IObservingConditions.SkyQuality Property
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IObservingConditionsSkyQuality Property

Sky quality at the observatory

Namespace:  ASCOM.DeviceInterface
Assembly:  ASCOM.DeviceInterfaces (in ASCOM.DeviceInterfaces.dll) Version: (
double SkyQuality { get; }

Property Value

Type: Double
Sky quality measured in magnitudes per square arc second
PropertyNotImplementedExceptionIf this property is not available.
NotConnectedExceptionIf the device is not connected and this information is only available when connected.

Optional property, can throw a PropertyNotImplementedException

Sky quality is typically measured in units of magnitudes per square arc second. A sky quality of 20 magnitudes per square arc second means that the overall sky appears with a brightness equivalent to having 1 magnitude 20 star in each square arc second of sky.

Examples of typical sky quality values were published by Sky and Telescope ( and, in slightly adpated form, are reproduced below:

Sky Quality (mag/arcsec2)Description
22.0 By convention, this is often assumed to be the average brightness of a moonless night sky that's completely free of artificial light pollution.
21.0 This is typical for a rural area with a medium-sized city not far away. It's comparable to the glow of the brightest section of the northern Milky Way, from Cygnus through Perseus.
20.0 This is typical for the outer suburbs of a major metropolis. The summer Milky Way is readily visible but severely washed out.
19.0 Typical for a suburb with widely spaced single-family homes. It's a little brighter than a remote rural site at the end of nautical twilight, when the Sun is 12° below the horizon.
18.0 Bright suburb or dark urban neighborhood. It's also a typical zenith skyglow at a rural site when the Moon is full. The Milky Way is invisible, or nearly so.
17.0 Typical near the center of a major city.
13.0 The zenith skyglow at the end of civil twilight, roughly a half hour after sunset, when the Sun is 6° below the horizon. Venus and Jupiter are easy to see, but bright stars are just beginning to appear.
7.0 The zenith skyglow at sunrise or sunset

See Also