-second.  The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2.5 times in brightness; a change of 5 magnitudes is defined as a change of exactly 100 times in brightness . On this scale, the sun has a magnitude of -27, the full moon of -13, Vega of 0, minor planets of 7 and comets of 22. (

The following letter from the International Dark Sky Association certifies the darkness of the UDSAC observatory location.

Photo of the Milky Way above the UDSAC observatory field

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