Glossary

SQM Sky Quality Scale

updated: 2025-04-29


SQM Definition

SQM refers to Sky Quality Meter, which is a hand held of mounted optical device that measures the sky brightness. The SQM values returned typically range from around 15  to max. 22 (where 21.6 is the theoretical maximum) and is measures in mag/sec2 .This gives the brightness of the sky of one square arc second. 


Sky Brightness Nomogram (from Dark Sky Awareness)

Click to enlarge


Limiting magnitudes with F = 2 (from: Limiting magnitude (Wikipedia)

SQM 18.5

SQM 19.5

SQM 20.5

SQM 21.5

5.0

5.3

5.7

6.0

where F  is a "field factor" specific to the observer and viewing situation will lie between about 1.4 and 2.4, with F = 2 being typical.

Description:


SQM = 17.0 is a typical reading for the center of a major U.S. city. It's usually pretty hard to find a spot near a city center without terrible lights overhead, but I have done some successful observing from the southern end of New York City's Central Park, which butts directly against midtown Manhattan, and from the Common in Boston, MA, which is surrounded by downtown on all sides. The top of a tall building would be another possibility. In such a setting I might be able to see stars as faint as magnitude 4.0 with considerable effort, but magnitude 3.5 is probably more realistic. The very brightest deep-sky objects are still visible with only modest optical aid; for instance M13 and the core of M31 are readily visible in 10x50 binoculars. But views of all deep-sky objects are severely degraded.


SQM = 18.0 is a typical reading on a good night at Danehy Park in Cambridge, MA, where I did the urban observations for my Urban/Suburban Messier Guide. (Enter lat,lon=42.3893, -71.1365 on the 2022 atlas.) You can refer to that guide for precisely what I could and could not see in terms of deep-sky objects. Naked-eye limiting stellar magnitude (NELM) around 4.5. The Double Cluster is visible to my unaided eye, but only because I know precisely where it is and what it looks like. The Milky Way is utterly out of the question.


SQM = 19.0 yields a NELM a bit better than 5.0, and the brighter parts of the summer Milky Way are visible. I can see M31 easily without optical aid when it's reasonably high in the sky. Deep-sky objects are still severely degraded, but star clusters can be pretty nice, and M31 appears perhaps one degree long in a telescope, as compared to bright skies, where just the more or less circular central core is visible.


SQM = 20.0 is beginning to get a lot more attractive. The NELM is around 5.5, meaning that all the important constellation stars are readily visible, and most of the Milky Way is pretty obvious. All the Messier objects are easy to see through modest-sized amateur instruments, and many of them are quite attractive. But it's still not dark enough to see the outer reaches of most galaxies. For instance it's impossible to see M51's spiral arms unless you have a really big telescope.


SQM = 21.0 is around where serious deep-sky observing begins. NELM should be at least 6.0, and the Milky Way shows in considerable detail, including numerous dark lanes. Galaxies are beginning to take on some serious structure, though their outer regions are still severely degraded. An experienced observer under skies this dark can see more than a novice would at a pristine site.


SQM = 21.4 is around the bright end of what I might plausibly call a dark sky. This is very typical reading at Vermont's famous Stellafane star party. At this point the Milky Way is fantastically detailed, and 8-inch scopes can show the spiral arms of quite a number of galaxies. But artificial skyglow is still stronger than natural sources even directly overhead, and it's much worse toward the horizon when looking toward the major light sources. Objects with low surface brightness, like Barnard's Galaxy or the California Nebula, are still pale shadows of their true selves.


SQM = 21.7 is getting dark for real. In fact a location with zero artificial light pollution can get this bright when natural sources such as airglow are unusually strong. In my experience, once you get this dark the quality of the experience depends more on the transparency than on the last decimal place of the reading on your SQM.


SQM = 22.0 is nominally as dark as skies can get, though people occasionally get legitimately darker readings.

(from: Rating Skies by SQM (Cloudy Nights))


See:

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